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Texas Instruments - TI.com Follow Us   |    Table of Contents Introduction Chapter 1: Operational amplifiers When is good enough good enough? Just use a 100Ω resistor: Understanding a rule of thumb for oscillating amplifiers Where did all that racket come from? Is your amplifier offset way out of whack? A good holiday-season project Don't be fooled by your amplifier's bandwidth Sallen-Key lowpass-filter stopband limitations Choosing antialiasing-filter amplifiers Closer to real-world analog filters Simply an adjustable low-pass filter Photo-sensing circuits: The eyes of the electronic world are watching Transimpedance-amplifier stability is key in light-sensing applications Transimpedance-amplifier noise issues Transimpedance-amplifier application: The pulse oximeter Remote photo sensing Single-supply amplifier outputs don't swing rail to rail The non-negotiable single-supply operational amplifier What does "rail to rail" input operation really mean? Rail-to-rail input amplifier application solutions Voltage- and current-feedback amps are almost the same Chapter 2: INAs and PGAs Understanding CMR and instrumentation amplifiers Common-mode range can bite hard Instrumentation amplifier input-circuit strategies Wringing out thermistor nonlinearities A good holiday-season project Author's biography   Introduction The Best of Baker's Best – Amplifiers Introduction, by Bonnie Baker Amplifiers have been around for a long time. These beasts were just coming alive in the 1930s – 1940s, at the hands of several individuals including Harold S. Black, George Philbrick, and H. W. Bode. Given the long time span between these beginning years and the present, you would think that we have finally come to terms with understanding this five-terminal http://www.ti.com/ww/en/analog/Amplifiers-eBook/[1/14/14 10:43:25 AM] Best of Baker's Best - Amplifiers eBook - TI.com Follow Us   |    Table of Contents Introduction Chapter 1: Operational amplifiers When is good enough good enough? Just use a 100Ω resistor: Understanding a rule of thumb for oscillating amplifiers Where did all that racket come from? Is your amplifier offset way out of whack? A good holiday-season project Don't be fooled by your amplifier's bandwidth Sallen-Key lowpass-filter stopband limitations Choosing antialiasing-filter amplifiers Closer to real-world analog filters Simply an adjustable low-pass filter Photo-sensing circuits: The eyes of the electronic world are watching Transimpedance-amplifier stability is key in light-sensing applications Transimpedance-amplifier noise issues Transimpedance-amplifier application: The pulse oximeter Remote photo sensing Single-supply amplifier outputs don't swing rail to rail The non-negotiable single-supply operational amplifier What does "rail to rail" input operation really mean? Rail-to-rail input amplifier application solutions Voltage- and current-feedback amps are almost the same Chapter 2: INAs and PGAs Understanding CMR and instrumentation amplifiers Common-mode range can bite hard Instrumentation amplifier input-circuit strategies Wringing out thermistor nonlinearities A good holiday-season project Author's biography   Introduction The Best of Baker's Best – Amplifiers Introduction by Bonnie Baker Amplifiers have been around for a long time. These beasts were just coming alive in the 1930s - 1940s, at the hands of several individuals including Harold S. Black, George Philbrick and H. W. Bode. Since the advent of the solid state operational amplifier (op amp), enabled by the invention of silicon transistors and the integrated circuit (IC), the amplifier has increasingly become a staple for signal conditioning in most electronic systems. Given the long time span between these beginning years and the present, you would think that we have finally come to terms with understanding this five-terminal device. Furthermore, isn't there something else out there that has successfully obsoleted these devices? Some people I have worked with are surprised when I tell them that the op amp is not dying out, but rather they are dynamic and permanent fixtures in the electronics world. This is particularly true as long as the real-world remains in the analog domain. Amplifiers, including instrumentation amplifiers (INA) and programmable gain amplifiers (PGA), are here to stay as standalone devices and as integrated functions inside complex ICs. And, to take it a step further, there is still a lot to learn about amplifiers and their appropriate placement in your application circuits. The following excerpts from the EDN Baker's Best column represent the best of the amplifier short articles. They have been compiled for your convenience. Please enjoy your read through and let me know what you think! 1 Back to top Chapter 1: Operational amplifiers When is good enough good enough? If you are having difficulty making product-selection decisions in a consumer circuit, such as the temperature-sensor circuit in Figure 1, you can quickly solve this problem by choosing the absolute best performing parts for each socket. Is this statement true or false? Using this type of logic may give you a confident feeling that your circuit will work correctly the first time. However, following such logic goes only so far when you try to justify the cost-versus-performance factors of the products you are using. In Figure 1, note that a 12-bit converter is at the end of the signal chain. So, are the highest performance analog products in front of the ADC appropriate? How do you determine which products are good enough for your system? Avoiding production-floor notifications or field failures may be your definition of "good enough." Instead of choosing the best products, you can use the RSS (root-sum-square) algebraic approach. One criterion is to keep the signal within the dynamic range of the full-scale range of the ADC. The product characteristics that influence the extent of the dynamic range are the system's cumulative offset and gain errors. As an example, assume that the maximum offset error of IC1 and IC2 is 0.5 mV. The offset error of the ADC is ±1 LSB or ±1.22 mV. (The full-scale range of the ADC is 5V.) The gain error of both the sensor cell and the IC1 amplifier configuration depends on the ±1% maximum resistor tolerances as well as on a maximum sensor-resistor tolerance of ±2%. The ADC's contributed gain error is 0.098% or equals 4.9 mV maximum at full scale. To determine the dynamic-range limitations of the circuit, if you combine all of these terms, you would calculate the combined RSS value of offset and gain, bringing these errors to the ADC's input. With the RSS formula, you take the square root of the sum of the squares of several terms that are statistically independent. You cannot use an RSS formula with entities that have correlated variations that are not statistically independent. For instance, the worst-case sensor-resistive offset error would be ±94 mV×10V/V. The contribution of the amplifier-gain stage, IC1 , is ±500 µV×10, the filter-stage (IC2 ) offset error is ±500 µV, and the ADC (IC3 ) offset error is ±1.22 mV. The cumulative possible offset error at the =940 mV. This calculation illustrates that the sensor cell contributes the most error with little ADC's input is impact from the amplifiers or the ADC. Using the same logic, you would use the RSS formula to determine gain-error contribution that limits the dynamic range from the four stages in this circuit. So, during your first consumer-product-selection attempt, you can use RSS calculations. These calculations can assist you in making logical and economical product decisions. Once you take this first step, make sure you use the same evaluation technique in your manufacturing process to quantify the effects of the processes–such as solder reflow–that you impose on these devices and the end-of-life effects due to environmental exposures. References 1. Sandler, Steven M, "A Comparison of Tolerance Analysis Methods," AEi Systems LLC, 1998. 2 Back to top Just use a 100Ω resistor: Understanding a rule of thumb for oscillating amplifiers So there I was, a new hire for a leading-edge analog-electronics company. As it was my first job, I was wide-eyed and excited–confronting new problems left and right. One problem I ran into involved the stability of an amplifier circuit. In this application, a buffer-amplifier circuit, with a capacitive load, sang like a bird. Because I had a huge community of experts around me, I ran around and asked for advice. The words of wisdom that came down to me were, "Oh, put a 100Ω load resistor between the amplifier output and the load capacitor." When I asked why, the engineer said, "Just do it. Trust me; it will work." So, I built a new circuit as suggested, and, lo and behold, the circuit still oscillated. This new circuit was still singing, but it was producing a new frequency. I returned to the engineer who gave me the first bit of great advice. His recommendation: "Change the 100Ω resistor to a 500Ω resistor," still with no explanation. It solved my problem, but, given my work load, I did not return to his suggestion for several years. Now, it has come back to haunt me. I need to know what is really going on! What I did not understand then but understand now is that a capacitor and a resistor that hang on the output of an amplifier change the amplifier's open-loop-gain curve. The combination of the load capacitor, CL ; the load resistor, RL ; and the amplifier's open-loop resistance, RO, introduces a pole to the open-loop-gain curve, and CL and RL then introduce a zero to the open-loop-gain curve (Figure 1). Creating this pole and zero does not disrupt the amplifier's stability as long as they cancel each other out before the open-loop-gain curve crosses the closed-loop-gain curve. If the open-loop-gain and closed-loop-gain curves cross with a 40-dB/decade closure rate, the amplifier circuit will be marginally unstable or, worse yet, will oscillate. You can find the pole and zero locations in this circuit in the following equations: What have I learned from this situation? It pays to understand why an engineer's rule of thumb works. If you comprehend the general guidelines, you will be OK. If you are not on top of the explanation, however, it will come back to bite you. References 1. Oljaca, Miro, and Bonnie Baker, "Start with the right op amp when driving SAR ADCs," EDN, Oct 16, 2008. 3 Back to top Where did all that racket come from? Since arriving in the market, CMOS single-supply amplifiers have been beneficial to single-supply-system designers worldwide. Key players affecting the THD+N (total-harmonic-distortion-plus-noise) characteristics of dual-supply amplifiers are input noise and output-stage crossover distortion. The THD+N performance of a single-supply amplifier also originates in the amplifier's input and output stages. However, the input stage's impact on THD+N complicates the nature of this specification with single-supply amplifiers. Several types of single-supply amplifier topologies can accept input signals across the power supplies. In the complementary-differentialinput-stage topology, when the amplifier's inputs are near the negative rail, the PMOS transistors are on, and the NMOS transistors are off (Figure 1). When the amplifier's inputs are closer to the positive rail, the NMOS transistors are on, and the PMOS transistors are off. This design topology has significant variations in the amplifier's offset voltage across the common-mode input range. In the input region near ground, the PMOS transistor's offset error is dominant. In the region near the positive power supply, the NMOS-transistor pair dominates the offset error. Both pairs are on as the amplifier's inputs pass between these two regions. The end result is that the input offset voltage changes between the stages. When the PMOS and NMOS transistors are both on, the common-mode voltage region is approximately 400 mV. This crossover-distortion phenomenon affects the amplifier's THD. If you configure the complementary-input amplifier in a noninverting configuration, the input crossover distortion can affect the amplifier's THD+N performance. For instance, in Figure 2 the THD+N is 0.0006% if you avoid using the input transition. If the THD+N tests include the amplifier's input crossover distortion, the THD+N is 0.004%. You can avoid this type of amplifier crossover distortion by usingg an inverting configuration. Another major THD+N contributor can be the operational amplifier's output stage. The output stage of a single-supply amplifier usually has an AB topology. As the output signal sweeps from rail to rail, the output stage displays a crossover distortion similar to the input-stage crossover distortion, in that the output stage switches from transistor to transistor. Generally, a higher level of quiescent current through the output stage reduces the amplifier's THD. The amplifier's input noise is another contributor to the THD+N specification. A high level of input noise, high closed-loop gains, or both can increase the amplifier's overall THD+N level. To optimize a complementary-input-single-supply amplifier's THD+N performance, place the amplifier in an inverting-gain configuration and keep the closed-loop gain low. If the system requires the amplifier to be configured as a noninverting buffer, an amplifier with a singledifferential input stage and charge pump is a more appropriate choice. References 1. "OPA350, OPA2350, OPA4350 High-Speed, Single-Supply, Rail-to-Rail Operational Amplifiers, MicroAmplifier Series," Texas Instruments, January 2005. 2. "OPA363, OPA2363, OPA364, OPA2364, OPA4364, 1.8V, 7MHz, 90dB CMRR, Single-Supply, Rail-to-Rail I/O Operational Amplifier," Texas Instruments, February 2003. 4 Back to top Is your amplifier offset way out of whack? Have you ever spent a great deal of time selecting the perfect operational amplifier for your circuit, only to find that the offset voltage is wrong at the manufacturer's bench-specified input? What if you find that it is more than 10 times higher than specification in your application circuit? Do you send the chip in for failure analysis or just toss the chip out and have another look at your list of amplifiers? As an alternative, I suggest that you try to explain the offset error by re-examining your amplifier's specifications. If you are using your amplifier as the key component in a transimpedance amplifier, an analog filter, a sample-and-hold circuit, an integrator, a capacitance transducer, or any other circuit with high-impedance components around your amplifier, you might find that the amplifier's input-bias current creates an offset-voltage error through the resistors in your circuit. In the bipolar-amplifier days, the term "input-bias current" was an accurate descriptor, and it still is. A bipolar amplifier's input-bias current is the same as the base current of the NPN or PNP transistors at the input of the amplifier. The magnitude of the bipolar amplifier's input-bias current ranges from a few nanoamperes for low-power devices to hundreds of nanoamperes for higher-power devices. The term "input-bias current" loses its meaning when you look at JFET or CMOS input amplifiers. With these types of amplifiers, the current sinking or sourcing from the amplifier's input pins is actually the leakage current from the input-ESD (electrostatic-discharge) cells (Figure 1). A more accurate descriptor for this current error is "input-leakage current." The magnitude of leakage current with JFET or CMOS amplifiers is less than 1 pA at 25°C. This specification is independent of the common-mode voltage and the magnitude-amplifier power. Almost all amplifiers have ESD cells for protection from an ESD event, but you will never see ESD-leakage current in bipolar amplifiers. The input-bias current swamps out the picoampere leakage current from the ESD cells. Input-bias and input-leakage current can change over temperature. However, depending on the operational-amplifier design, the bipolar input-bias current can be fairly stable. The JFET and CMOS input amplifiers may not be, however. Because the leakage current is from the reverse-biased ESD diodes, the leakage current increases approximately two times per 10°C change. In ensuring that the input-leakage current remains low with JFET and CMOS amplifiers, you must understand the impact of your PCB (printed-circuit board) on the picoampere levels of current. For instance, a small amount of dust, oil, or water molecules can increase leakage current and masquerade as input-bias current. The good news is that, if you exercise special care, you can build a PCB that will adhere to a 1-pA performance specification. The most effective way you can reduce or minimize the effects of input-bias or input-leakage current is to check your circuit configurations. As you examine your circuits, look at the voltage characteristics of each node and make sure that you understand the impact of all of the current paths in your circuit. 5 Back to top A good holiday-season project Which of the bulbs on a Christmas tree is the brightest? If you had the time and desire to answer this question, you could use a single photodiode to determine the brightness of your bulbs. Finding that brightest bulb among the many and the background light would be a laborious task, however, unless you expanded your design task to using two photodiodes. The two photodiodes let you find a light's position by monitoring the difference between their output signals. If you use three op amps in a differential-photodiode-amp configuration, you will see greater accuracy in proximity and difference (Figure 1). The configurations of A 1 and A 2 act as traditional current-to-voltage converters or transimpedance amps. A 3 and R2 form a difference amp, subtracting the output voltages of A 1 and A 2 . In this circuit, the incident light on the photodiode causes current to flow through the diode from cathode to anode. Because the inverting input of A 1 and A 2 has high impedance, the photodiode's currents flow through the R1 feedback resistors. The voltage at the inverting input of the amp tracks the voltage at the amp's noninverting input. Consequently, the amp's outputs change in voltage along with the IR drop across the R1 resistors. The output voltages of A 1 and A 2 contain both difference and common-mode signals. A 3 rejects the common-mode signal and delivers the differential-voltage signal to the circuit output at V OUT . The key performance parameters for A 1 and A 2 are input capacitance, bias current, offset, noise, and temperature drift. The goal is to select amps in which these parameters are as low as possible. A 1 and A 2 require low-inputcurrent CMOS or FET op amps. You can implement a differential amp discretely or with an off-the-shelf product. As long as the resistors surrounding A 3 are equal, the dctransfer function of this circuit is 1V/V. If the resistors around A 3 are not equal, a noticeable gain error can occur between the two input signals. You can easily compensate for this type of error by replacing any of the four resistors with a potentiometer. More important, however, this type of mismatch can introduce nonlinearities in the system when the common-mode voltage of the two inputs changes. You define the common-mode voltage of the input signals as (VA1OUT +V A2OUT )/2. Ideally, the differential amp rejects common-mode-voltage changes. The calculated CMR (common-moderejection) error due to resistor mismatches is 100×(1+R 2 /R1 )/(% of mismatch error). An equal illumination on the two photodiodes makes the output voltage 0V. D1 and D2 respond linearly to illumination intensity, which makes the magnitude of the output voltage a direct measurement of the difference between the direct light impinging on D1 and D2 . A single photodiode provides some measure of a light's intensity through the magnitude of the diode's output signal. However, backgroundlight conditions can influence this magnitude, requiring calibrated and impractical measurement conditions. Adding a matched photodiode and monitoring the difference between the two diode outputs removes the equal offsets the two diodes produce. Background light adds only an offset to the photodiode's outputs, and the differential amp removes this effect. References 1. Photodiode Monitoring with Op Amps, sboa035. Texas Instruments, January 1995. 2. Grame, Jerald. Photodiode Amplifiers: Op Amp Solutions. McGraw-Hill, ISBN: 0-07-024247, 2006. 3. Precision Unity Gain Differential Amplifier, SBOS145, Texas Instruments, August 1993. 6 Back to top Don't be fooled by your amplifier's bandwidth As we design our SAR-converter analog circuits, we may be tempted to match the voltage-feedback amplifier's data-sheet bandwidth to the bandwidth of our analog signal source. We keep the amplifier bandwidths as low as possible, because faster amplifiers on the board potentially can produce layout headaches. These layout headaches come from putting higher-speed amplifiers in the circuit that are producing fast rise and fall times. The faster signals have the potential to produce EMI signals that other devices or traces on the board can receive, resulting in unexpected noise. Given this scenario, it makes sense to keep the bandwidth of the circuit's amplifiers as low as possible. With a maximum signal frequency input from dc to 20 kHz, one would think that the required amplifiers would have very low unity-gain bandwidths or a gain-bandwidth product (GBWP). On the contrary, you need higher-speed amplifiers in the circuit for two basic reasons. The primary reason is to accommodate the lost bandwidth in amplifier circuits that have a closed-loop noise gain greater than one. The other reason is to make sure the bits in your system at the end of the signal path can reliably convert the signal throughout the entire system's frequency range. Figure 1 shows an example of how the amplifier's closed-loop gain affects the system's overall bandwidth. The GBWP of this amplifier is 20 MHz. The amplifier's closed-loop gain is 10V/V, or 20 dB. In our circuit, we already require an amplifier that has a bandwidth that is 10 times higher than our input signal. You can see this by comparing the intersect of the closed-loop gain (f 3 dB ) with the amplifier GBWP frequency (f GBWP ). Figure 1: The system bandwidth becomes lower due to the closed-loop (ACL) intercept point with the amplifier's open-loop gain (AOL ) and the need to accommodate bit accuracy across the entire frequency range at f 1 . The amplifier characteristics shown in Figure 1 appear to be a perfect fit, but we are looking for the correct amplifier for a 16-bit system. One sticky point with this circuit is that the amplifier's closed-loop gain is not equal to 20 dB all the way up to 2 MHz. In fact, the closed-loop gain at 2 MHz is 17 dB. This is down 3 dB, which is approximately 70.7% lower than the closed-loop gain, or a 29.3% increase is needed to get up to that curve. Of course, the circuit's closed-loop gain does not instantaneously change from 20 dB to 17 dB at 2 MHz. Instead, it gradually gets closer to the closed-loop gain curve starting about a decade before 2 MHz. A 20-MHz bandwidth for an amplifier should be good enough, but let's do the math. This intersection has a simple, first-order attenuation. The correct formula to determine the attenuation back from the 3-dB point toward 0 Hz where A CL1 is the target closed-loop gain, A CL_DC is the closed-loop gain at dc, f 1 is the target is f 1 =f 3 dB × frequency, and f 3 dB is the corner 3-dB frequency of this amplifier system. If you make A CL1 equal to 9.9988752, which produces a ˜0.0112% error at the full-scale frequency, the bandwidth of the closed-loop system is approximately 100 times lower than f 3 dB . With our amplifier circuit, we are not so lucky. We have a 20-kHz signal that we need to increase by 10 times. At the end of the signal chain, we have a 16-bit converter. We find that a 20-kHz amplifier does not work for our circuit. With a signal gain of 10V/V, or 20 dB, the amplifier bandwidth needs to be at least 10 times higher than the signal. We also find that the amplifier bandwidth needs to be at least 100 times higher to maintain ADC integrity. This situation places our amplifier unity-gain bandwidth at 20 MHz. So much for lower- speed amplifiers. References 1. High-Speed, Single-Supply, Rail-to-Rail OPERATIONAL AMPLIFIERS MicroAmplifier ™ Series, Texas Instruments, March 1999. 7 Back to top Sallen-Key lowpass-filter stopband limitations When you design an analog, lowpass, antialiasing filter, you would expect its gain amplitude to continuously decrease beyond the filter's cutoff frequency. For the most part, this assumption is a safe one, but it's not necessarily true with the classic Sallen-Key lowpass-filter design. The Sallen-Key filter attenuates any signals in the frequency range above the cutoff frequency to a point, and then the response turns around and starts to increase in gain with frequency. Figure 1 illustrates the behavior of three Sallen-Key lowpass filters using signal-supply amplifiers. In the top three curves, the diagram captures the open-loop gain of each amplifier as the response crosses 0 dB. During this test, the configuration for all three amplifiers is a dc noise gain of 1000V/V, or 60 dB. In the diagram, the bandwidth of op amps A, B, and C are 38 MHz, 2 MHz, and 300 kHz, respectively. A second set of three curves in Figure 1 shows the frequency response of second-order, Sallen-Key lowpass filters for each amplifier. The data indicates that the lowpass filters are performing as you would expect for a little more than a decade after the cutoff frequency of 1 kHz. Although the approximation method does not impact or correct this unexpected behavior, these filters use a Butterworth design. After the cutoff frequency, all three of the filter's responses show a slope of -40 dB/decade. You would expect this response from a second-order lowpass filter. Then, at some point, the filter gain begins to increase at a rate of 20 dB/decade. The difference in the frequency response at the point at which the three amplifiers change to a positive slope depends on the amplifier's output impedance. As the amplifier's open-loop gain decreases, its closed-loop output resistance increases. Eventually, each filter's response flattens at the 0-dB crossing frequency of the op amp's open-loop gain. It is no coincidence that the "flattening" of the filter response occurs at this crossing. As the frequency increases beyond this point, the amplifier's gain is less than 0 dB. If you use a Sallen-Key lowpass filter, some characterization is in order. You can reduce the impact of the upward trend in the filter's response by following the offending active filter with a passive RC lowpass filter. The caveat to this action is that the following filter may interfere with the phase response of your intended filter, which may cause additional ringing in the time domain. Further, this action also creates a stage whose output is not low-impedance. Alternative filters can solve the problem without adding an RC filter. When an inverting filter is an acceptable alternative, you can use a multiple-feedback circuit, which does not display this reversal in the gain response at higher frequencies and does not swing the input stage's common-mode voltage. 8 Back to top Choosing antialiasing-filter amplifiers When you digitize an analog signal, you use a lowpass filter to prevent aliasing errors from out-of-band noise. Doing so attenuates superimposed, high-frequency noise on the analog signal before it reaches the ADC. If the noise on the input signal is more than half the sampling frequency of the converter, the magnitude of that noise stays the same, but the frequency changes as it aliases back onto your signal of interest. You cannot use a digital filter to reduce in-band noise after digitizing the signal. Selecting the correct operational amplifier for an active lowpass-filter circuit can appear overwhelming as you read an amplifier's data sheet and view all of the specifications. Before selecting the amplifier, however, you need to determine the filter-cutoff frequency, f CUT (or -3-dB frequency). You can use filterdesign programs to determine the filter's capacitor and resistor values. Next, you should initially consider only two important specifications when selecting an amplifier for your active lowpass filter: gain-bandwidth product and slew rate. In Figure 1, for Q i of less than 1, the gain-bandwidth product of the amplifier, f AMP, must be at least 100×gain×f CUT ×k i , where Q i is the quality factor of the ith partial filter and ki is the ratio of the partial-filter-corner frequency to the overall, where ai is the ith coefficient in the partialfilter-corner frequency. For Q i greater than 1, f AMP=100×gain×(f CUT /a i )× filter-transfer function. The gain-bandwidth product of the operational amplifiers appears in the specification table of the respective productdata sheet. You should also evaluate the effects of amplifier slew rate. Doing so ensures that your filter does not create signal distortions due to slew limitations. The slew rate depends on internal IC currents and capacitances. When you send large signals through the amplifier, internal currents charge these internal capacitors. The speed of this charging process depends on the amplifier's internal resistances, capacitances, and currents. To ensure that your active filter does not enter into a slew condition, you need to select an amplifier such that the slew rate (πV OUT P-Pf CUT ), where V OUT P-P is the expected peak-to-peak output-voltage swing below your filter's cutoff frequency. The most common topologies for active, second-order, lowpass filters are the noninverting Sallen-Key and the inverting multiple feedback. If you need a higher order filter, you can cascade both of these topologies. When using the Sallen-Key circuit, input-common-mode-voltage range (VCMR) and input bias current (I B) can also affect you. In this configuration, V CMR limits the range of your input signal. Additionally, the input bias current conducts through the external source resistance. The voltage drop that the input-bias-current error causes appears as an additional input-offset voltage. Also, be aware that this circuit has high-frequency feedthrough. References 1. Bishop, J, B Trump, and RM Stitt, "FilterPro MFB and Sallen-Key Low-Pass Filter Design Program," Application Note (SBFA001A), Texas Instruments , November 2001. 2. Mancini, Ron, Op Amps for Everyone, ISBN-0-7506-7701-5, Elsevier-Newnes, April 2003. 9 Back to top Closer to real-world analog filters There are two basic filter topologies that everyone recommends when they start to design their signal chain: multiple-feedback (MFB) or Sallen-Key topologies. But what are the differences and why would you choose one over the other? The MFB topology (sometimes called infinite gain or Rauch) is often preferred because it has low sensitivity to amplifier variations, such as the open-loop gain or input range (Figure 1). The MFB topology creates a second-order filter. A MFB single-stage (second-order filter) provides two poles while inverting the signal. In designs where an even number of stages is required, such as a fourth- or eighth-order filter, the output polarity is the same as the input signal. A sixth-order filter consisting of three MFB stages inverts the signal three times, resulting in an inverted output after the third stage. This inversion may or may not be a concern in the application circuit, particularly if you have differential input/output stages. Figure 1: MFB second-order single-supply low-pass filter. The gain of this circuit is equal to −R2 /R1 , where V CM =V +/2. In Figure 1, the gain circuit is negative and equal to the resistor ratio of R2 and R1 . This arrangement allows a variety of negative gains. Generally, the MFB topology allows low sensitivity to component tolerances. The value required for C1 in an MFB design can be quite low while the designer is trying to use reasonable resistor values. The caveat is that low capacitor values can result in significant errors due to parasitic capacitance and the input capacitance of the amplifier. There are instances where the Sallen-Key topology is a better choice. A single, second-order Sallen-Key stage also provides two poles but is a non-inverting circuit. This may be preferable over a MFB single stage, but this is not the only potential advantage. As a rule of thumb, the Sallen-Key topology is better if: 1.    gain accuracy is important, and 2.    a unity-gain filter is needed, and 3.    a low Q is required (for example, Q < 3) At unity-gain, the Sallen-Key topology has excellent gain accuracy. This is because the op amp is in a unity-gain buffer configuration with a high open-loop gain. Consequently, the gain is independent of the resistors in the circuit. With the MFB topology, the R2 /R1 resistor ratio and the resistor errors determine the gain. The unity-gain Sallen-Key topology also requires fewer components–two resistors versus three for the MFB (Figure 2). Figure 2: A Sallen-Key second-order, dual-supply, low-pass unity gain filter. The gain of this circuit is equal to 1V/V. Your circuit also may require a low Q, the quality factor. The relationship between Q and the damping factor is Q = 1/2 ζ, where ζ is the damping factor. The higher the Q, the more easily the circuit oscillates, particularly at the 3 dB corner frequency. Figure 3: Sallen-Key second-order, single-supply filter. The gain of this circuit is 1+R 2 /R1 . V CM =V +/2. The Sallen-Key topology may be preferable for low-Q, high-frequency filter sections. Selecting the correct amplifier, for these circuits, presents an interesting challenge. It is not simply a matter of picking an amplifier with a gain-bandwidth-product that is 100 times higher than the filter's corner frequency. The amplifier bandwidth calculation involves variations of the amplifier's bandwidth and variations of the passive components over time, to mention a few. Sometimes amplifier manufacturers, such as Texas Instruments' WEBENCH Filter Designer Web software, provide a list of the appropriate amplifiers for the related filter. This program provides the actual amplifier suggestion along with options, if you would rather use an amplifier other than the suggested device. Filter Designer also provides SPICE simulations of the chosen filter's sine wave response, step response, and closed-loop ac response. The WEBENCH Filter Designer program seems to have broken through some filter design barriers. All I can say is filter design has come a long way, baby. References 1. WEBENCH Filter Designer program, Texas Instruments . 10 Back to top Simply an adjustable low-pass filter A low-pass filter is the most common filter found in data acquisition systems. Typically this type of filter is used to reduce analog-to-digital converter (ADC) aliasing errors and noise outside the signal bandwidth. A signal path requires this type of dedicated filter to match the signal's requirements. If the circuit has a front-end multiplexer, it is possible to have a variety of signals that reach the ADC where each signal source has its own set of filter requirements. Consequently, a variety of different filters and corner frequency requirements may be required in the circuit prior to the multiplexer. These filters use independent operational amplifiers (op amps) in combination with fixed resistors and capacitors. An alternative filter design solution is to have one programmable filter after the multiplexer (Figure 1). The obvious advantage is reduction in chip count, from multiple op amps to one single amplifier. Subsequently, the cost is lower when a single filter serves many analog inputs. You can use a dual digital potentiometer, two capacitors, and a single amplifier to configure a low-pass, second-order Butterworth response with a programmable corner frequency range of 1:100. Table 1 summarizes the digital potentiometer programmed settings. Figure 1: A second-order, analog filter using a dual digital potentiometer, two capacitors, and one operational amplifier reduces chip count. With this circuit, it is possible to program second-order Bessel or Chebyshev filters with a programmable corner frequency range of 1:100. Additionally, you can realize a combination of Butterworth, Bessel, and Chebyshev filters with the same circuit using a 1:10 corner frequency range. Figure 1 shows the details of a single-supply, unity gain, second-order programmable low-pass Sallen-key filter. The OPA314 is a singlesupply, rail-to-rail op amp. The filter implementation requires two resistors and two capacitors. The dual TPL0102-100, a 100 kΩ 8-bit digital potentiometer, replaces the two resistors in this circuit. The capacitors are hard-wired in. A reprogramming of the dual digital potentiometer in Figure 1 changes the filter's frequency cut-off and approximation (Butterworth, Bessel, vs. Chebyshev) of this second-order, low-pass filter. You can calculate the appropriate resistance and capacitance with a little research, a sharp pencil, and a good eraser. An alternative to this tedious design exercise is to determine the capacitor and resistor values using TI's WEBENCH Filter Designer software. In the input screen, enable the low-pass filter button and type in a filter bandwidth with Ao = 1 V/V, fc = 100, fs = 1000, and Asb = -35 dB. This will produce a second-order Butterworth filter. In this view, you can also select the supply requirements of Single Supply = +5 V. Then press the green button, "Start Filter Design." The next page displays a list of filter response options that meet your requirements. Select a second-order Butterworth from the list and click "Open Design." The Filter Designer Design Summary view (page 3) allows the user to adjust the capacitor seed value to the desired value of C1 and C2 on the left side of the screen, under the "Filter Topology Specifications" section. Change to Capacitor seed value to 15 e-9 or 15 nF, then press the "update" button. When this capacitor seed is set, the software changes the resistors in the circuit to appropriate values. To produce the remaining filters in Table 1, return to page one and set the conditions for the next filter. As you do this make sure that fs is ten times higher than fc. Table 1 shows the digital potentiometer program setting for Butterworth filter with corner frequencies ranging from 100 to 10 kHz (C1 = 33 nF, C2 = 15 nF). In this design, you have learned to quickly produce an adjustable analog filter. This is one technique to design a programmable anti-aliasing filter. Can you think of another way to do this? Send in your ideas and we can discuss them! 11 Back to top Photo-sensing circuits: The eyes of the electronic world are watching Silicon photo sensors have been in electronic circuits since the inception of the era of silicon electronics. More than likely, scientists quickly discovered the photo-sensing characteristics of silicon in the lab, as they worked from the daylight hours into the evening. To this day, IC designers regularly cover their wafers under test to shield out extraneous light. Although the light sensitivity of silicon is an undesirable byproduct of the silicon, system designers have exploited this transfer of light into electrical energy in various systems. Consequently, a wide variety of applications use silicon to sense the intensity and characteristics of light. In these systems, a silicon sensor converts light into charge or an electrical current. These silicon sensors are the "eyes" in the electronics world that users can employ to analyze blood, search noninvasively for tumors, detect smoke, position equipment, or perform chromatography, to name a few applications. Basically, system designers understand how to convert light into a current, but the real challenge is determining how to convert the low-level currents from the photo sensor into a useful electrical representation. To further exacerbate the difficulty of the design, the required accuracy in these applications continues to increase. The traditional design topology of the transimpedance amplifier captures this low-level signal in a hybrid approach that starts with an amplifier and a high-value resistor in the feedback loop. The circuit design uses resistance to provide a real-time, linear representation of the light source. This circuit places the photodiode across the amplifier's inverting input and ground of the operational amplifier. A resistor with a value of 100 kΩ to 10 MΩ connects the inverting input of the amplifier to the output. You then connect the noninverting input to ground (Figure 1). Light excitement on the photo sensor generates charge. The only path of escape for this charge is through the highvalue resistor in the amplifier's feedback loop. The simplistic approach in Figure 1 is not without its design challenges. The operational amplifier must have relatively low-picoampere input-bias currents and low input capacitance. An appropriate amplifier for this circuit would have a FET- or a CMOS-input stage with lowvoltage noise and microvolt-offset specifications. In the end, the designer optimizes the stability, bandwidth, low-noise performance, and layout of this transimpedance-amplifier circuit. The final design method is not always intuitively obvious. The photo sensor, operational amplifier, amplifier-feedback element, and these parts' parasitics combine to create quite a rat's nest of formulas for consideration. The signal after the transimpedance amplifier requires a multipole analog filter. In this manner, combining the input and filtering stages separates the signal of interest from the noise floor. A sampling ADC digitizes the signal after the analog filter. Photo-sensing circuits have changed over the years. The first approach was purely analog, using the transimpedance amplifier and following it with a lowpass filter. From the classic transimpedance amplifier, the switched integrator has gained favor. The switched integrator was the first step toward bringing the digital portion of the circuit closer to the signal source. The migration of the photo-sensing-application product has moved on to totally integrated systems, such as the charged digitizing ADC. 12 Back to top Transimpedance-amplifier stability is key in light-sensing applications A variety of precision applications sense light and convert that information into a useful digital word. At the system's front end, a preamplifier converts the photodiode's current-output signal to a usable voltage level. Figure 1 shows the front-end circuit of this system, which comprises a photodiode, an operational amplifier, and a feedback network. (Figure 1) The transfer function of this system is: where A OL (jΩ) is the open-loop gain of the amplifier over frequency; β is the system-feedback factor, equaling 1/(1+Z F /ZIN); Z IN is the distributed input impedance, equaling RPD||jΩ(C PD+CCM +CDIFF ); and Z F is the distributed feedback impedance, equaling RF ||jΩ(C RF +CF ). A good tool for determining stability is a Bode plot. The appropriate Bode plot for this design includes the amplifier's open-loop gain and the 1/β curve. System elements determining the noise-gain frequency response are the photodiode's parasitics and the operational amplifier's input capacitance, as well as RF , CRF , and CF in the amplifier's feedback loop. Figure 2 shows the frequency response of the 1/β curve and the amplifier's open-loop-gain response: f P=1/(2π(R PD||RF )(CPD+CCM +CDIFF +CF +CRF )), and f Z +1/(2π(R F )(CF +CRF )). The A OL (jΩ) curve intersects the 1/β curve at an interesting point. The closure rate between the two curves suggests the system's phase margin and, in turn, predicts the stability. For instance, the closure rate of the two curves is 20 dB/decade. Here, the amplifier contributes an approximately -90° phase shift, and the feedback factor contributes an approximately 0° phase shift. By adding the 1/β phase shift from the A OL (jΩ) phase shift, the system's phase shift is -90°, and its margin is 90°, resulting in a stable system. If the closure rate of these two curves is 40 dB/decade, indicating a phase shift of -180° and a phase margin of 0°, the circuit will oscillate or ring with a step-function input. One way to correct circuit instability is to add a feedback capacitor, CF , or to change the amplifier to have a different frequency response or different input capacitance. A conservative calculation that allows variation in amplifier bandwidth, input capacitance, and feedback-resistor value places the system's pole of 1/β at half the frequency where the two curves intersect: where f GBW is the gain-bandwidth product of the amplifier. In this design, the system's phase margin is 65°, and the step function's overshoot is 5%. References 1. Baker, Bonnie, "Photo-sensing circuits: The eyes of the electronic world are watching," EDN, Aug 7, 2008. 13 Back to top Transimpedance-amplifier noise issues How much noise is too much noise in a photodiode-preamplifier circuit? You can derive the noise performance of a transimpedance amplifier (Figure 1a) with calculations or by using a Spice simulation. When calculating the noise performance of the circuit, consider six regions in the frequency spectrum (Figure 1b) and add each region with a root-sum-square equation or the following equation: The first five regions are equal to the multiple of the areas under the closed-loop-gain and amplifier-noise-density curves. The area under the noise-density curve in the e1 , flicker-noise (1/f), region is V 1/f:fB-fA =A N , where A N is the amplifier's input-noise-density at 1 Hz and f B is the corner frequency where the flicker noise tapers off. For many CMOS or FET amplifiers, the flicker-noise region usually ranges from dc to 100 or 1000 Hz. A calculation proves that the contribution to noise in this low-frequency region is relatively low: where RF is the feedback resistor and RPD is the device's parallel resistance. In the e2 region, multiply the broadband noise of the amplifier, the closed-loop dc-noise gain (1+R F /RPD), and the square root of the region's bandwidth. Again, the contributed noise in this region is usually relatively low because of its location in the lower frequency range. Calculate the noise contribution and the e3 region in the same manner with f P=1/[2π(RPD||RF ) (CPD+CCM +CDIFF +CF +CRF )] and f Z =1/[2π(RF ) (CF +CRF )]. where CPD is the device's capacitance and CDIFF is the differential amplifier's capacitance. The noise in regions e4 and e5 uses the higher-frequency gain of the closed-loop-gain curve with the value of C1 being the parallel combination of the input capacitors, or [CP-R1||2C CM ||CDIFF ], and C2 is the parallel combination of CF and CRF . The sixth part of the noise equation, e6 , represents the noise contribution of the feedback resistor. The amplifier does not gain the contribution of noise from the feedback resistor: where K is Boltzmann's constant, which is 1.38×10-23 ; T is temperature in Kelvin; RF is the feedback resistor in ohms; and BW is the bandwidth of interest. When asking how much noise is too much noise in this photodiode-preamp circuit, consider that a 12-bit system operating with a 5V input range has an LSB of 1.22 mV. The LSB for a 16-bit system with the same input-voltage range is 76.29 µV. Both LSBs are peak-to-peak numbers, and the values in this column are root-mean-square values. References 1. Baker, Bonnie, "Photo-sensing circuits: The eyes of the electronic world are watching," EDN, Aug 7, 2008. 2. "Noise Analysis of FET Transimpedance Amplifiers," SBOA060, Texas Instruments, February 1994. 3. Baker, Bonnie, "RMS and peak-to-peak noise trade-off," EDN, May 15, 2008. 14 Back to top Transimpedance-amplifier application: The pulse oximeter Have you ever been in need of a second opinion when your sanity is at stake? The pulse oximeter may be able to provide that second opinion if your brain is oxygen-deprived. This condition could affect you if you are a pilot, hiking in the high altitude of a mountain range, or even undergoing surgery. The pulse oximeter is a noninvasive instrument that monitors SpO 2 (saturation of hemoglobin with oxygen) in your blood. You measure the oxygen in the blood by alternating the on-times of a red LED with a 650-mm wavelength and an NIR (near-infrared) LED with a 940-mm wavelength, taking the ratio between the intensities from a photodiode, and comparing that ratio with an SpO 2 look-up table in the microcontroller. The transimpedance amplifier appears in medical and laboratory instrumentation, position and proximity sensors, photographic analyzers, bar-code scanners, and even smoke detectors. In the medical field, you will primarily find transimpedance amplifiers in the CT (computedtomography)-scanner front end and the pulse oximeter. Figure 1 shows a simplified block diagram of a pulse oximeter. In the circuit in Figure 1, the red LED is on for 50 µsec, both LEDs are off for 450 µsec, the NIR LED is on for 50 µsec, and then both LEDs are off for 450 µsec. The system repeats this cycle continuously. The transimpedance amplifier, A 1 , converts the photodiode current generated by the LEDs to a voltage at the output. The signal then travels through a bandpass filter and gain stage to the 12-bit ADC. The signal also travels through a lowpass filter to regulate the driver power to the LEDs. The microcontroller acquires the signals from the 12-bit ADC, computes the ratio of the red- and NIR-LED signals, and compares the results with a look-up table. The LCD shows a percentage of oxygenated hemoglobin versus nonoxygenated hemoglobin and your heart rate. When you choose your device for the pulse-oximeter transimpedance-amplifier circuit, you need to make sure that the amplifier's input-bias current is very low or in a picoamp region at 25°C. The amplifier's input-bias current creates an output-voltage error by conducting through the high-impedance resistor, RF , in the amplifier's feedback loop. FET- or CMOS-amplifier input devices usually meet this requirement. A second consideration is that the low-frequency voltage noise of your amplifier must be very low. When you consider the input-voltage noise of the amplifier, scrutinize the impact of the flicker noise. After the transimpedance amplifier, a bandpass filter eliminates the noise above 5 Hz. Finally, the amplifier's initial offset error and overtemperature should be in the microvolt region if you want to minimize linearity errors. It may be worthwhile to use an autozero amplifier. A normal output for the pulse oximeter is approximately 97%±2%, ranging from 95 to 100%. The alarms on the pulse oximeter usually sound when the SpO 2 level drops below 90%. If there is a shortage of oxygen in your system, you may experience poor judgment or loss of motor function. If a pulse oximeter indicates that your oxygen levels are stable, you may want to explore other diagnostic avenues, or perhaps you just dance to the beat of a different drummer. Good luck! References 1. Medical Instruments Applications Guide, Texas Instruments, 2010. 2. Townsend, Neil, MD, "Pulse Oximetry," Medical Electronics, Michaelmas 2001. 15 Back to top Remote photo sensing Photodiodes transform a basic physical occurrence, light, into an electrical form, current. Design engineers methodically convert the photodetector's current to a usable voltage, which makes the manipulation of the photodiode signal manageable. There are many ways to approach the photosensing-circuit problem, but one issue came to mind as I read the comments in the Talkback section for a recent column. A reader requested a circuit that reduces the noise impact of a photodiode with a large parasitic capacitance. A classic photo-sensing system circuit has a photodiode, an operational amplifier, and a feedback resistor/capacitor pair at the front end. Recalling a circuit from another column, in this circuit, the photodiode, amplifier, and feedback-capacitance elements limit the bandwidth of the circuit. Yet another column details the stability of this circuit. When sensing with a photodiode with a large parasitic capacitance or from a remote site, the input of the amplifier has a large capacitance across its input. The result of this added capacitance increases the circuit's noise gain unless you increase the amplifier's feedback capacitor. If the feedback capacitor, CF , increases, the bandwidth of the circuit decreases. To fix this problem, you can use a bootstrap circuit (Figure 1). Photodiodes with a relatively low diode capacitance do not benefit from this circuit. A unity-gain buffer, A 2 , removes the cable's capacitance and the photodiode's parasitic capacitance from the input of the transimpedance amplifier, A 1 . When designing this circuit, you'll find that the type of amplifier you select for A 2 is somewhat easy. The only important performance specifications are low input capacitance, low noise, a wider bandwidth than A 1 , and low output impedance. In this design, A 2 's input capacitance is the only capacitance that plays a role in the ac-transfer function of the transimpedance system. The input capacitance of the buffer replaces the summation of the input capacitance of A 1 , the cable capacitance, and the parasitic capacitance of the photodiode. A good rule of thumb is to have CA2 <<(CA1 +CCA+CPD), where CA1 and CA2 represent the sum of their input differential and common-mode capacitance. With this design, you exchange one noise problem, A 1 , with another, A 2 . The unity-gain buffer removes the noise effect from A 1 . A good approach is to make the noise of A 2 less than or equal to that of A 1 . The difference between the input signal and the output signal in this system falls across the cable/diode capacitance. You can keep this difference low selecting A 2 with wider bandwidth than A 1 and keeping A 2 's output impedance low. A 2 's gain roll-off introduces an upper limit for the bandwidth improvement, making A 2 's bandwidth much greater than that of A 1 . This circuit requires stability optimization as you balance CF and the input capacitance of A 2 . References 1. Baker, Bonnie, "A good holiday-season project," EDN, Dec 15, 2010. 2. Baker, Bonnie, "Photo-sensing circuits: The eyes of the electronic world are watching," EDN, Aug 7, 2008. 3. Baker, Bonnie, "Transimpedance-amplifier stability is key in light-sensing applications," EDN, Sept 4, 2008. 4. Graeme, Jerald G, Photodiode Amplifiers: Op Amp Solutions, McGraw-Hill, 1996, ISBN 0-07-024247-X. 5. Kurz, Dov, and Avner Cohen, "Bootstrapping reduces amplifier input capacitance," EDN, March 20, 1978. 16 Single-supply amplifier outputs don't swing rail to rail http://www.ti.com/ww/en/analog/Amplifiers-eBook/[1/21/14 9:27:15 AM] Back to top and f 3 dB is the corner 3-dB frequency of this amplifier system. Rail-to-Rail OPERATIONAL AMPLIFIERS MicroAmplifier ™ Series. One sticky point with this circuit is that the amplifier's closed-loop gain is not equal to 20 dB all the way up to 2 MHz. The two photodiodes let you find a light's position by monitoring the difference between their output signals. In the region near the positive power supply.22 mV. In the input region near ground. the term "input-bias current" was an accurate descriptor. Bode.9988752. In the complementary-differentialinput-stage topology. and 300 kHz. The calculated CMR (common-moderejection) error due to resistor mismatches is 100×(1+R 2 /R1 )/(% of mismatch error). References 1. For instance. We keep the amplifier bandwidths as low as possible. we have a 16-bit converter. This is down 3 dB. Input-bias and input-leakage current can change over temperature. Figure 1 illustrates the behavior of three Sallen-Key lowpass filters using signal-supply amplifiers. The end result is that the input offset voltage changes between the stages. 2. The GBWP of this amplifier is 20 MHz. because faster amplifiers on the board potentially can produce layout headaches. an integrator. Trust me. The faster signals have the potential to produce EMI signals that other devices or traces on the board can receive. If the open-loop-gain and closed-loop-gain curves cross with a 40-dB/decade closure rate. The most effective way you can reduce or minimize the effects of input-bias or input-leakage current is to check your circuit configurations. 90dB CMRR. you can use RSS calculations. bringing these errors to the ADC's input. and the differential amp removes this effect. a new hire for a leading-edge analog-electronics company. OPA2350. Black. However. Ideally. Precision Unity Gain Differential Amplifier. and Bonnie Baker. To determine the dynamic-range limitations of the circuit. or a 29. backgroundlight conditions can influence this magnitude. the configuration for all three amplifiers is a dc noise gain of 1000V/V. when the amplifier's inputs are near the negative rail. In ensuring that the input-leakage current remains low with JFET and CMOS amplifiers. However. As long as the resistors surrounding A 3 are equal. Oljaca. if you exercise special care. March 1999. the amplifier bandwidth needs to be at least 10 times higher than the signal. but I have a different opinion. We find that a 20-kHz amplifier does not work for our circuit. One criterion is to keep the signal within the dynamic range of the full-scale range of the ADC. You can avoid this type of amplifier crossover distortion by usingg an inverting configuration. January 2005. If the system requires the amplifier to be configured as a noninverting buffer. The Sallen-Key filter attenuates any signals in the frequency range above the cutoff frequency to a point. the bandwidth of op amps A. which produces a ˜0. in that the output stage switches from transistor to transistor. the differential amp rejects common-mode-voltage changes. but you will never see ESD-leakage current in bipolar amplifiers. Please enjoy your read through and let me know what you think! 1 Back to top Chapter 1: Operational amplifiers When is good enough good enough? If you are having difficulty making product-selection decisions in a consumer circuit. you could use a single photodiode to determine the brightness of your bulbs. or both can increase the amplifier's overall THD+N level. Jerald. At the end of the signal chain. sboa035. Oct 16. resulting in unexpected noise. the worst-case sensor-resistive offset error would be ±94 mV×10V/V.3% increase is needed to get up to that curve. bias current. So much for lower. The cumulative possible offset error at the =940 mV. such as the temperature-sensor circuit in Figure 1. The combination of the load capacitor. make sure you use the same evaluation technique in your manufacturing process to quantify the effects of the processes–such as solder reflow–that you impose on these devices and the end-of-life effects due to environmental exposures. If the THD+N tests include the amplifier's input crossover distortion. In the top three curves. OPA364. References 1. A single photodiode provides some measure of a light's intensity through the magnitude of the diode's output signal.com/ww/en/analog/Amplifiers-eBook/[1/14/14 10:58:49 AM] . The product characteristics that influence the extent of the dynamic range are the system's cumulative offset and gain errors. Both pairs are on as the amplifier's inputs pass between these two regions. You can implement a differential amp discretely or with an off-the-shelf product. The good news is that. Almost all amplifiers have ESD cells for protection from an ESD event. unless you expanded your design task to using two photodiodes. We also find that the amplifier bandwidth needs to be at least 100 times higher to maintain ADC integrity. and CL and RL then introduce a zero to the open-loop-gain curve (Figure 1). Figure 1 shows an example of how the amplifier's closed-loop gain affects the system's overall bandwidth. I need to know what is really going on! What I did not understand then but understand now is that a capacitor and a resistor that hang on the output of an amplifier change the amplifier's open-loop-gain curve. but we are looking for the correct amplifier for a 16-bit system. the photodiode's currents flow through the R1 feedback resistors. the input stage's impact on THD+N complicates the nature of this specification with single-supply amplifiers. A 3 rejects the common-mode signal and delivers the differential-voltage signal to the circuit output at V OUT ." EDN. With the RSS formula. 2008. The following excerpts from the EDN Baker's Best column represent the best of the amplifier short articles. During this test. or 20 dB. Is this statement true or false? Using this type of logic may give you a confident feeling that your circuit will work correctly the first time. For instance. the NMOS-transistor pair dominates the offset error. When the amplifier's inputs are closer to the positive rail. McGraw-Hill. So. at the hands of several individuals including Harold S. References 1.8V.TI. For the most part." AEi Systems LLC. As an example. lowpass. worse yet. The THD+N performance of a single-supply amplifier also originates in the amplifier's input and output stages. lo and behold. it gradually gets closer to the closed-loop gain curve starting about a decade before 2 MHz. Grame.0006% if you avoid using the input transition. if you combine all of these terms. the THD+N is 0. Sandler.5 mV. Miro. When the PMOS and NMOS transistors are both on. and temperature drift. And. I was wide-eyed and excited–confronting new problems left and right. Adding a matched photodiode and monitoring the difference between the two diode outputs removes the equal offsets the two diodes produce. you would use the RSS formula to determine gain-error contribution that limits the dynamic range from the four stages in this circuit. look at the voltage characteristics of each node and make sure that you understand the impact of all of the current paths in your circuit. a buffer-amplifier circuit. high closed-loop gains. George Philbrick. there is still a lot to learn about the amplifier and their appropriate placements in your application circuits." Texas Instruments. The voltage at the inverting input of the amp tracks the voltage at the amp's noninverting input. On the contrary. however. The JFET and CMOS input amplifiers may not be. Background light adds only an offset to the photodiode's outputs. an amplifier with a singledifferential input stage and charge pump is a more appropriate choice. References 1. The amplifier's closed-loop gain is 10V/V. including instrumentation amplifiers (INA) and programmable gain amplifiers (PGA). They have been complied for your convenience. this type of mismatch can introduce nonlinearities in the system when the common-mode voltage of the two inputs changes. it has come back to haunt me. CMOS single-supply amplifiers have been beneficial to single-supply-system designers worldwide. So. The offset error of the ADC is ±1 LSB or ±1. As the output signal sweeps from rail to rail. this assumption is a safe one. You can easily compensate for this type of error by replacing any of the four resistors with a potentiometer.com Follow Us   |    Table of Contents Introduction Chapter 1: Operational amplifiers When is good enough good enough? Just use a 100Ω resistor: Understanding a rule of thumb for oscillating amplifiers Where did all that racket come from? Is your amplifier offset way out of whack? A good holiday-season project Don't be fooled by your amplifier's bandwidth Sallen-Key lowpass-filter stopband limitations Choosing antialiasing-filter amplifiers Closer to real-world analog filters Simply an adjustable low-pass filter Photo-sensing circuits: The eyes of the electronic world are watching Transimpedance-amplifier stability is key in light-sensing applications Transimpedance-amplifier noise issues Transimpedance-amplifier application: The pulse oximeter Remote photo sensing Single-supply amplifier outputs don't swing rail to rail The non-negotiable single-supply operational amplifier What does "rail to rail" input operation really mean? Rail-to-rail input amplifier application solutions Voltage. January 1995. 7MHz. This situation places our amplifier unity-gain bandwidth at 20 MHz. "OPA350. requiring calibrated and impractical measurement conditions. OPA4350 High-Speed. High-Speed. you would think that we have finally come to terms with understanding this five-terminal device. The input-bias current swamps out the picoampere leakage current from the ESD cells. you can quickly solve this problem by choosing the absolute best performing parts for each socket. In Figure 1. Now. the load resistor." still with no explanation. The output stage of a single-supply amplifier usually has an AB topology. Instead. However. The amplifier characteristics shown in Figure 1 appear to be a perfect fit. Rail-to-Rail I/O Operational Amplifier. I suggest that you try to explain the offset error by re-examining your amplifier's specifications. noise. You can find the pole and zero locations in this circuit in the following equations: What have I learned from this situation? It pays to understand why an engineer's rule of thumb works. only to find that the offset voltage is wrong at the manufacturer's bench-specified input? What if you find that it is more than 10 times higher than specification in your application circuit? Do you send the chip in for failure analysis or just toss the chip out and have another look at your list of amplifiers? As an alternative. place the amplifier in an inverting-gain configuration and keep the closed-loop gain low. are here to stay as standalone devices and as integrated functions inside complex ICs. the incident light on the photodiode causes current to flow through the diode from cathode to anode. or 60 dB.) The gain error of both the sensor cell and the IC1 amplifier configuration depends on the ±1% maximum resistor tolerances as well as on a maximum sensor-resistor tolerance of ±2%. With a signal gain of 10V/V. To optimize a complementary-input-single-supply amplifier's THD+N performance. a noticeable gain error can occur between the two input signals. In the diagram. http://www. which makes the magnitude of the output voltage a direct measurement of the difference between the direct light impinging on D1 and D2 . but. Texas Instruments. Given the long time span between these beginning years and the present. This specification is independent of the common-mode voltage and the magnitude-amplifier power. following such logic goes only so far when you try to justify the cost-versus-performance factors of the products you are using. The words of wisdom that came down to me were. the current sinking or sourcing from the amplifier's input pins is actually the leakage current from the input-ESD (electrostatic-discharge) cells (Figure 1). I built a new circuit as suggested. you must understand the impact of your PCB (printed-circuit board) on the picoampere levels of current. The goal is to select amps in which these parameters are as low as possible. Using the same logic. CL . This design topology has significant variations in the amplifier's offset voltage across the common-mode input range. the PMOS transistors are on. during your first consumer-product-selection attempt. 6 Back to top Don't be fooled by your amplifier's bandwidth As we design our SAR-converter analog circuits. Once you take this first step. however. however. The term "input-bias current" loses its meaning when you look at JFET or CMOS input amplifiers. note that a 12-bit converter is at the end of the signal chain. but rather they are permanent fixtures in the electronics world. For instance. or 20 dB. Amplifiers. "OPA363. we are not so lucky. These layout headaches come from putting higher-speed amplifiers in the circuit that are producing fast rise and fall times. 1. The amplifier's input noise is another contributor to the THD+N specification. The output voltages of A 1 and A 2 contain both difference and common-mode signals. a small amount of dust. "A Comparison of Tolerance Analysis Methods. You may say that it will become obsolete soon. you would expect its gain amplitude to continuously decrease beyond the filter's cutoff frequency. 2 MHz. A high level of input noise. the filter-stage (IC2 ) offset error is ±500 µV. the input crossover distortion can affect the amplifier's THD+N performance. one would think that the required amplifiers would have very low unity-gain bandwidths or a gain-bandwidth product (GBWP). and. MicroAmplifier Series. In fact. This calculation illustrates that the sensor cell contributes the most error with little ADC's input is impact from the amplifiers or the ADC. ISBN: 0-07-024247. D1 and D2 respond linearly to illumination intensity. and the NMOS transistors are off (Figure 1). With a maximum signal frequency input from dc to 20 kHz. February 2003. a higher level of quiescent current through the output stage reduces the amplifier's THD. We have a 20-kHz signal that we need to increase by 10 times. Texas Instruments. Rail-to-Rail Operational Amplifiers. we already require an amplifier that has a bandwidth that is 10 times higher than our input signal. "Oh." Texas Instruments. Generally. the circuit still oscillated. As you examine your circuits. the diagram captures the open-loop gain of each amplifier as the response crosses 0 dB. Furthermore. A CL_DC is the closed-loop gain at dc. Key players affecting the THD+N (total-harmonic-distortion-plus-noise) characteristics of dual-supply amplifiers are input noise and output-stage crossover distortion. 2006. introduces a pole to the open-loop-gain curve. the amp's outputs change in voltage along with the IR drop across the R1 resistors. with a capacitive load. SBOS145.7% lower than the closed-loop gain. This intersection has a simple. However. References 1.22 mV.004%. and the PMOS transistors are off. put a 100Ω load resistor between the amplifier output and the load capacitor." So. by Bonnie Baker Amplifiers have been around for a long time. the bandwidth of the closed-loop system is approximately 100 times lower than f 3 dB . oil. You cannot use an RSS formula with entities that have correlated variations that are not statistically independent. IC1 . and it still is. If you make A CL1 equal to 9. 5 Back to top A good holiday-season project Which of the bulbs on a Christmas tree is the brightest? If you had the time and desire to answer this question. This crossover-distortion phenomenon affects the amplifier's THD. The magnitude of the bipolar amplifier's input-bias current ranges from a few nanoamperes for low-power devices to hundreds of nanoamperes for higher-power devices. f 1 is the target is f 1 =f 3 dB × frequency. The ADC's contributed gain error is 0. Consequently. which is approximately 70. isn't there something else out there that has successfully obsoleted these devices? I have worked with people who are surprised when I tell them that the operational amplifier is not dying. It solved my problem. A more accurate descriptor for this current error is "input-leakage current. the common-mode voltage region is approximately 400 mV. you can build a PCB that will adhere to a 1-pA performance specification. but it's not necessarily true with the classic Sallen-Key lowpass-filter design. In this circuit. A 3 and R2 form a difference amp. you might find that the amplifier's input-bias current creates an offset-voltage error through the resistors in your circuit. These beasts were just coming alive in the 1930s – 1940s. depending on the operational-amplifier design. is ±500 µV×10. Single-Supply." When I asked why. a capacitance transducer. With our amplifier circuit. assume that the maximum offset error of IC1 and IC2 is 0. the leakage current increases approximately two times per 10°C change. the output stage displays a crossover distortion similar to the input-stage crossover distortion. the amplifier circuit will be marginally unstable or. OPA4364. 7 Back to top Sallen-Key lowpass-filter stopband limitations When you design an analog. If you comprehend the general guidelines. Given this scenario. One problem I ran into involved the stability of an amplifier circuit. Single-Supply.and current-feedback amps are almost the same Chapter 2: INAs and PGAs Understanding CMR and instrumentation amplifiers Common-mode range can bite hard Instrumentation amplifier input-circuit strategies Wringing out thermistor nonlinearities A good holiday-season project Author's biography   Introduction The Best of Baker's Best – Amplifiers Introduction. OPA2364. If you are not on top of the explanation. W. You define the common-mode voltage of the input signals as (VA1OUT +V A2OUT )/2. I did not return to his suggestion for several years. The primary reason is to accommodate the lost bandwidth in amplifier circuits that have a closed-loop noise gain greater than one. "Start with the right op amp when driving SAR ADCs. You can see this by comparing the intersect of the closed-loop gain (f 3 dB ) with the amplifier GBWP frequency (f GBWP ). These calculations can assist you in making logical and economical product decisions. and the amplifier's open-loop resistance. the closed-loop gain at 2 MHz is 17 dB. and the ADC (IC3 ) offset error is ±1. RO. If you use three op amps in a differential-photodiode-amp configuration. sang like a bird. you will be OK. August 1993. Because the leakage current is from the reverse-biased ESD diodes. Single-Supply.9 mV maximum at full scale. will oscillate." The magnitude of leakage current with JFET or CMOS amplifiers is less than 1 pA at 25°C. B. 2 Back to top Just use a 100Ω resistor: Understanding a rule of thumb for oscillating amplifiers So there I was. Steven M. Several types of single-supply amplifier topologies can accept input signals across the power supplies. 2. An equal illumination on the two photodiodes makes the output voltage 0V. you will see greater accuracy in proximity and difference (Figure 1). or water molecules can increase leakage current and masquerade as input-bias current. respectively. Because the inverting input of A 1 and A 2 has high impedance. The key performance parameters for A 1 and A 2 are input capacitance. the circuit's closed-loop gain does not instantaneously change from 20 dB to 17 dB at 2 MHz. Photodiode Monitoring with Op Amps. the bipolar input-bias current can be fairly stable. I returned to the engineer who gave me the first bit of great advice. A 1 and A 2 require low-inputcurrent CMOS or FET op amps. In our circuit. Another major THD+N contributor can be the operational amplifier's output stage. given my work load. If the resistors around A 3 are not equal. in Figure 2 the THD+N is 0. With these types of amplifiers.098% or equals 4. 3. the PMOS transistor's offset error is dominant. an analog filter. A bipolar amplifier's input-bias current is the same as the base current of the NPN or PNP transistors at the input of the amplifier. This is particularly true as long as the real-world remains in the analog domain. first-order attenuation. This new circuit was still singing. or any other circuit with high-impedance components around your amplifier. Finding that brightest bulb among the many and the background light would be a laborious task. 3 Back to top Where did all that racket come from? Since arriving in the market. but it was producing a new frequency. Figure 1: The system bandwidth becomes lower due to the closed-loop (ACL) intercept point with the amplifier's open-loop gain (AOL ) and the need to accommodate bit accuracy across the entire frequency range at f 1 . however. Of course. we may be tempted to match the voltage-feedback amplifier's data-sheet bandwidth to the bandwidth of our analog signal source. The correct formula to determine the attenuation back from the 3-dB point toward 0 Hz where A CL1 is the target closed-loop gain. it makes sense to keep the bandwidth of the circuit's amplifiers as low as possible. and H. Texas Instruments. offset. Creating this pole and zero does not disrupt the amplifier's stability as long as they cancel each other out before the open-loop-gain curve crosses the closed-loop-gain curve. (The full-scale range of the ADC is 5V. If you configure the complementary-input amplifier in a noninverting configuration. The other reason is to make sure the bits in your system at the end of the signal path can reliably convert the signal throughout the entire system's frequency range. His recommendation: "Change the 100Ω resistor to a 500Ω resistor. More important. the NMOS transistors are on. the engineer said. In this application. you can use the RSS (root-sum-square) algebraic approach. If you are using your amplifier as the key component in a transimpedance amplifier. The configurations of A 1 and A 2 act as traditional current-to-voltage converters or transimpedance amps. antialiasing filter. In the bipolar-amplifier days. you need higher-speed amplifiers in the circuit for two basic reasons.ti. and then the response turns around and starts to increase in gain with frequency." Instead of choosing the best products. The contribution of the amplifier-gain stage. and C are 38 MHz. a sample-and-hold circuit. subtracting the output voltages of A 1 and A 2 . 1998. 4 Back to top Is your amplifier offset way out of whack? Have you ever spent a great deal of time selecting the perfect operational amplifier for your circuit. I ran around and asked for advice. As it was my first job. Because I had a huge community of experts around me.0112% error at the full-scale frequency.Texas Instruments . you would calculate the combined RSS value of offset and gain. OPA2363. are the highest performance analog products in front of the ADC appropriate? How do you determine which products are good enough for your system? Avoiding production-floor notifications or field failures may be your definition of "good enough. you take the square root of the sum of the squares of several terms that are statistically independent. it will work.speed amplifiers. but let's do the math. A 20-MHz bandwidth for an amplifier should be good enough. to take it a step further. "Just do it. RL . it will come back to bite you. the dctransfer function of this circuit is 1V/V. Photodiode Amplifiers: Op Amp Solutions. Because I had a huge community of experts around me. Rail-to-Rail Operational Amplifiers. OPA4350 High-Speed. and Bonnie Baker. The THD+N performance of a single-supply amplifier also originates in the amplifier's input and output stages. the circuit still oscillated. OPA2350.com/ww/en/analog/Amplifiers-eBook/[1/14/14 10:48:43 AM] ." When I asked why. The term "input-bias current" loses its meaning when you look at JFET or CMOS input amplifiers. you might find that the amplifier's input-bias current creates an offset-voltage error through the resistors in your circuit. the engineer said. 1. a buffer-amplifier circuit. If you comprehend the general guidelines. OPA4364. and it still is. will oscillate. References 1. When the PMOS and NMOS transistors are both on. This new circuit was still singing. I suggest that you try to explain the offset error by re-examining your amplifier's specifications. 4 Back to top Is your amplifier offset way out of whack? Have you ever spent a great deal of time selecting the perfect operational amplifier for your circuit. CMOS single-supply amplifiers have been beneficial to single-supply-system designers worldwide. Key players affecting the THD+N (total-harmonic-distortion-plus-noise) characteristics of dual-supply amplifiers are input noise and output-stage crossover distortion. 3 Back to top Where did all that racket come from? Since arriving in the market. the output stage displays a crossover distortion similar to the input-stage crossover distortion. If you configure the complementary-input amplifier in a noninverting configuration. an integrator. only to find that the offset voltage is wrong at the manufacturer's bench-specified input? What if you find that it is more than 10 times higher than specification in your application circuit? Do you send the chip in for failure analysis or just toss the chip out and have another look at your list of amplifiers? As an alternative. You can avoid this type of amplifier crossover distortion by usingg an inverting configuration. If the THD+N tests include the amplifier's input crossover distortion. In the bipolar-amplifier days. The amplifier's input noise is another contributor to the THD+N specification. it will work. you will be OK. the THD+N is 0. Both pairs are on as the amplifier's inputs pass between these two regions. I did not return to his suggestion for several years. As it was my first job. Trust me. Several types of single-supply amplifier topologies can accept input signals across the power supplies. I returned to the engineer who gave me the first bit of great advice. worse yet.TI. References 1.0006% if you avoid using the input transition. the NMOS transistors are on. If you are using your amplifier as the key component in a transimpedance amplifier. however.ti. The combination of the load capacitor." So. RL . If the open-loop-gain and closed-loop-gain curves cross with a 40-dB/decade closure rate. lo and behold. I need to know what is really going on! What I did not understand then but understand now is that a capacitor and a resistor that hang on the output of an amplifier change the amplifier's open-loop-gain curve. the current http://www. MicroAmplifier Series. His recommendation: "Change the 100Ω resistor to a 500Ω resistor.Texas Instruments . and the NMOS transistors are off (Figure 1). OPA2363. Miro. For instance." Texas Instruments. the common-mode voltage region is approximately 400 mV. Oct 16. The words of wisdom that came down to me were. One problem I ran into involved the stability of an amplifier circuit. 90dB CMRR.com 2 Back to top Just use a 100Ω resistor: Understanding a rule of thumb for oscillating amplifiers So there I was. February 2003. A bipolar amplifier's input-bias current is the same as the base current of the NPN or PNP transistors at the input of the amplifier. the load resistor. In the input region near ground. RO. Single-Supply. the PMOS transistors are on. OPA2364. In the region near the positive power supply. an amplifier with a singledifferential input stage and charge pump is a more appropriate choice. "Start with the right op amp when driving SAR ADCs. The output stage of a single-supply amplifier usually has an AB topology. Single-Supply. When the amplifier's inputs are closer to the positive rail. This design topology has significant variations in the amplifier's offset voltage across the common-mode input range. Another major THD+N contributor can be the operational amplifier's output stage. "OPA350. "OPA363. or any other circuit with high-impedance components around your amplifier. high closed-loop gains. I built a new circuit as suggested. You can find the pole and zero locations in this circuit in the following equations: What have I learned from this situation? It pays to understand why an engineer's rule of thumb works. and the amplifier's open-loop resistance. OPA364. To optimize a complementary-input-single-supply amplifier's THD+N performance. Now. the PMOS transistor's offset error is dominant." still with no explanation. the input crossover distortion can affect the amplifier's THD+N performance. a new hire for a leading-edge analog-electronics company." EDN." Texas Instruments. I was wide-eyed and excited–confronting new problems left and right.004%. Creating this pole and zero does not disrupt the amplifier's stability as long as they cancel each other out before the open-loop-gain curve crosses the closed-loop-gain curve. with a capacitive load. in that the output stage switches from transistor to transistor. It solved my problem. but. the term "input-bias current" was an accurate descriptor. 2008. introduces a pole to the open-loop-gain curve. an analog filter. January 2005. or both can increase the amplifier's overall THD+N level. the amplifier circuit will be marginally unstable or. Oljaca. given my work load. Generally. However. a higher level of quiescent current through the output stage reduces the amplifier's THD. a capacitance transducer. sang like a bird. a sample-and-hold circuit. but it was producing a new frequency. the input stage's impact on THD+N complicates the nature of this specification with single-supply amplifiers. In this application. CL . A high level of input noise. when the amplifier's inputs are near the negative rail. and. "Oh. the NMOS-transistor pair dominates the offset error. 2. in Figure 2 the THD+N is 0. If you are not on top of the explanation. it has come back to haunt me. and the PMOS transistors are off. With these types of amplifiers.8V. As the output signal sweeps from rail to rail. "Just do it. This crossover-distortion phenomenon affects the amplifier's THD. I ran around and asked for advice. place the amplifier in an inverting-gain configuration and keep the closed-loop gain low. and CL and RL then introduce a zero to the open-loop-gain curve (Figure 1). The magnitude of the bipolar amplifier's input-bias current ranges from a few nanoamperes for low-power devices to hundreds of nanoamperes for higher-power devices. If the system requires the amplifier to be configured as a noninverting buffer. Rail-to-Rail I/O Operational Amplifier. put a 100Ω load resistor between the amplifier output and the load capacitor. it will come back to bite you. 7MHz. In the complementary-differentialinput-stage topology. The end result is that the input offset voltage changes between the stages. " Texas Instruments. it has come back to haunt me. "Just do it.004%. A high level of input noise. the load resistor. or any other circuit with high-impedance components around your amplifier. This design topology has significant variations in the amplifier's offset voltage across the common-mode input range. For instance. in that the output stage switches from transistor to transistor. January 2005. and CL and RL then introduce a zero to the open-loop-gain curve (Figure 1). In the input region near ground. Another major THD+N contributor can be the operational amplifier's output stage. In the bipolar-amplifier days. the NMOS-transistor pair dominates the offset error. the engineer said. Several types of single-supply amplifier topologies can accept input signals across the power supplies. I ran around and asked for advice. The combination of the load capacitor. Rail-to-Rail Operational Amplifiers. and it still is. I returned to the engineer who gave me the first bit of great advice. Because I had a huge community of experts around me. I did not return to his suggestion for several years. When the amplifier's inputs are closer to the positive rail. The term "input-bias current" loses its meaning when you look at JFET or CMOS input amplifiers. To optimize a complementary-input-single-supply amplifier's THD+N performance. 4 Back to top Is your amplifier offset way out of whack? Have you ever spent a great deal of time selecting the perfect operational amplifier for your circuit. 90dB CMRR. If you are not on top of the explanation. February 2003. Generally. If you comprehend the general guidelines. an amplifier with a singledifferential input stage and charge pump is a more appropriate choice. a sample-and-hold circuit." So. References 1. The THD+N performance of a single-supply amplifier also originates in the amplifier's input and output stages. a buffer-amplifier circuit. a capacitance transducer. "Start with the right op amp when driving SAR ADCs. The amplifier's input noise is another contributor to the THD+N specification. CL . and the PMOS transistors are off. Now. The output stage of a single-supply amplifier usually has an AB topology. CMOS single-supply amplifiers have been beneficial to single-supply-system designers worldwide. an analog filter. in Figure 2 the THD+N is 0. "OPA350. The words of wisdom that came down to me were. In the complementary-differentialinput-stage topology. OPA4350 High-Speed. the input stage's impact on THD+N complicates the nature of this specification with single-supply amplifiers.0006% if you avoid using the input transition. I built a new circuit as suggested. RL . and Bonnie Baker. however. or both can increase the amplifier's overall THD+N level. "OPA363. OPA2350. I need to know what is really going on! What I did not understand then but understand now is that a capacitor and a resistor that hang on the output of an amplifier change the amplifier's open-loop-gain curve. When the PMOS and NMOS transistors are both on." still with no explanation. The magnitude of the bipolar amplifier's input-bias current ranges from a few nanoamperes for low-power devices to hundreds of nanoamperes for higher-power devices. RO. References 1. "Oh. Rail-to-Rail I/O Operational Amplifier. In the region near the positive power supply. One problem I ran into involved the stability of an amplifier circuit. In this application. introduces a pole to the open-loop-gain curve. the amplifier circuit will be marginally unstable or. it will work. Oct 16. Creating this pole and zero does not disrupt the amplifier's stability as long as they cancel each other out before the open-loop-gain curve crosses the closed-loop-gain curve. place the amplifier in an inverting-gain configuration and keep the closed-loop gain low. OPA2363.8V. sang like a bird. It solved my problem. 7MHz. If the THD+N tests include the amplifier's input crossover distortion. MicroAmplifier Series.com 2 Back to top Just use a 100Ω resistor: Understanding a rule of thumb for oscillating amplifiers So there I was. Key players affecting the THD+N (total-harmonic-distortion-plus-noise) characteristics of dual-supply amplifiers are input noise and output-stage crossover distortion. If you configure the complementary-input amplifier in a noninverting configuration. it will come back to bite you. put a 100Ω load resistor between the amplifier output and the load capacitor. 2008. an integrator. Trust me. Miro. only to find that the offset voltage is wrong at the manufacturer's bench-specified input? What if you find that it is more than 10 times higher than specification in your application circuit? Do you send the chip in for failure analysis or just toss the chip out and have another look at your list of amplifiers? As an alternative. If you are using your amplifier as the key component in a transimpedance amplifier. the output stage displays a crossover distortion similar to the input-stage crossover distortion. the NMOS transistors are on. OPA4364. the current http://www. This new circuit was still singing.Texas Instruments ." EDN. You can find the pole and zero locations in this circuit in the following equations: What have I learned from this situation? It pays to understand why an engineer's rule of thumb works. given my work load. 3 Back to top Where did all that racket come from? Since arriving in the market. This crossover-distortion phenomenon affects the amplifier's THD. However. the term "input-bias current" was an accurate descriptor." When I asked why. OPA2364. 1. As it was my first job. the PMOS transistors are on. you will be OK. If the system requires the amplifier to be configured as a noninverting buffer. The end result is that the input offset voltage changes between the stages. and the NMOS transistors are off (Figure 1). but it was producing a new frequency. You can avoid this type of amplifier crossover distortion by usingg an inverting configuration. the THD+N is 0. will oscillate. Oljaca. and the amplifier's open-loop resistance. you might find that the amplifier's input-bias current creates an offset-voltage error through the resistors in your circuit. worse yet. Both pairs are on as the amplifier's inputs pass between these two regions.TI. with a capacitive load. the common-mode voltage region is approximately 400 mV. high closed-loop gains. His recommendation: "Change the 100Ω resistor to a 500Ω resistor. A bipolar amplifier's input-bias current is the same as the base current of the NPN or PNP transistors at the input of the amplifier.ti. and. As the output signal sweeps from rail to rail. I suggest that you try to explain the offset error by re-examining your amplifier's specifications. lo and behold. With these types of amplifiers.com/ww/en/analog/Amplifiers-eBook/[1/14/14 10:48:43 AM] . the PMOS transistor's offset error is dominant. a higher level of quiescent current through the output stage reduces the amplifier's THD. the input crossover distortion can affect the amplifier's THD+N performance. If the open-loop-gain and closed-loop-gain curves cross with a 40-dB/decade closure rate. a new hire for a leading-edge analog-electronics company." Texas Instruments. Single-Supply. OPA364. but. the circuit still oscillated. 2. I was wide-eyed and excited–confronting new problems left and right. when the amplifier's inputs are near the negative rail. Single-Supply. offset. Using the same logic. George Philbrick. We keep the amplifier bandwidths as low as possible. the dctransfer function of this circuit is 1V/V." Texas Instruments. noise.098% or equals 4. the amplifier circuit will be marginally unstable or. So much for lower. His recommendation: "Change the 100Ω resistor to a 500Ω resistor. the closed-loop gain at 2 MHz is 17 dB. January 2005. High-Speed. The correct formula to determine the attenuation back from the 3-dB point toward 0 Hz where A CL1 is the target closed-loop gain.5 mV. the photodiode's currents flow through the R1 feedback resistors. If you make A CL1 equal to 9. you could use a single photodiode to determine the brightness of your bulbs. requiring calibrated and impractical measurement conditions. The input-bias current swamps out the picoampere leakage current from the ESD cells. Ideally. if you exercise special care. but I have a different opinion. or 60 dB. A 1 and A 2 require low-inputcurrent CMOS or FET op amps. A CL_DC is the closed-loop gain at dc. with a capacitive load. The output voltages of A 1 and A 2 contain both difference and common-mode signals. the bandwidth of op amps A. the diagram captures the open-loop gain of each amplifier as the response crosses 0 dB. at the hands of several individuals including Harold S. Adding a matched photodiode and monitoring the difference between the two diode outputs removes the equal offsets the two diodes produce. the differential amp rejects common-mode-voltage changes. (The full-scale range of the ADC is 5V. With the RSS formula. Figure 1: The system bandwidth becomes lower due to the closed-loop (ACL) intercept point with the amplifier's open-loop gain (AOL ) and the need to accommodate bit accuracy across the entire frequency range at f 1 . the current sinking or sourcing from the amplifier's input pins is actually the leakage current from the input-ESD (electrostatic-discharge) cells (Figure 1). March 1999. Rail-to-Rail OPERATIONAL AMPLIFIERS MicroAmplifier ™ Series. In this application. If you configure the complementary-input amplifier in a noninverting configuration. A bipolar amplifier's input-bias current is the same as the base current of the NPN or PNP transistors at the input of the amplifier. the PMOS transistors are on. Given this scenario. As an example.22 mV. 7MHz. One sticky point with this circuit is that the amplifier's closed-loop gain is not equal to 20 dB all the way up to 2 MHz. It solved my problem. As long as the resistors surrounding A 3 are equal. Background light adds only an offset to the photodiode's outputs. I was wide-eyed and excited–confronting new problems left and right. there is still a lot to learn about the amplifier and their appropriate placements in your application circuits. in Figure 2 the THD+N is 0. We have a 20-kHz signal that we need to increase by 10 times. To optimize a complementary-input-single-supply amplifier's THD+N performance. the amp's outputs change in voltage along with the IR drop across the R1 resistors. Texas Instruments. the NMOS transistors are on. or 20 dB. are here to stay as standalone devices and as integrated functions inside complex ICs. MicroAmplifier Series. the input crossover distortion can affect the amplifier's THD+N performance. To determine the dynamic-range limitations of the circuit. the filter-stage (IC2 ) offset error is ±500 µV. Trust me. As you examine your circuits. an integrator. So. we already require an amplifier that has a bandwidth that is 10 times higher than our input signal. The amplifier's input noise is another contributor to the THD+N specification. however. you might find that the amplifier's input-bias current creates an offset-voltage error through the resistors in your circuit. and C are 38 MHz. backgroundlight conditions can influence this magnitude. The key performance parameters for A 1 and A 2 are input capacitance. With these types of amplifiers. Single-Supply. the worst-case sensor-resistive offset error would be ±94 mV×10V/V. note that a 12-bit converter is at the end of the signal chain. IC1 . The faster signals have the potential to produce EMI signals that other devices or traces on the board can receive. by Bonnie Baker Amplifiers have been around for a long time. Given the long time span between these beginning years and the present.9988752. For the most part. When the amplifier's inputs are closer to the positive rail. 6 Back to top Don't be fooled by your amplifier's bandwidth As we design our SAR-converter analog circuits. A 3 rejects the common-mode signal and delivers the differential-voltage signal to the circuit output at V OUT . I ran around and asked for advice. this type of mismatch can introduce nonlinearities in the system when the common-mode voltage of the two inputs changes. When the PMOS and NMOS transistors are both on. such as the temperature-sensor circuit in Figure 1. This specification is independent of the common-mode voltage and the magnitude-amplifier power. depending on the operational-amplifier design. Generally. sang like a bird. "Start with the right op amp when driving SAR ADCs. And. With a maximum signal frequency input from dc to 20 kHz. Once you take this first step." Instead of choosing the best products. but you will never see ESD-leakage current in bipolar amplifiers." Texas Instruments. "OPA363. and the amplifier's open-loop resistance. An equal illumination on the two photodiodes makes the output voltage 0V. These beasts were just coming alive in the 1930s – 1940s. "Oh. This new circuit was still singing. However. As the output signal sweeps from rail to rail. The Sallen-Key filter attenuates any signals in the frequency range above the cutoff frequency to a point. Single-Supply." still with no explanation." The magnitude of leakage current with JFET or CMOS amplifiers is less than 1 pA at 25°C. will oscillate. August 1993. assume that the maximum offset error of IC1 and IC2 is 0. a small amount of dust. in that the output stage switches from transistor to transistor. This is down 3 dB. you must understand the impact of your PCB (printed-circuit board) on the picoampere levels of current. and the NMOS transistors are off (Figure 1). For instance. In the complementary-differentialinput-stage topology. Single-Supply. are the highest performance analog products in front of the ADC appropriate? How do you determine which products are good enough for your system? Avoiding production-floor notifications or field failures may be your definition of "good enough. Oljaca. but it's not necessarily true with the classic Sallen-Key lowpass-filter design. it gradually gets closer to the closed-loop gain curve starting about a decade before 2 MHz. 5 Back to top A good holiday-season project Which of the bulbs on a Christmas tree is the brightest? If you had the time and desire to answer this question. to take it a step further. For instance. Key players affecting the THD+N (total-harmonic-distortion-plus-noise) characteristics of dual-supply amplifiers are input noise and output-stage crossover distortion. you would expect its gain amplitude to continuously decrease beyond the filter's cutoff frequency. high closed-loop gains. it makes sense to keep the bandwidth of the circuit's amplifiers as low as possible. which is approximately 70. One problem I ran into involved the stability of an amplifier circuit. including instrumentation amplifiers (INA) and programmable gain amplifiers (PGA). More important. you can quickly solve this problem by choosing the absolute best performing parts for each socket. This is particularly true as long as the real-world remains in the analog domain. worse yet.7% lower than the closed-loop gain. Grame. If the open-loop-gain and closed-loop-gain curves cross with a 40-dB/decade closure rate. The cumulative possible offset error at the =940 mV. given my work load. during your first consumer-product-selection attempt. Steven M. if you combine all of these terms. Is this statement true or false? Using this type of logic may give you a confident feeling that your circuit will work correctly the first time.8V. the circuit's closed-loop gain does not instantaneously change from 20 dB to 17 dB at 2 MHz. One criterion is to keep the signal within the dynamic range of the full-scale range of the ADC. The output stage of a single-supply amplifier usually has an AB topology. A more accurate descriptor for this current error is "input-leakage current. but it was producing a new frequency. This situation places our amplifier unity-gain bandwidth at 20 MHz. and Bonnie Baker. a new hire for a leading-edge analog-electronics company. 2006. Consequently.Texas Instruments . which makes the magnitude of the output voltage a direct measurement of the difference between the direct light impinging on D1 and D2 . However. 90dB CMRR. Instead. The amplifier's closed-loop gain is 10V/V. bias current. In the diagram. the common-mode voltage region is approximately 400 mV. and the PMOS transistors are off.and current-feedback amps are almost the same Chapter 2: INAs and PGAs Understanding CMR and instrumentation amplifiers Common-mode range can bite hard Instrumentation amplifier input-circuit strategies Wringing out thermistor nonlinearities A good holiday-season project Author's biography   Introduction The Best of Baker's Best – Amplifiers Introduction. We find that a 20-kHz amplifier does not work for our circuit. Figure 1 illustrates the behavior of three Sallen-Key lowpass filters using signal-supply amplifiers. a noticeable gain error can occur between the two input signals. SBOS145. Both pairs are on as the amplifier's inputs pass between these two regions. introduces a pole to the open-loop-gain curve. the bipolar input-bias current can be fairly stable. Finding that brightest bulb among the many and the background light would be a laborious task. If the THD+N tests include the amplifier's input crossover distortion. This intersection has a simple. you take the square root of the sum of the squares of several terms that are statistically independent. In the top three curves. "OPA350. You can see this by comparing the intersect of the closed-loop gain (f 3 dB ) with the amplifier GBWP frequency (f GBWP ). or 20 dB." So.0006% if you avoid using the input transition. We also find that the amplifier bandwidth needs to be at least 100 times higher to maintain ADC integrity. You define the common-mode voltage of the input signals as (VA1OUT +V A2OUT )/2. 3. the amplifier bandwidth needs to be at least 10 times higher than the signal. Amplifiers. and H. Rail-to-Rail I/O Operational Amplifier. but rather they are permanent fixtures in the electronics world. W. The offset error of the ADC is ±1 LSB or ±1. Almost all amplifiers have ESD cells for protection from an ESD event. Black. 2. These calculations can assist you in making logical and economical product decisions. OPA2350. you can use RSS calculations." EDN. References 1. So. which produces a ˜0. the input stage's impact on THD+N complicates the nature of this specification with single-supply amplifiers. A 20-MHz bandwidth for an amplifier should be good enough. In our circuit. the NMOS-transistor pair dominates the offset error. we may be tempted to match the voltage-feedback amplifier's data-sheet bandwidth to the bandwidth of our analog signal source. however. lo and behold. The combination of the load capacitor. RO. Because I had a huge community of experts around me. or any other circuit with high-impedance components around your amplifier. or a 29. 3 Back to top Where did all that racket come from? Since arriving in the market. Because the inverting input of A 1 and A 2 has high impedance. The THD+N performance of a single-supply amplifier also originates in the amplifier's input and output stages.3% increase is needed to get up to that curve. the incident light on the photodiode causes current to flow through the diode from cathode to anode. and the ADC (IC3 ) offset error is ±1. put a 100Ω load resistor between the amplifier output and the load capacitor. If you comprehend the general guidelines. If you are not on top of the explanation. CMOS single-supply amplifiers have been beneficial to single-supply-system designers worldwide. The term "input-bias current" loses its meaning when you look at JFET or CMOS input amplifiers. In this circuit. The voltage at the inverting input of the amp tracks the voltage at the amp's noninverting input. you will see greater accuracy in proximity and difference (Figure 1). following such logic goes only so far when you try to justify the cost-versus-performance factors of the products you are using.22 mV. the configuration for all three amplifiers is a dc noise gain of 1000V/V.004%. The most effective way you can reduce or minimize the effects of input-bias or input-leakage current is to check your circuit configurations. we have a 16-bit converter. At the end of the signal chain. an amplifier with a singledifferential input stage and charge pump is a more appropriate choice. If the system requires the amplifier to be configured as a noninverting buffer. and then the response turns around and starts to increase in gain with frequency. OPA364. 1998. and temperature drift. If the resistors around A 3 are not equal. Oct 16. This crossover-distortion phenomenon affects the amplifier's THD. In the region near the positive power supply. a sample-and-hold circuit. With a signal gain of 10V/V. "Just do it. it will work. The words of wisdom that came down to me were. Photodiode Amplifiers: Op Amp Solutions. I built a new circuit as suggested. I suggest that you try to explain the offset error by re-examining your amplifier's specifications. unless you expanded your design task to using two photodiodes. As it was my first job. These layout headaches come from putting higher-speed amplifiers in the circuit that are producing fast rise and fall times. this assumption is a safe one. you need higher-speed amplifiers in the circuit for two basic reasons. The configurations of A 1 and A 2 act as traditional current-to-voltage converters or transimpedance amps. Jerald. 4 Back to top Is your amplifier offset way out of whack? Have you ever spent a great deal of time selecting the perfect operational amplifier for your circuit. the output stage displays a crossover distortion similar to the input-stage crossover distortion. but. D1 and D2 respond linearly to illumination intensity. and 300 kHz. or water molecules can increase leakage current and masquerade as input-bias current. Precision Unity Gain Differential Amplifier. The good news is that. you can build a PCB that will adhere to a 1-pA performance specification. only to find that the offset voltage is wrong at the manufacturer's bench-specified input? What if you find that it is more than 10 times higher than specification in your application circuit? Do you send the chip in for failure analysis or just toss the chip out and have another look at your list of amplifiers? As an alternative." AEi Systems LLC. With our amplifier circuit. first-order attenuation. The contribution of the amplifier-gain stage. when the amplifier's inputs are near the negative rail. References 1. The calculated CMR (common-moderejection) error due to resistor mismatches is 100×(1+R 2 /R1 )/(% of mismatch error). Sandler. You cannot use an RSS formula with entities that have correlated variations that are not statistically independent. ISBN: 0-07-024247. but let's do the math. Now. Please enjoy your read through and let me know what you think! 1 Back to top Chapter 1: Operational amplifiers When is good enough good enough? If you are having difficulty making product-selection decisions in a consumer circuit." When I asked why. If you use three op amps in a differential-photodiode-amp configuration. CL . I did not return to his suggestion for several years. A high level of input noise. an analog filter. Another major THD+N contributor can be the operational amplifier's output stage. The end result is that the input offset voltage changes between the stages. Furthermore. Bode. On the contrary. The following excerpts from the EDN Baker's Best column represent the best of the amplifier short articles. bringing these errors to the ADC's input. a higher level of quiescent current through the output stage reduces the amplifier's THD. a capacitance transducer. and. you can use the RSS (root-sum-square) algebraic approach. The two photodiodes let you find a light's position by monitoring the difference between their output signals. place the amplifier in an inverting-gain configuration and keep the closed-loop gain low.0112% error at the full-scale frequency. 2008. Figure 1 shows an example of how the amplifier's closed-loop gain affects the system's overall bandwidth. one would think that the required amplifiers would have very low unity-gain bandwidths or a gain-bandwidth product (GBWP).TI. In ensuring that the input-leakage current remains low with JFET and CMOS amplifiers. because faster amplifiers on the board potentially can produce layout headaches. A 3 and R2 form a difference amp. The product characteristics that influence the extent of the dynamic range are the system's cumulative offset and gain errors. and f 3 dB is the corner 3-dB frequency of this amplifier system. a buffer-amplifier circuit. If you are using your amplifier as the key component in a transimpedance amplifier. The JFET and CMOS input amplifiers may not be. you will be OK. resulting in unexpected noise. However. look at the voltage characteristics of each node and make sure that you understand the impact of all of the current paths in your circuit. the bandwidth of the closed-loop system is approximately 100 times lower than f 3 dB . sboa035. OPA4350 High-Speed. lowpass.ti. A single photodiode provides some measure of a light's intensity through the magnitude of the diode's output signal. For instance. it will come back to bite you.com Follow Us   |    Table of Contents Introduction Chapter 1: Operational amplifiers When is good enough good enough? Just use a 100Ω resistor: Understanding a rule of thumb for oscillating amplifiers Where did all that racket come from? Is your amplifier offset way out of whack? A good holiday-season project Don't be fooled by your amplifier's bandwidth Sallen-Key lowpass-filter stopband limitations Choosing antialiasing-filter amplifiers Closer to real-world analog filters Simply an adjustable low-pass filter Photo-sensing circuits: The eyes of the electronic world are watching Transimpedance-amplifier stability is key in light-sensing applications Transimpedance-amplifier noise issues Transimpedance-amplifier application: The pulse oximeter Remote photo sensing Single-supply amplifier outputs don't swing rail to rail The non-negotiable single-supply operational amplifier What does "rail to rail" input operation really mean? Rail-to-rail input amplifier application solutions Voltage. is ±500 µV×10. B. however.9 mV maximum at full scale. The magnitude of the bipolar amplifier's input-bias current ranges from a few nanoamperes for low-power devices to hundreds of nanoamperes for higher-power devices. it has come back to haunt me. OPA2364. the leakage current increases approximately two times per 10°C change. but we are looking for the correct amplifier for a 16-bit system. In the bipolar-amplifier days. During this test. McGraw-Hill. you would use the RSS formula to determine gain-error contribution that limits the dynamic range from the four stages in this circuit. Miro. This design topology has significant variations in the amplifier's offset voltage across the common-mode input range. 2. oil. and it still is. You can easily compensate for this type of error by replacing any of the four resistors with a potentiometer. January 1995. You may say that it will become obsolete soon. The goal is to select amps in which these parameters are as low as possible. References 1. OPA4364. the load resistor. the circuit still oscillated. the engineer said. Input-bias and input-leakage current can change over temperature. Texas Instruments. I need to know what is really going on! What I did not understand then but understand now is that a capacitor and a resistor that hang on the output of an amplifier change the amplifier's open-loop-gain curve. The other reason is to make sure the bits in your system at the end of the signal path can reliably convert the signal throughout the entire system's frequency range.com/ww/en/analog/Amplifiers-eBook/[1/14/14 10:58:49 AM] . However. References 1. the PMOS transistor's offset error is dominant. subtracting the output voltages of A 1 and A 2 . 7 Back to top Sallen-Key lowpass-filter stopband limitations When you design an analog. This calculation illustrates that the sensor cell contributes the most error with little ADC's input is impact from the amplifiers or the ADC. OPA2363. Because the leakage current is from the reverse-biased ESD diodes. You can avoid this type of amplifier crossover distortion by usingg an inverting configuration. Of course.) The gain error of both the sensor cell and the IC1 amplifier configuration depends on the ±1% maximum resistor tolerances as well as on a maximum sensor-resistor tolerance of ±2%. and CL and RL then introduce a zero to the open-loop-gain curve (Figure 1). Photodiode Monitoring with Op Amps. and the differential amp removes this effect. The GBWP of this amplifier is 20 MHz. isn't there something else out there that has successfully obsoleted these devices? I have worked with people who are surprised when I tell them that the operational amplifier is not dying. In fact. In Figure 1. Several types of single-supply amplifier topologies can accept input signals across the power supplies. "A Comparison of Tolerance Analysis Methods. antialiasing filter. Rail-to-Rail Operational Amplifiers. You can find the pole and zero locations in this circuit in the following equations: What have I learned from this situation? It pays to understand why an engineer's rule of thumb works. 2 MHz. The primary reason is to accommodate the lost bandwidth in amplifier circuits that have a closed-loop noise gain greater than one. the THD+N is 0. I returned to the engineer who gave me the first bit of great advice. http://www. however. make sure you use the same evaluation technique in your manufacturing process to quantify the effects of the processes–such as solder reflow–that you impose on these devices and the end-of-life effects due to environmental exposures. Texas Instruments. Creating this pole and zero does not disrupt the amplifier's stability as long as they cancel each other out before the open-loop-gain curve crosses the closed-loop-gain curve. The ADC's contributed gain error is 0. References 1. you would calculate the combined RSS value of offset and gain. You can implement a differential amp discretely or with an off-the-shelf product. RL . f 1 is the target is f 1 =f 3 dB × frequency. respectively. we are not so lucky. They have been complied for your convenience. or both can increase the amplifier's overall THD+N level.speed amplifiers. 2 Back to top Just use a 100Ω resistor: Understanding a rule of thumb for oscillating amplifiers So there I was. The amplifier characteristics shown in Figure 1 appear to be a perfect fit. February 2003. you would think that we have finally come to terms with understanding this five-terminal device. In the input region near ground. the term "input-bias current" was an accurate descriptor. 1. The correct formula to determine the attenuation back from the 3-dB point toward 0 Hz where A CL1 is the target closed-loop gain.TI. one would think that the required amplifiers would have very low unity-gain bandwidths or a gain-bandwidth product (GBWP). Jerald. A single photodiode provides some measure of a light's intensity through the magnitude of the diode's output signal. January 1995. The input-bias current swamps out the picoampere leakage current from the ESD cells. Ideally. However. 5 Back to top A good holiday-season project Which of the bulbs on a Christmas tree is the brightest? If you had the time and desire to answer this question.com/ww/en/analog/Amplifiers-eBook/[1/14/14 10:48:43 AM] . This intersection has a simple. The configurations of A 1 and A 2 act as traditional current-to-voltage converters or transimpedance amps. The goal is to select amps in which these parameters are as low as possible. Of course. however. oil. With a maximum signal frequency input from dc to 20 kHz. the bipolar input-bias current can be fairly stable. Almost all amplifiers have ESD cells for protection from an ESD event. August 1993. If the resistors around A 3 are not equal. it makes sense to keep the bandwidth of the circuit's amplifiers as low as possible. As long as the resistors surrounding A 3 are equal. In ensuring that the input-leakage current remains low with JFET and CMOS amplifiers. The primary reason is to accommodate the lost bandwidth in amplifier circuits that have a closed-loop noise gain greater than one.Texas Instruments . You can implement a differential amp discretely or with an off-the-shelf product. The JFET and CMOS input amplifiers may not be. A CL_DC is the closed-loop gain at dc. The faster signals have the potential to produce EMI signals that other devices or traces on the board can receive. The most effective way you can reduce or minimize the effects of input-bias or input-leakage current is to check your circuit configurations. the bandwidth of the closed-loop system is approximately 100 times lower than f 3 dB . Given this scenario. we already require an amplifier that has a bandwidth that is 10 times higher than our input signal. the leakage current increases approximately two times per 10°C change. A more accurate descriptor for this current error is "input-leakage current. An equal illumination on the two photodiodes makes the output voltage 0V. sboa035. we may be tempted to match the voltage-feedback amplifier's data-sheet bandwidth to the bandwidth of our analog signal source. or water molecules can increase leakage current and masquerade as input-bias current. A 3 rejects the common-mode signal and delivers the differential-voltage signal to the circuit output at V OUT . and temperature drift. the circuit's closed-loop gain does not instantaneously change from 20 dB to 17 dB at 2 MHz. 3. The calculated CMR (common-moderejection) error due to resistor mismatches is 100×(1+R 2 /R1 )/(% of mismatch error). unless you expanded your design task to using two photodiodes. You can easily compensate for this type of error by replacing any of the four resistors with a potentiometer. References 1. but we are looking for the correct amplifier for a 16-bit system. bias current. it gradually gets closer to the closed-loop gain curve starting about a decade before 2 MHz. Texas Instruments. a small amount of dust. These layout headaches come from putting higher-speed amplifiers in the circuit that are producing fast rise and fall times. Input-bias and input-leakage current can change over temperature. As you examine your circuits. However. In our circuit. you need higher-speed amplifiers in the circuit for two basic reasons. Background light adds only an offset to the photodiode's outputs. Because the leakage current is from the reverse-biased ESD diodes. however. because faster amplifiers on the board potentially can produce layout headaches. this type of mismatch can introduce nonlinearities in the system when the common-mode voltage of the two inputs changes. The amplifier characteristics shown in Figure 1 appear to be a perfect fit. subtracting the output voltages of A 1 and A 2 . On the contrary. resulting in unexpected noise. D1 and D2 respond linearly to illumination intensity. f 1 is the target is f 1 =f 3 dB × frequency. and f 3 dB is the corner 3-dB frequency of this amplifier system. first-order attenuation. One sticky point with this circuit is that the amplifier's closed-loop gain is not equal to 20 dB all the way up to 2 MHz.7% lower than the closed-loop gain. backgroundlight conditions can influence this magnitude. You define the common-mode voltage of the input signals as (VA1OUT +V A2OUT )/2.3% increase is needed to get up to that curve. you can build a PCB that will adhere to a 1-pA performance specification. which is approximately 70. depending on the operational-amplifier design. however.0112% error at the full-scale frequency." The magnitude of leakage current with JFET or CMOS amplifiers is less than 1 pA at 25°C. ISBN: 0-07-024247. If you use three op amps in a differential-photodiode-amp configuration. This is down 3 dB. The key performance parameters for A 1 and A 2 are input capacitance. which produces a ˜0. Finding that brightest bulb among the many and the background light would be a laborious task. requiring calibrated and impractical measurement conditions. More important. SBOS145. Grame. A 3 and R2 form a difference amp. Consequently. you must understand the impact of your PCB (printed-circuit board) on the picoampere levels of current. http://www. 6 Back to top Don't be fooled by your amplifier's bandwidth As we design our SAR-converter analog circuits. A 1 and A 2 require low-inputcurrent CMOS or FET op amps. you could use a single photodiode to determine the brightness of your bulbs. Adding a matched photodiode and monitoring the difference between the two diode outputs removes the equal offsets the two diodes produce. look at the voltage characteristics of each node and make sure that you understand the impact of all of the current paths in your circuit. Because the inverting input of A 1 and A 2 has high impedance.com sinking or sourcing from the amplifier's input pins is actually the leakage current from the input-ESD (electrostatic-discharge) cells (Figure 1). a noticeable gain error can occur between the two input signals. The two photodiodes let you find a light's position by monitoring the difference between their output signals.9988752. A 20-MHz bandwidth for an amplifier should be good enough. but you will never see ESD-leakage current in bipolar amplifiers. Figure 1: The system bandwidth becomes lower due to the closed-loop (ACL) intercept point with the amplifier's open-loop gain (AOL ) and the need to accommodate bit accuracy across the entire frequency range at f 1 . 2006. The voltage at the inverting input of the amp tracks the voltage at the amp's noninverting input. McGraw-Hill. offset. the incident light on the photodiode causes current to flow through the diode from cathode to anode. you will see greater accuracy in proximity and difference (Figure 1). Figure 1 shows an example of how the amplifier's closed-loop gain affects the system's overall bandwidth. but let's do the math. which makes the magnitude of the output voltage a direct measurement of the difference between the direct light impinging on D1 and D2 . In fact. This specification is independent of the common-mode voltage and the magnitude-amplifier power. If you make A CL1 equal to 9. The good news is that. or a 29. Precision Unity Gain Differential Amplifier. The output voltages of A 1 and A 2 contain both difference and common-mode signals. We keep the amplifier bandwidths as low as possible. Instead. The other reason is to make sure the bits in your system at the end of the signal path can reliably convert the signal throughout the entire system's frequency range. You can see this by comparing the intersect of the closed-loop gain (f 3 dB ) with the amplifier GBWP frequency (f GBWP ). the differential amp rejects common-mode-voltage changes. For instance. 2. In this circuit. Texas Instruments. the dctransfer function of this circuit is 1V/V. Photodiode Amplifiers: Op Amp Solutions. The GBWP of this amplifier is 20 MHz. if you exercise special care. the amp's outputs change in voltage along with the IR drop across the R1 resistors. the photodiode's currents flow through the R1 feedback resistors. noise. or 20 dB. the closed-loop gain at 2 MHz is 17 dB. Photodiode Monitoring with Op Amps. and the differential amp removes this effect.ti. The amplifier's closed-loop gain is 10V/V. the bandwidth of op amps A. oil. However. you will be OK. you can use the RSS (root-sum-square) algebraic approach. Rail-to-Rail Operational Amplifiers. "OPA350. isn't there something else out there that has successfully obsoleted these devices? I have worked with people who are surprised when I tell them that the operational amplifier is not dying. January 2005. In Figure 1. A 1 and A 2 require low-inputcurrent CMOS or FET op amps. You cannot use an RSS formula with entities that have correlated variations that are not statistically independent.Texas Instruments . The end result is that the input offset voltage changes between the stages. 4 Back to top Is your amplifier offset way out of whack? Have you ever spent a great deal of time selecting the perfect operational amplifier for your circuit. Using the same logic. Trust me. We also find that the amplifier bandwidth needs to be at least 100 times higher to maintain ADC integrity.0006% if you avoid using the input transition. Single-Supply. but it's not necessarily true with the classic Sallen-Key lowpass-filter design. This is down 3 dB.9 mV maximum at full scale." AEi Systems LLC. In the region near the positive power supply.8V. noise. you can build a PCB that will adhere to a 1-pA performance specification. the amplifier circuit will be marginally unstable or. but I have a different opinion. look at the voltage characteristics of each node and make sure that you understand the impact of all of the current paths in your circuit. or 20 dB. which is approximately 70. unless you expanded your design task to using two photodiodes. In this application. we are not so lucky. bias current. including instrumentation amplifiers (INA) and programmable gain amplifiers (PGA). MicroAmplifier Series. When the PMOS and NMOS transistors are both on. Texas Instruments. you need higher-speed amplifiers in the circuit for two basic reasons. The other reason is to make sure the bits in your system at the end of the signal path can reliably convert the signal throughout the entire system's frequency range. Figure 1: The system bandwidth becomes lower due to the closed-loop (ACL) intercept point with the amplifier's open-loop gain (AOL ) and the need to accommodate bit accuracy across the entire frequency range at f 1 . a sample-and-hold circuit. RO. assume that the maximum offset error of IC1 and IC2 is 0. You define the common-mode voltage of the input signals as (VA1OUT +V A2OUT )/2. We have a 20-kHz signal that we need to increase by 10 times. and the differential amp removes this effect. Single-Supply. His recommendation: "Change the 100Ω resistor to a 500Ω resistor. To determine the dynamic-range limitations of the circuit. This crossover-distortion phenomenon affects the amplifier's THD. and the ADC (IC3 ) offset error is ±1. In the diagram. this assumption is a safe one. If you are using your amplifier as the key component in a transimpedance amplifier. you would use the RSS formula to determine gain-error contribution that limits the dynamic range from the four stages in this circuit. Several types of single-supply amplifier topologies can accept input signals across the power supplies. the circuit still oscillated. you take the square root of the sum of the squares of several terms that are statistically independent. the input stage's impact on THD+N complicates the nature of this specification with single-supply amplifiers. and H. If the THD+N tests include the amplifier's input crossover distortion. but we are looking for the correct amplifier for a 16-bit system. This new circuit was still singing. I did not return to his suggestion for several years. 7MHz. Of course. OPA364. 2 Back to top Just use a 100Ω resistor: Understanding a rule of thumb for oscillating amplifiers So there I was. but let's do the math. And. the filter-stage (IC2 ) offset error is ±500 µV. When the amplifier's inputs are closer to the positive rail. however.22 mV. during your first consumer-product-selection attempt.9988752. you could use a single photodiode to determine the brightness of your bulbs. The words of wisdom that came down to me were. The JFET and CMOS input amplifiers may not be. we may be tempted to match the voltage-feedback amplifier's data-sheet bandwidth to the bandwidth of our analog signal source. I ran around and asked for advice. Precision Unity Gain Differential Amplifier. and C are 38 MHz. with a capacitive load. The combination of the load capacitor. Miro. References 1. the closed-loop gain at 2 MHz is 17 dB. Because I had a huge community of experts around me. You can avoid this type of amplifier crossover distortion by usingg an inverting configuration. These layout headaches come from putting higher-speed amplifiers in the circuit that are producing fast rise and fall times. such as the temperature-sensor circuit in Figure 1. This specification is independent of the common-mode voltage and the magnitude-amplifier power. the photodiode's currents flow through the R1 feedback resistors. Bode. January 1995. This design topology has significant variations in the amplifier's offset voltage across the common-mode input range. OPA2363. A single photodiode provides some measure of a light's intensity through the magnitude of the diode's output signal. Background light adds only an offset to the photodiode's outputs. In the bipolar-amplifier days. and the PMOS transistors are off. sboa035. make sure you use the same evaluation technique in your manufacturing process to quantify the effects of the processes–such as solder reflow–that you impose on these devices and the end-of-life effects due to environmental exposures. because faster amplifiers on the board potentially can produce layout headaches. I suggest that you try to explain the offset error by re-examining your amplifier's specifications. in that the output stage switches from transistor to transistor. I returned to the engineer who gave me the first bit of great advice. the THD+N is 0. the amplifier bandwidth needs to be at least 10 times higher than the signal. One sticky point with this circuit is that the amplifier's closed-loop gain is not equal to 20 dB all the way up to 2 MHz. More important. in Figure 2 the THD+N is 0. The correct formula to determine the attenuation back from the 3-dB point toward 0 Hz where A CL1 is the target closed-loop gain. 6 Back to top Don't be fooled by your amplifier's bandwidth As we design our SAR-converter analog circuits. Key players affecting the THD+N (total-harmonic-distortion-plus-noise) characteristics of dual-supply amplifiers are input noise and output-stage crossover distortion. One problem I ran into involved the stability of an amplifier circuit. This situation places our amplifier unity-gain bandwidth at 20 MHz. This intersection has a simple. Figure 1 shows an example of how the amplifier's closed-loop gain affects the system's overall bandwidth. These calculations can assist you in making logical and economical product decisions. As long as the resistors surrounding A 3 are equal. the load resistor. February 2003." So. RL . the incident light on the photodiode causes current to flow through the diode from cathode to anode. OPA2364. The amplifier's input noise is another contributor to the THD+N specification. the diagram captures the open-loop gain of each amplifier as the response crosses 0 dB. introduces a pole to the open-loop-gain curve. References 1.) The gain error of both the sensor cell and the IC1 amplifier configuration depends on the ±1% maximum resistor tolerances as well as on a maximum sensor-resistor tolerance of ±2%. The THD+N performance of a single-supply amplifier also originates in the amplifier's input and output stages. the bipolar input-bias current can be fairly stable. I need to know what is really going on! What I did not understand then but understand now is that a capacitor and a resistor that hang on the output of an amplifier change the amplifier's open-loop-gain curve. With these types of amplifiers.com Follow Us   |    Table of Contents Introduction Chapter 1: Operational amplifiers When is good enough good enough? Just use a 100Ω resistor: Understanding a rule of thumb for oscillating amplifiers Where did all that racket come from? Is your amplifier offset way out of whack? A good holiday-season project Don't be fooled by your amplifier's bandwidth Sallen-Key lowpass-filter stopband limitations Choosing antialiasing-filter amplifiers Closer to real-world analog filters Simply an adjustable low-pass filter Photo-sensing circuits: The eyes of the electronic world are watching Transimpedance-amplifier stability is key in light-sensing applications Transimpedance-amplifier noise issues Transimpedance-amplifier application: The pulse oximeter Remote photo sensing Single-supply amplifier outputs don't swing rail to rail The non-negotiable single-supply operational amplifier What does "rail to rail" input operation really mean? Rail-to-rail input amplifier application solutions Voltage. IC1 . However. to take it a step further. the PMOS transistor's offset error is dominant.5 mV. The product characteristics that influence the extent of the dynamic range are the system's cumulative offset and gain errors. Adding a matched photodiode and monitoring the difference between the two diode outputs removes the equal offsets the two diodes produce. if you exercise special care. In the top three curves. but you will never see ESD-leakage current in bipolar amplifiers. Single-Supply. B. and. I was wide-eyed and excited–confronting new problems left and right. a buffer-amplifier circuit. put a 100Ω load resistor between the amplifier output and the load capacitor. D1 and D2 respond linearly to illumination intensity. As the output signal sweeps from rail to rail. Because the leakage current is from the reverse-biased ESD diodes. The following excerpts from the EDN Baker's Best column represent the best of the amplifier short articles. and the NMOS transistors are off (Figure 1). 5 Back to top A good holiday-season project Which of the bulbs on a Christmas tree is the brightest? If you had the time and desire to answer this question. Because the inverting input of A 1 and A 2 has high impedance. The contribution of the amplifier-gain stage. you would calculate the combined RSS value of offset and gain. you can use RSS calculations. Black. Rail-to-Rail OPERATIONAL AMPLIFIERS MicroAmplifier ™ Series. and it still is. the term "input-bias current" was an accurate descriptor. On the contrary. The ADC's contributed gain error is 0." Instead of choosing the best products. and Bonnie Baker. it has come back to haunt me. George Philbrick. A 3 rejects the common-mode signal and delivers the differential-voltage signal to the circuit output at V OUT . "Start with the right op amp when driving SAR ADCs. Oct 16. which produces a ˜0. you must understand the impact of your PCB (printed-circuit board) on the picoampere levels of current. For instance. will oscillate. offset. Another major THD+N contributor can be the operational amplifier's output stage. Steven M." EDN. and CL and RL then introduce a zero to the open-loop-gain curve (Figure 1). Jerald. backgroundlight conditions can influence this magnitude. subtracting the output voltages of A 1 and A 2 . Instead. August 1993.ti. (The full-scale range of the ADC is 5V. you would expect its gain amplitude to continuously decrease beyond the filter's cutoff frequency. You can easily compensate for this type of error by replacing any of the four resistors with a potentiometer. The term "input-bias current" loses its meaning when you look at JFET or CMOS input amplifiers." Texas Instruments. are the highest performance analog products in front of the ADC appropriate? How do you determine which products are good enough for your system? Avoiding production-floor notifications or field failures may be your definition of "good enough. lowpass. The primary reason is to accommodate the lost bandwidth in amplifier circuits that have a closed-loop noise gain greater than one. A 20-MHz bandwidth for an amplifier should be good enough. You can implement a differential amp discretely or with an off-the-shelf product. If you are not on top of the explanation. or both can increase the amplifier's overall THD+N level. References 1. and temperature drift. or 60 dB. For instance. 7 Back to top Sallen-Key lowpass-filter stopband limitations When you design an analog. depending on the operational-amplifier design. an integrator. it gradually gets closer to the closed-loop gain curve starting about a decade before 2 MHz. 2 MHz. With a signal gain of 10V/V. One criterion is to keep the signal within the dynamic range of the full-scale range of the ADC." The magnitude of leakage current with JFET or CMOS amplifiers is less than 1 pA at 25°C. The most effective way you can reduce or minimize the effects of input-bias or input-leakage current is to check your circuit configurations. It solved my problem. 1998. At the end of the signal chain. References 1. the configuration for all three amplifiers is a dc noise gain of 1000V/V. the current sinking or sourcing from the amplifier's input pins is actually the leakage current from the input-ESD (electrostatic-discharge) cells (Figure 1). You can see this by comparing the intersect of the closed-loop gain (f 3 dB ) with the amplifier GBWP frequency (f GBWP ). In this circuit. an amplifier with a singledifferential input stage and charge pump is a more appropriate choice. The good news is that. Grame. To optimize a complementary-input-single-supply amplifier's THD+N performance. and f 3 dB is the corner 3-dB frequency of this amplifier system. you would think that we have finally come to terms with understanding this five-terminal device. you will see greater accuracy in proximity and difference (Figure 1). The configurations of A 1 and A 2 act as traditional current-to-voltage converters or transimpedance amps. However. the input crossover distortion can affect the amplifier's THD+N performance. at the hands of several individuals including Harold S. They have been complied for your convenience. An equal illumination on the two photodiodes makes the output voltage 0V. however. A bipolar amplifier's input-bias current is the same as the base current of the NPN or PNP transistors at the input of the amplifier. 2. These beasts were just coming alive in the 1930s – 1940s. The amplifier characteristics shown in Figure 1 appear to be a perfect fit. During this test. The calculated CMR (common-moderejection) error due to resistor mismatches is 100×(1+R 2 /R1 )/(% of mismatch error). and the amplifier's open-loop resistance. This is particularly true as long as the real-world remains in the analog domain. Texas Instruments.7% lower than the closed-loop gain. note that a 12-bit converter is at the end of the signal chain. 2. a new hire for a leading-edge analog-electronics company. but. Texas Instruments. an analog filter. Sandler. 3. A 3 and R2 form a difference amp. one would think that the required amplifiers would have very low unity-gain bandwidths or a gain-bandwidth product (GBWP). The goal is to select amps in which these parameters are as low as possible. Is this statement true or false? Using this type of logic may give you a confident feeling that your circuit will work correctly the first time. the worst-case sensor-resistive offset error would be ±94 mV×10V/V. by Bonnie Baker Amplifiers have been around for a long time.TI. We find that a 20-kHz amplifier does not work for our circuit. In the complementary-differentialinput-stage topology. If you make A CL1 equal to 9. 90dB CMRR. first-order attenuation. or water molecules can increase leakage current and masquerade as input-bias current. or a 29. Furthermore. The input-bias current swamps out the picoampere leakage current from the ESD cells. Finding that brightest bulb among the many and the background light would be a laborious task. The Sallen-Key filter attenuates any signals in the frequency range above the cutoff frequency to a point.speed amplifiers." When I asked why. OPA2350. If you use three op amps in a differential-photodiode-amp configuration. Figure 1 illustrates the behavior of three Sallen-Key lowpass filters using signal-supply amplifiers. The voltage at the inverting input of the amp tracks the voltage at the amp's noninverting input. 3 Back to top Where did all that racket come from? Since arriving in the market. only to find that the offset voltage is wrong at the manufacturer's bench-specified input? What if you find that it is more than 10 times higher than specification in your application circuit? Do you send the chip in for failure analysis or just toss the chip out and have another look at your list of amplifiers? As an alternative. If the system requires the amplifier to be configured as a noninverting buffer. Photodiode Amplifiers: Op Amp Solutions. W. respectively. a higher level of quiescent current through the output stage reduces the amplifier's THD. f 1 is the target is f 1 =f 3 dB × frequency. are here to stay as standalone devices and as integrated functions inside complex ICs. As an example. The faster signals have the potential to produce EMI signals that other devices or traces on the board can receive.3% increase is needed to get up to that curve.004%. References 1. is ±500 µV×10. Now. requiring calibrated and impractical measurement conditions. the differential amp rejects common-mode-voltage changes. A high level of input noise." Texas Instruments. 1. or any other circuit with high-impedance components around your amplifier. High-Speed. following such logic goes only so far when you try to justify the cost-versus-performance factors of the products you are using. Input-bias and input-leakage current can change over temperature. SBOS145. If you configure the complementary-input amplifier in a noninverting configuration. the amp's outputs change in voltage along with the IR drop across the R1 resistors. but rather they are permanent fixtures in the electronics world.098% or equals 4. place the amplifier in an inverting-gain configuration and keep the closed-loop gain low. "A Comparison of Tolerance Analysis Methods. I built a new circuit as suggested. In fact. A CL_DC is the closed-loop gain at dc. OPA4350 High-Speed. The output stage of a single-supply amplifier usually has an AB topology. you can quickly solve this problem by choosing the absolute best performing parts for each socket.22 mV. sang like a bird. 2006. Please enjoy your read through and let me know what you think! 1 Back to top Chapter 1: Operational amplifiers When is good enough good enough? If you are having difficulty making product-selection decisions in a consumer circuit. The magnitude of the bipolar amplifier's input-bias current ranges from a few nanoamperes for low-power devices to hundreds of nanoamperes for higher-power devices. ISBN: 0-07-024247. The two photodiodes let you find a light's position by monitoring the difference between their output signals. a capacitance transducer. Rail-to-Rail I/O Operational Amplifier. Ideally. "Just do it. the NMOS transistors are on. there is still a lot to learn about the amplifier and their appropriate placements in your application circuits. So. Both pairs are on as the amplifier's inputs pass between these two regions. A more accurate descriptor for this current error is "input-leakage current. So much for lower. So. the circuit's closed-loop gain does not instantaneously change from 20 dB to 17 dB at 2 MHz. the dctransfer function of this circuit is 1V/V. The key performance parameters for A 1 and A 2 are input capacitance. or 20 dB. and 300 kHz. "OPA363. Consequently. March 1999. For instance. In ensuring that the input-leakage current remains low with JFET and CMOS amplifiers. Once you take this first step. The GBWP of this amplifier is 20 MHz. We keep the amplifier bandwidths as low as possible. You may say that it will become obsolete soon. CL . the leakage current increases approximately two times per 10°C change. With our amplifier circuit. the engineer said. 2008. high closed-loop gains. For the most part. a noticeable gain error can occur between the two input signals. The amplifier's closed-loop gain is 10V/V. OPA4364. Almost all amplifiers have ESD cells for protection from an ESD event. a small amount of dust. The cumulative possible offset error at the =940 mV. CMOS single-supply amplifiers have been beneficial to single-supply-system designers worldwide. However. Generally. The output voltages of A 1 and A 2 contain both difference and common-mode signals.0112% error at the full-scale frequency. In the input region near ground. antialiasing filter. Given this scenario. the PMOS transistors are on. In our circuit. bringing these errors to the ADC's input. it will work. the output stage displays a crossover distortion similar to the input-stage crossover distortion. which makes the magnitude of the output voltage a direct measurement of the difference between the direct light impinging on D1 and D2 . Given the long time span between these beginning years and the present. the bandwidth of the closed-loop system is approximately 100 times lower than f 3 dB . we have a 16-bit converter. this type of mismatch can introduce nonlinearities in the system when the common-mode voltage of the two inputs changes. As you examine your circuits. if you combine all of these terms. If the resistors around A 3 are not equal. You can find the pole and zero locations in this circuit in the following equations: What have I learned from this situation? It pays to understand why an engineer's rule of thumb works." still with no explanation. lo and behold. but it was producing a new frequency. we already require an amplifier that has a bandwidth that is 10 times higher than our input signal. McGraw-Hill. you might find that the amplifier's input-bias current creates an offset-voltage error through the resistors in your circuit. This calculation illustrates that the sensor cell contributes the most error with little ADC's input is impact from the amplifiers or the ADC.com/ww/en/analog/Amplifiers-eBook/[1/14/14 10:58:49 AM] . the common-mode voltage region is approximately 400 mV. Amplifiers. If the open-loop-gain and closed-loop-gain curves cross with a 40-dB/decade closure rate. however. worse yet. "Oh. As it was my first job.and current-feedback amps are almost the same Chapter 2: INAs and PGAs Understanding CMR and instrumentation amplifiers Common-mode range can bite hard Instrumentation amplifier input-circuit strategies Wringing out thermistor nonlinearities A good holiday-season project Author's biography   Introduction The Best of Baker's Best – Amplifiers Introduction. If you comprehend the general guidelines. and then the response turns around and starts to increase in gain with frequency. it makes sense to keep the bandwidth of the circuit's amplifiers as low as possible. it will come back to bite you. Oljaca. the NMOS-transistor pair dominates the offset error. The offset error of the ADC is ±1 LSB or ±1. when the amplifier's inputs are near the negative rail. With a maximum signal frequency input from dc to 20 kHz. given my work load. Photodiode Monitoring with Op Amps. http://www. Creating this pole and zero does not disrupt the amplifier's stability as long as they cancel each other out before the open-loop-gain curve crosses the closed-loop-gain curve. With the RSS formula. however. resulting in unexpected noise. 9 Back to top Closer to real-world analog filters There are two basic filter topologies that everyone recommends when they start to design their signal chain: multiple-feedback (MFB) or Sallen-Key topologies. The Sallen-Key filter attenuates any signals in the frequency range above the cutoff frequency to a point. 7 Back to top Sallen-Key lowpass-filter stopband limitations When you design an analog. the filter gain begins to increase at a rate of 20 dB/decade. April 2003. you need to select an amplifier such that the slew rate (πV OUT P-Pf CUT ). input-common-mode-voltage range (VCMR) and input bias current (I B) can also affect you. So much for lower. 8 Back to top Choosing antialiasing-filter amplifiers When you digitize an analog signal. 2. With a signal gain of 10V/V. For Q i greater than 1. Figure 1 illustrates the behavior of three Sallen-Key lowpass filters using signal-supply amplifiers. But what are the differences and why would you choose one over the other? The MFB topology (sometimes called infinite gain or Rauch) is often preferred because it has low sensitivity to amplifier variations. The caveat to this action is that the following filter may interfere with the phase response of your intended filter. its closed-loop output resistance increases. J. you can cascade both of these topologies. f CUT (or –3-dB frequency). The MFB topology creates a second-order filter. lowpass. A second set of three curves in Figure 1 shows the frequency response of second-order. the bandwidth of op amps A. be aware that this circuit has high-frequency feedthrough. In the diagram. During this test. Selecting the correct operational amplifier for an active lowpass-filter circuit can appear overwhelming as you read an amplifier's data sheet and view all of the specifications. This inversion may or may not be a concern in the application circuit.com/ww/en/analog/Amplifiers-eBook/[1/14/14 10:48:43 AM] . The voltage drop that the input-bias-current error causes appears as an additional input-offset voltage. antialiasing filter. capacitances. In the top three curves. http://www. Also. Sallen-Key lowpass filters for each amplifier. the output polarity is the same as the input signal. When using the Sallen-Key circuit. and then the response turns around and starts to increase in gain with frequency. You cannot use a digital filter to reduce in-band noise after digitizing the signal. particularly if you have differential input/output stages. the configuration for all three amplifiers is a dc noise gain of 1000V/V. In this configuration. In designs where an even number of stages is required. When you send large signals through the amplifier. A sixth-order filter consisting of three MFB stages inverts the signal three times. When an inverting filter is an acceptable alternative. Doing so attenuates superimposed. and 300 kHz. this assumption is a safe one. As the frequency increases beyond this point. we have a 16-bit converter. which may cause additional ringing in the time domain. the gain-bandwidth product of the amplifier. Further. The data indicates that the lowpass filters are performing as you would expect for a little more than a decade after the cutoff frequency of 1 kHz." Application Note (SBFA001A). The speed of this charging process depends on the amplifier's internal resistances. you would expect its gain amplitude to continuously decrease beyond the filter's cutoff frequency. If the noise on the input signal is more than half the sampling frequency of the converter. Doing so ensures that your filter does not create signal distortions due to slew limitations. which does not display this reversal in the gain response at higher frequencies and does not swing the input stage's common-mode voltage. If you need a higher order filter. B. respectively. At the end of the signal chain. Bishop. Eventually. such as the open-loop gain or input range (Figure 1). Next. You would expect this response from a second-order lowpass filter. where V OUT P-P is the expected peak-to-peak output-voltage swing below your filter's cutoff frequency. for Q i of less than 1.speed amplifiers. high-frequency noise on the analog signal before it reaches the ADC. Rail-to-Rail OPERATIONAL AMPLIFIERS MicroAmplifier ™ Series.com With our amplifier circuit. you should initially consider only two important specifications when selecting an amplifier for your active lowpass filter: gain-bandwidth product and slew rate. It is no coincidence that the "flattening" of the filter response occurs at this crossing. Single-Supply. Op Amps for Everyone. the input bias current conducts through the external source resistance. High-Speed. Then. References 1. some characterization is in order. the amplifier's gain is less than 0 dB. or 60 dB. Mancini.ti. Elsevier-Newnes. and currents. but the frequency changes as it aliases back onto your signal of interest. You can reduce the impact of the upward trend in the filter's response by following the offending active filter with a passive RC lowpass filter.or eighth-order filter. This situation places our amplifier unity-gain bandwidth at 20 MHz. You should also evaluate the effects of amplifier slew rate. B Trump. References 1. such as a fourth. all three of the filter's responses show a slope of –40 dB/decade. Texas Instruments . We have a 20-kHz signal that we need to increase by 10 times. In Figure 1. The difference in the frequency response at the point at which the three amplifiers change to a positive slope depends on the amplifier's output impedance. second-order. 2 MHz. The most common topologies for active. March 1999. the amplifier bandwidth needs to be at least 10 times higher than the signal. each filter's response flattens at the 0-dB crossing frequency of the op amp's open-loop gain. Ron. Before selecting the amplifier. For the most part. you use a lowpass filter to prevent aliasing errors from out-of-band noise. you can use a multiple-feedback circuit. You can use filterdesign programs to determine the filter's capacitor and resistor values. lowpass filters are the noninverting Sallen-Key and the inverting multiple feedback. November 2001. Additionally. "FilterPro MFB and Sallen-Key Low-Pass Filter Design Program. We also find that the amplifier bandwidth needs to be at least 100 times higher to maintain ADC integrity. V CMR limits the range of your input signal. where Q i is the quality factor of the ith partial filter and ki is the ratio of the partial-filter-corner frequency to the overall. f AMP. or 20 dB. at some point. ISBN-0-7506-7701-5. We find that a 20-kHz amplifier does not work for our circuit. f AMP=100×gain×(f CUT /a i )× filter-transfer function. Texas Instruments. Although the approximation method does not impact or correct this unexpected behavior. this action also creates a stage whose output is not low-impedance. The gain-bandwidth product of the operational amplifiers appears in the specification table of the respective productdata sheet. A MFB single-stage (second-order filter) provides two poles while inverting the signal. but it's not necessarily true with the classic Sallen-Key lowpass-filter design.TI. and C are 38 MHz. you need to determine the filter-cutoff frequency. As the amplifier's open-loop gain decreases. however. and RM Stitt.Texas Instruments . we are not so lucky. the diagram captures the open-loop gain of each amplifier as the response crosses 0 dB. To ensure that your active filter does not enter into a slew condition. where ai is the ith coefficient in the partialfilter-corner frequency. Alternative filters can solve the problem without adding an RC filter. these filters use a Butterworth design. the magnitude of that noise stays the same. The slew rate depends on internal IC currents and capacitances. internal currents charge these internal capacitors. resulting in an inverted output after the third stage. After the cutoff frequency. If you use a Sallen-Key lowpass filter. must be at least 100×gain×f CUT ×k i . com A second set of three curves in Figure 1 shows the frequency response of second-order. position equipment." The Filter Designer Design Summary view (page 3) allows the user to adjust the capacitor seed value to the desired value of C1 and C2 on the left side of the screen. be aware that this circuit has high-frequency feedthrough. and Chebyshev filters with the same circuit using a 1:10 corner frequency range. The caveat is that low capacitor values can result in significant errors due to parasitic capacitance and the input capacitance of the amplifier. combining the input and filtering stages separates the signal of interest from the noise floor. at some point. A reprogramming of the dual digital potentiometer in Figure 1 changes the filter's frequency cut-off and approximation (Butterworth. it is possible to have a variety of signals that reach the ADC where each signal source has its own set of filter requirements. where ai is the ith coefficient in the partialfilter-corner frequency. As you do this make sure that fs is ten times higher than fc. and RM Stitt. a sharp pencil. the switched integrator has gained favor. To produce the remaining filters in Table 1. system designers have exploited this transfer of light into electrical energy in various systems. the input bias current conducts through the external source resistance. Filter Designer also provides SPICE simulations of the chosen filter's sine wave response. Change to Capacitor seed value to 15 e-9 or 15 nF. its closed-loop output resistance increases. presents an interesting challenge.ti. the system's phase margin is 65°. Consequently. Your circuit also may require a low Q. the Sallen-Key topology is better if: 1. and feedback-resistor value places the system's pole of 1/β at half the frequency where the two curves intersect: where f GBW is the gain-bandwidth product of the amplifier. Then press the green button. Figure 2: A Sallen-Key second-order. and BW is the bandwidth of interest. The closure rate between the two curves suggests the system's phase margin and. amplifier-feedback element. you need to determine the filter-cutoff frequency.22 mV. and layout of this transimpedance-amplifier circuit.TI. In this view. two capacitors. low-noise performance.    a unity-gain filter is needed. indicating a phase shift of –180° and a phase margin of 0°. and one operational amplifier reduces chip count. Selecting the correct operational amplifier for an active lowpass-filter circuit can appear overwhelming as you read an amplifier's data sheet and view all of the specifications. a silicon sensor converts light into charge or an electrical current. the closure rate of the two curves is 20 dB/decade. 2008. you can also select the supply requirements of Single Supply = +5 V. This may be preferable over a MFB single stage. each filter's response flattens at the 0-dB crossing frequency of the op amp's open-loop gain. you should initially consider only two important specifications when selecting an amplifier for your active lowpass filter: gain-bandwidth product and slew rate. resulting in a stable system. a 100 kΩ 8-bit digital potentiometer. Both LSBs are peak-to-peak numbers. At the system's front end. References 1. But what are the differences and why would you choose one over the other? The MFB topology (sometimes called infinite gain or Rauch) is often preferred because it has low sensitivity to amplifier variations. second-order programmable low-pass Sallen-key filter. When an inverting filter is an acceptable alternative. you need to select an amplifier such that the slew rate (πV OUT P-Pf CUT ). B Trump. Elsevier-Newnes. detect smoke. CRF . The speed of this charging process depends on the amplifier's internal resistances. Here. and a feedback network. Figure 1: MFB second-order single-supply low-pass filter. equaling 1/(1+Z F /ZIN). When calculating the noise performance of the circuit. scientists quickly discovered the photo-sensing characteristics of silicon in the lab. fs = 1000. equaling RPD||jΩ(C PD+CCM +CDIFF ). you can cascade both of these topologies. A resistor with a value of 100 kΩ to 10 MΩ connects the inverting input of the amplifier to the output. Figure 1 shows the details of a single-supply. April 2003. f AMP=100×gain×(f CUT /a i )× filter-transfer function. Typically this type of filter is used to reduce analog-to-digital converter (ADC) aliasing errors and noise outside the signal bandwidth. 12 Back to top Transimpedance-amplifier stability is key in light-sensing applications A variety of precision applications sense light and convert that information into a useful digital word. "Photo-sensing circuits: The eyes of the electronic world are watching. and C2 is the parallel combination of CF and CRF . If the closure rate of these two curves is 40 dB/decade. 10 Back to top Simply an adjustable low-pass filter A low-pass filter is the most common filter found in data acquisition systems. An appropriate amplifier for this circuit would have a FET. As the frequency increases beyond this point. but the frequency changes as it aliases back onto your signal of interest. the required accuracy in these applications continues to increase. You cannot use a digital filter to reduce in-band noise after digitizing the signal. "Photo-sensing circuits: The eyes of the electronic world are watching. the gain circuit is negative and equal to the resistor ratio of R2 and R1 . Light excitement on the photo sensor generates charge. You can calculate the appropriate resistance and capacitance with a little research. After the cutoff frequency. Aug 7. The Sallen-Key topology may be preferable for low-Q. f AMP. Baker. flicker-noise (1/f). The switched integrator was the first step toward bringing the digital portion of the circuit closer to the signal source. the designer optimizes the stability. replaces the two resistors in this circuit. The area under the noise-density curve in the e1 . The migration of the photo-sensing-application product has moved on to totally integrated systems. When using the Sallen-Key circuit. Subsequently. The most common topologies for active. such as Texas Instruments' WEBENCH Filter Designer Web software. With this circuit. where CPD is the device's capacitance and CDIFF is the differential amplifier's capacitance. WEBENCH Filter Designer program. The WEBENCH Filter Designer program seems to have broken through some filter design barriers. The amplifier bandwidth calculation involves variations of the amplifier's bandwidth and variations of the passive components over time. Z IN is the distributed input impedance. resulting in an inverted output after the third stage. the more easily the circuit oscillates. In the end. all three of the filter's responses show a slope of –40 dB/decade. To ensure that your active filter does not enter into a slew condition. The filter implementation requires two resistors and two capacitors. 13 Back to top Transimpedance-amplifier noise issues How much noise is too much noise in a photodiode-preamplifier circuit? You can derive the noise performance of a transimpedance amplifier (Figure 1a) with calculations or by using a Spice simulation. but this is not the only potential advantage. If you use a Sallen-Key lowpass filter.    a low Q is required (for example. equaling RF ||jΩ(C RF +CF ). "Start Filter Design. The MFB topology creates a second-order filter. These filters use independent operational amplifiers (op amps) in combination with fixed resistors and capacitors. Baker. Before selecting the amplifier. You can reduce the impact of the upward trend in the filter's response by following the offending active filter with a passive RC lowpass filter. For many CMOS or FET amplifiers. The amplifier does not gain the contribution of noise from the feedback resistor: where K is Boltzmann's constant. 9 Back to top Closer to real-world analog filters There are two basic filter topologies that everyone recommends when they start to design their signal chain: multiple-feedback (MFB) or Sallen-Key topologies. References 1. To this day. It is not simply a matter of picking an amplifier with a gain-bandwidth-product that is 100 times higher than the filter's corner frequency. The traditional design topology of the transimpedance amplifier captures this low-level signal in a hybrid approach that starts with an amplifier and a high-value resistor in the feedback loop. Figure 1 shows the front-end circuit of this system. from multiple op amps to one single amplifier. "FilterPro MFB and Sallen-Key Low-Pass Filter Design Program. region is V 1/f:fB–fA =A N . and the values in this column are root-mean-square values. an operational amplifier. A signal path requires this type of dedicated filter to match the signal's requirements. the cost is lower when a single filter serves many analog inputs. particularly at the 3 dB corner frequency. All I can say is filter design has come a long way. e6 . The A OL (jΩ) curve intersects the 1/β curve at an interesting point. 2. and a good eraser. As the amplifier's open-loop gain decreases. Ron. to mention a few. System elements determining the noise-gain frequency response are the photodiode's parasitics and the operational amplifier's input capacitance. The data indicates that the lowpass filters are performing as you would expect for a little more than a decade after the cutoff frequency of 1 kHz. Doing so attenuates superimposed. such as a fourth. search noninvasively for tumors.or a CMOS-input stage with lowvoltage noise and microvolt-offset specifications. where A N is the amplifier's input-noise-density at 1 Hz and f B is the corner frequency where the flicker noise tapers off. for these circuits. the R2 /R1 resistor ratio and the resistor errors determine the gain. such as the open-loop gain or input range (Figure 1). If the noise on the input signal is more than half the sampling frequency of the converter. as well as RF . lowpass filters are the noninverting Sallen-Key and the inverting multiple feedback. a wide variety of applications use silicon to sense the intensity and characteristics of light. This arrangement allows a variety of negative gains. where ζ is the damping factor. low-pass filter. A sampling ADC digitizes the signal after the analog filter. which is 1. a preamplifier converts the photodiode's current-output signal to a usable voltage level. bandwidth. Generally. Additionally. Bonnie. high-frequency filter sections. There are instances where the Sallen-Key topology is a better choice. In the e2 region. provide a list of the appropriate amplifiers for the related filter. the quality factor. enable the low-pass filter button and type in a filter bandwidth with Ao = 1 V/V. "RMS and peak-to-peak noise trade-off. the software changes the resistors in the circuit to appropriate values. where Q i is the quality factor of the ith partial filter and ki is the ratio of the partial-filter-corner frequency to the overall. "Noise Analysis of FET Transimpedance Amplifiers. You can use a dual digital potentiometer. It is no coincidence that the "flattening" of the filter response occurs at this crossing. Table 1 shows the digital potentiometer program setting for Butterworth filter with corner frequencies ranging from 100 to 10 kHz (C1 = 33 nF. and the feedback factor contributes an approximately 0° phase shift. for Q i of less than 1. this action also creates a stage whose output is not low-impedance. and CF in the amplifier's feedback loop. Photo-sensing circuits have changed over the years. Figure 1: A second-order. the amplifier contributes an approximately –90° phase shift. The simplistic approach in Figure 1 is not without its design challenges. The operational amplifier must have relatively low-picoampere input-bias currents and low input capacitance. The relationship between Q and the damping factor is Q = 1/2 ζ. multiply the broadband noise of the amplifier. and its margin is 90°. Sallen-Key lowpass filters for each amplifier. you use a lowpass filter to prevent aliasing errors from out-of-band noise." EDN. you have learned to quickly produce an adjustable analog filter. You then connect the noninverting input to ground (Figure 1). low-pass unity gain filter. you can realize a combination of Butterworth. linear representation of the light source. system designers understand how to convert light into a current." EDN. rail-to-rail op amp. or perform chromatography. f CUT (or –3-dB frequency).29 µV. and 2. A MFB single-stage (second-order filter) provides two poles while inverting the signal. The final design method is not always intuitively obvious. the circuit will oscillate or ring with a step-function input. capacitances. References 1. The sixth part of the noise equation. fc = 100. where V OUT P-P is the expected peak-to-peak output-voltage swing below your filter's cutoff frequency. high-frequency noise on the analog signal before it reaches the ADC. second-order Butterworth response with a programmable corner frequency range of 1:100. two capacitors. which comprises a photodiode. Although the approximation method does not impact or correct this unexpected behavior. The OPA314 is a singlesupply. Doing so ensures that your filter does not create signal distortions due to slew limitations. Figure 2 shows the frequency response of the 1/β curve and the amplifier's open-loop-gain response: f P=1/(2π(R PD||RF )(CPD+CCM +CDIFF +CF +CRF )). if you would rather use an amplifier other than the suggested device. Then. Texas Instruments . to name a few applications. 8 Back to top Choosing antialiasing-filter amplifiers When you digitize an analog signal. The value required for C1 in an MFB design can be quite low while the designer is trying to use reasonable resistor values. represents the noise contribution of the feedback resistor. In this design. the magnitude of that noise stays the same. Chebyshev) of this second-order. If the circuit has a front-end multiplexer. consider six regions in the frequency spectrum (Figure 1b) and add each region with a root-sum-square equation or the following equation: The first five regions are equal to the multiple of the areas under the closed-loop-gain and amplifier-noise-density curves. Consequently. and the square root of the region's bandwidth. The appropriate Bode plot for this design includes the amplifier's open-loop gain and the 1/β curve. Bessel. These silicon sensors are the "eyes" in the electronics world that users can employ to analyze blood. dual-supply. In Figure 1. Next. Q < 3) At unity-gain. The first approach was purely analog. The noise in regions e4 and e5 uses the higher-frequency gain of the closed-loop-gain curve with the value of C1 being the parallel combination of the input capacitors. Texas Instruments. and the step function's overshoot is 5%. 3. This is one technique to design a programmable anti-aliasing filter. A conservative calculation that allows variation in amplifier bandwidth. When asking how much noise is too much noise in this photodiode-preamp circuit. the flicker-noise region usually ranges from dc to 100 or 1000 Hz. Additionally. and f Z +1/(2π(R F )(CF +CRF )). Sometimes amplifier manufacturers. some characterization is in order. the filter gain begins to increase at a rate of 20 dB/decade. as they worked from the daylight hours into the evening. input-common-mode-voltage range (VCMR) and input bias current (I B) can also affect you. IC designers regularly cover their wafers under test to shield out extraneous light. If you need a higher order filter. The difference in the frequency response at the point at which the three amplifiers change to a positive slope depends on the amplifier's output impedance. consider that a 12-bit system operating with a 5V input range has an LSB of 1. Bishop. β is the system-feedback factor. November 2001. An alternative filter design solution is to have one programmable filter after the multiplexer (Figure 1). Select a second-order Butterworth from the list and click "Open Design. This will produce a second-order Butterworth filter. For instance. return to page one and set the conditions for the next filter. Again. Selecting the correct amplifier. vs. Alternative filters can solve the problem without adding an RC filter. and Z F is the distributed feedback impedance. and closed-loop ac response. The gain of this circuit is 1+R 2 /R1 . In this configuration. (Figure 1) The transfer function of this system is: where A OL (jΩ) is the open-loop gain of the amplifier over frequency. CF ." SBOA060. References 1. This circuit places the photodiode across the amplifier's inverting input and ground of the operational amplifier. V CMR limits the range of your input signal. The slew rate depends on internal IC currents and capacitances. Basically. internal currents charge these internal capacitors. The gain of this circuit is equal to 1V/V. must be at least 100×gain×f CUT ×k i . To further exacerbate the difficulty of the design. step response. 2. Mancini. then press the "update" button. the Sallen-Key topology has excellent gain accuracy. or [CP–R1 ||2C CM ||CDIFF ]. Bessel. J. May 15. such as the charged digitizing ADC. second-order. From the classic transimpedance amplifier. The photo sensor. For Q i greater than 1. a variety of different filters and corner frequency requirements may be required in the circuit prior to the multiplexer. In the input screen. or to change the amplifier to have a different frequency response or different input capacitance. In these systems. Texas Instruments . The LSB for a 16-bit system with the same input-voltage range is 76. Bonnie. but the real challenge is determining how to convert the low-level currents from the photo sensor into a useful electrical representation. The gain-bandwidth product of the operational amplifiers appears in the specification table of the respective productdata sheet. the amplifier's gain is less than 0 dB. A sixth-order filter consisting of three MFB stages inverts the signal three times.38×10–23 . which may cause additional ringing in the time domain. which does not display this reversal in the gain response at higher frequencies and does not swing the input stage's common-mode voltage. Can you think of another way to do this? Send in your ideas and we can discuss them! 11 Back to top Photo-sensing circuits: The eyes of the electronic world are watching Silicon photo sensors have been in electronic circuits since the inception of the era of silicon electronics." The next page displays a list of filter response options that meet your requirements. in turn. In this manner. Further. you can use a multiple-feedback circuit. C2 = 15 nF). 14 Transimpedance-amplifier application: The pulse oximeter http://www. Table 1 summarizes the digital potentiometer programmed settings. Bonnie. The only path of escape for this charge is through the highvalue resistor in the amplifier's feedback loop. You would expect this response from a second-order lowpass filter. and 3. 2008. Eventually. February 1994. analog filter using a dual digital potentiometer.or eighth-order filter. The circuit design uses resistance to provide a real-time. these filters use a Butterworth design. and a single amplifier to configure a low-pass. under the "Filter Topology Specifications" section. The obvious advantage is reduction in chip count. In this design. More than likely. using the transimpedance amplifier and following it with a lowpass filter. the gain-bandwidth product of the amplifier. Consequently. A calculation proves that the contribution to noise in this low-frequency region is relatively low: where RF is the feedback resistor and RPD is the device's parallel resistance. Aug 7. predicts the stability. Baker. In designs where an even number of stages is required. A good tool for determining stability is a Bode plot. The unity-gain Sallen-Key topology also requires fewer components–two resistors versus three for the MFB (Figure 2). the system's phase shift is –90°. Calculate the noise contribution and the e3 region in the same manner with f P=1/[2π(RPD||RF ) (CPD+CCM +CDIFF +CF +CRF )] and f Z =1/[2π(RF ) (CF +CRF )]. baby. input capacitance. second-order Sallen-Key stage also provides two poles but is a non-inverting circuit. As a rule of thumb. An alternative to this tedious design exercise is to determine the capacitor and resistor values using TI's WEBENCH Filter Designer software. the output polarity is the same as the input signal. Also. A single. This program provides the actual amplifier suggestion along with options. With the MFB topology. and Asb = –35 dB. V CM =V +/2. it is possible to program second-order Bessel or Chebyshev filters with a programmable corner frequency range of 1:100. the gain is independent of the resistors in the circuit. unity gain. the closed-loop dc-noise gain (1+R F /RPD)." Application Note (SBFA001A). When this capacitor seed is set. 2008. ISBN-0-7506-7701-5. single-supply filter.Texas Instruments . however. Figure 3: Sallen-Key second-order. RF is the feedback resistor in ohms.    gain accuracy is important. This inversion may or may not be a concern in the application circuit. In Figure 1. the contributed noise in this region is usually relatively low because of its location in the lower frequency range. You should also evaluate the effects of amplifier slew rate. When you send large signals through the amplifier. The gain of this circuit is equal to −R2 /R1 . The higher the Q. and currents. You can use filterdesign programs to determine the filter's capacitor and resistor values." EDN. Although the light sensitivity of silicon is an undesirable byproduct of the silicon. T is temperature in Kelvin. The dual TPL0102-100. This is because the op amp is in a unity-gain buffer configuration with a high open-loop gain. operational amplifier. The voltage drop that the input-bias-current error causes appears as an additional input-offset voltage.com/ww/en/analog/Amplifiers-eBook/[1/14/14 10:58:49 AM] Back to top . the MFB topology allows low sensitivity to component tolerances. Op Amps for Everyone. The capacitors are hard-wired in. By adding the 1/β phase shift from the A OL (jΩ) phase shift. The signal after the transimpedance amplifier requires a multipole analog filter. where V CM =V +/2. The caveat to this action is that the following filter may interfere with the phase response of your intended filter. One way to correct circuit instability is to add a feedback capacitor. particularly if you have differential input/output stages. and these parts' parasitics combine to create quite a rat's nest of formulas for consideration. or [CP–R1 ||2C CM ||CDIFF ]. Z IN is the distributed input impedance. References 1. the software changes the resistors in the circuit to appropriate values. Before selecting the amplifier. Doing so attenuates superimposed. predicts the stability. and layout of this transimpedance-amplifier circuit. Aug 7.or a CMOS-input stage with lowvoltage noise and microvolt-offset specifications. Aug 7. two capacitors. where ζ is the damping factor. The relationship between Q and the damping factor is Q = 1/2 ζ. a preamplifier converts the photodiode's current-output signal to a usable voltage level. the input bias current conducts through the external source resistance. the Sallen-Key topology is better if: 1. The circuit design uses resistance to provide a real-time. The speed of this charging process depends on the amplifier's internal resistances. These filters use independent operational amplifiers (op amps) in combination with fixed resistors and capacitors.TI. Figure 1 shows the front-end circuit of this system. C2 = 15 nF). from multiple op amps to one single amplifier. The traditional design topology of the transimpedance amplifier captures this low-level signal in a hybrid approach that starts with an amplifier and a high-value resistor in the feedback loop. You cannot use a digital filter to reduce in-band noise after digitizing the signal. Filter Designer also provides SPICE simulations of the chosen filter's sine wave response. fs = 1000. J. You can reduce the impact of the upward trend in the filter's response by following the offending active filter with a passive RC lowpass filter. the closed-loop dc-noise gain (1+R F /RPD). f CUT (or –3-dB frequency). for Q i of less than 1.    a low Q is required (for example. input-common-mode-voltage range (VCMR) and input bias current (I B) can also affect you. presents an interesting challenge.29 µV. β is the system-feedback factor. The OPA314 is a singlesupply. f AMP. an operational amplifier." Application Note (SBFA001A). The Sallen-Key topology may be preferable for low-Q. For instance. you use a lowpass filter to prevent aliasing errors from out-of-band noise. the switched integrator has gained favor. the contributed noise in this region is usually relatively low because of its location in the lower frequency range. the output polarity is the same as the input signal. Table 1 summarizes the digital potentiometer programmed settings. ISBN-0-7506-7701-5. If the circuit has a front-end multiplexer. When you send large signals through the amplifier. References 1. flicker-noise (1/f). The signal after the transimpedance amplifier requires a multipole analog filter. must be at least 100×gain×f CUT ×k i . From the classic transimpedance amplifier. capacitances. This program provides the actual amplifier suggestion along with options." SBOA060. to name a few applications. where A N is the amplifier's input-noise-density at 1 Hz and f B is the corner frequency where the flicker noise tapers off. It is no coincidence that the "flattening" of the filter response occurs at this crossing. The value required for C1 in an MFB design can be quite low while the designer is trying to use reasonable resistor values. Alternative filters can solve the problem without adding an RC filter. After the cutoff frequency. Typically this type of filter is used to reduce analog-to-digital converter (ADC) aliasing errors and noise outside the signal bandwidth. The operational amplifier must have relatively low-picoampere input-bias currents and low input capacitance. B Trump. If you need a higher order filter. To ensure that your active filter does not enter into a slew condition. the flicker-noise region usually ranges from dc to 100 or 1000 Hz. equaling 1/(1+Z F /ZIN).22 mV. Bonnie. you need to select an amplifier such that the slew rate (πV OUT P-Pf CUT ). and the feedback factor contributes an approximately 0° phase shift. bandwidth. internal currents charge these internal capacitors. "RMS and peak-to-peak noise trade-off. for these circuits. system designers have exploited this transfer of light into electrical energy in various systems. or to change the amplifier to have a different frequency response or different input capacitance. f AMP=100×gain×(f CUT /a i )× filter-transfer function. the R2 /R1 resistor ratio and the resistor errors determine the gain. and the values in this column are root-mean-square values. Here. By adding the 1/β phase shift from the A OL (jΩ) phase shift. 9 Back to top Closer to real-world analog filters There are two basic filter topologies that everyone recommends when they start to design their signal chain: multiple-feedback (MFB) or Sallen-Key topologies. In this view. The unity-gain Sallen-Key topology also requires fewer components–two resistors versus three for the MFB (Figure 2). Can you think of another way to do this? Send in your ideas and we can discuss them! 11 Back to top Photo-sensing circuits: The eyes of the electronic world are watching Silicon photo sensors have been in electronic circuits since the inception of the era of silicon electronics. Selecting the correct amplifier. May 15. Texas Instruments ." EDN.Texas Instruments . The caveat to this action is that the following filter may interfere with the phase response of your intended filter. two capacitors. One way to correct circuit instability is to add a feedback capacitor. A signal path requires this type of dedicated filter to match the signal's requirements. using the transimpedance amplifier and following it with a lowpass filter. The switched integrator was the first step toward bringing the digital portion of the circuit closer to the signal source. you need to determine the filter-cutoff frequency. In the e2 region. The amplifier bandwidth calculation involves variations of the amplifier's bandwidth and variations of the passive components over time. "Noise Analysis of FET Transimpedance Amplifiers. 2. and these parts' parasitics combine to create quite a rat's nest of formulas for consideration. A resistor with a value of 100 kΩ to 10 MΩ connects the inverting input of the amplifier to the output. Although the light sensitivity of silicon is an undesirable byproduct of the silicon. low-pass filter. Next. and feedback-resistor value places the system's pole of 1/β at half the frequency where the two curves intersect: where f GBW is the gain-bandwidth product of the amplifier. Figure 1: A second-order. however. The data indicates that the lowpass filters are performing as you would expect for a little more than a decade after the cutoff frequency of 1 kHz. The gain of this circuit is equal to 1V/V. as they worked from the daylight hours into the evening. The only path of escape for this charge is through the highvalue resistor in the amplifier's feedback loop. you can realize a combination of Butterworth. some characterization is in order. combining the input and filtering stages separates the signal of interest from the noise floor. but the frequency changes as it aliases back onto your signal of interest. Figure 3: Sallen-Key second-order. the cost is lower when a single filter serves many analog inputs. which does not display this reversal in the gain response at higher frequencies and does not swing the input stage's common-mode voltage. "Start Filter Design. Consequently.    a unity-gain filter is needed. You can calculate the appropriate resistance and capacitance with a little research. The slew rate depends on internal IC currents and capacitances. such as a fourth. consider six regions in the frequency spectrum (Figure 1b) and add each region with a root-sum-square equation or the following equation: The first five regions are equal to the multiple of the areas under the closed-loop-gain and amplifier-noise-density curves. equaling RF ||jΩ(C RF +CF ). the magnitude of that noise stays the same. second-order Sallen-Key stage also provides two poles but is a non-inverting circuit. all three of the filter's responses show a slope of –40 dB/decade. Baker. When asking how much noise is too much noise in this photodiode-preamp circuit. input capacitance. 10 Back to top Simply an adjustable low-pass filter A low-pass filter is the most common filter found in data acquisition systems. and 2. Texas Instruments. the circuit will oscillate or ring with a step-function input.38×10–23 . and a single amplifier to configure a low-pass. 3. the gain is independent of the resistors in the circuit. Light excitement on the photo sensor generates charge. Bessel." The Filter Designer Design Summary view (page 3) allows the user to adjust the capacitor seed value to the desired value of C1 and C2 on the left side of the screen. Chebyshev) of this second-order.com A second set of three curves in Figure 1 shows the frequency response of second-order. Then press the green button. equaling RPD||jΩ(C PD+CCM +CDIFF ). as well as RF . With the MFB topology. dual-supply. where V CM =V +/2. under the "Filter Topology Specifications" section. it is possible to have a variety of signals that reach the ADC where each signal source has its own set of filter requirements. you can cascade both of these topologies. The closure rate between the two curves suggests the system's phase margin and. Bonnie. The appropriate Bode plot for this design includes the amplifier's open-loop gain and the 1/β curve. and one operational amplifier reduces chip count. Texas Instruments . Sometimes amplifier manufacturers. The filter implementation requires two resistors and two capacitors. As a rule of thumb. In designs where an even number of stages is required. More than likely. the quality factor. and Z F is the distributed feedback impedance. which may cause additional ringing in the time domain. The gain-bandwidth product of the operational amplifiers appears in the specification table of the respective productdata sheet. you can also select the supply requirements of Single Supply = +5 V. the closure rate of the two curves is 20 dB/decade. A calculation proves that the contribution to noise in this low-frequency region is relatively low: where RF is the feedback resistor and RPD is the device's parallel resistance. search noninvasively for tumors. Table 1 shows the digital potentiometer program setting for Butterworth filter with corner frequencies ranging from 100 to 10 kHz (C1 = 33 nF. indicating a phase shift of –180° and a phase margin of 0°. the more easily the circuit oscillates. With this circuit. But what are the differences and why would you choose one over the other? The MFB topology (sometimes called infinite gain or Rauch) is often preferred because it has low sensitivity to amplifier variations. and Chebyshev filters with the same circuit using a 1:10 corner frequency range. V CM =V +/2.or eighth-order filter. which comprises a photodiode. This arrangement allows a variety of negative gains. An alternative to this tedious design exercise is to determine the capacitor and resistor values using TI's WEBENCH Filter Designer software. the MFB topology allows low sensitivity to component tolerances. It is not simply a matter of picking an amplifier with a gain-bandwidth-product that is 100 times higher than the filter's corner frequency. References 1. resulting in a stable system. Op Amps for Everyone. second-order Butterworth response with a programmable corner frequency range of 1:100. a variety of different filters and corner frequency requirements may be required in the circuit prior to the multiplexer. November 2001. Figure 1: MFB second-order single-supply low-pass filter. a 100 kΩ 8-bit digital potentiometer. In the input screen. The amplifier does not gain the contribution of noise from the feedback resistor: where K is Boltzmann's constant. provide a list of the appropriate amplifiers for the related filter. where ai is the ith coefficient in the partialfilter-corner frequency. lowpass filters are the noninverting Sallen-Key and the inverting multiple feedback. 8 Back to top Choosing antialiasing-filter amplifiers When you digitize an analog signal. The most common topologies for active. This is one technique to design a programmable anti-aliasing filter. Sallen-Key lowpass filters for each amplifier. or perform chromatography. system designers understand how to convert light into a current." EDN. For many CMOS or FET amplifiers. RF is the feedback resistor in ohms. such as the open-loop gain or input range (Figure 1). A single. Eventually. the designer optimizes the stability. CRF . and a feedback network. System elements determining the noise-gain frequency response are the photodiode's parasitics and the operational amplifier's input capacitance. If you use a Sallen-Key lowpass filter. This inversion may or may not be a concern in the application circuit. multiply the broadband noise of the amplifier. The area under the noise-density curve in the e1 . When using the Sallen-Key circuit.com/ww/en/analog/Amplifiers-eBook/[1/14/14 10:58:49 AM] Back to top . which is 1. A good tool for determining stability is a Bode plot. Baker. this action also creates a stage whose output is not low-impedance. where V OUT P-P is the expected peak-to-peak output-voltage swing below your filter's cutoff frequency. enable the low-pass filter button and type in a filter bandwidth with Ao = 1 V/V. 2008. Photo-sensing circuits have changed over the years. its closed-loop output resistance increases. You can use filterdesign programs to determine the filter's capacitor and resistor values. April 2003. the amplifier's gain is less than 0 dB. 14 Transimpedance-amplifier application: The pulse oximeter http://www. The final design method is not always intuitively obvious. at some point. high-frequency filter sections. The gain of this circuit is equal to −R2 /R1 . When calculating the noise performance of the circuit. the Sallen-Key topology has excellent gain accuracy. Also. 13 Back to top Transimpedance-amplifier noise issues How much noise is too much noise in a photodiode-preamplifier circuit? You can derive the noise performance of a transimpedance amplifier (Figure 1a) with calculations or by using a Spice simulation.ti. 12 Back to top Transimpedance-amplifier stability is key in light-sensing applications A variety of precision applications sense light and convert that information into a useful digital word. Bessel. and the step function's overshoot is 5%. These silicon sensors are the "eyes" in the electronics world that users can employ to analyze blood. "Photo-sensing circuits: The eyes of the electronic world are watching. Bonnie. The sixth part of the noise equation. single-supply filter. Figure 2: A Sallen-Key second-order. February 1994. In this design. A sampling ADC digitizes the signal after the analog filter. The dual TPL0102-100. The higher the Q. the filter gain begins to increase at a rate of 20 dB/decade. In the end. If the closure rate of these two curves is 40 dB/decade. unity gain. and RM Stitt. particularly if you have differential input/output stages. such as Texas Instruments' WEBENCH Filter Designer Web software. When an inverting filter is an acceptable alternative. Again. The noise in regions e4 and e5 uses the higher-frequency gain of the closed-loop-gain curve with the value of C1 being the parallel combination of the input capacitors. When this capacitor seed is set. but the real challenge is determining how to convert the low-level currents from the photo sensor into a useful electrical representation. detect smoke. Additionally. The migration of the photo-sensing-application product has moved on to totally integrated systems. In this design. you should initially consider only two important specifications when selecting an amplifier for your active lowpass filter: gain-bandwidth product and slew rate. This may be preferable over a MFB single stage. You would expect this response from a second-order lowpass filter. and a good eraser. return to page one and set the conditions for the next filter. Subsequently. This will produce a second-order Butterworth filter. CF . the gain-bandwidth product of the amplifier. be aware that this circuit has high-frequency feedthrough. fc = 100. The WEBENCH Filter Designer program seems to have broken through some filter design barriers. each filter's response flattens at the 0-dB crossing frequency of the op amp's open-loop gain. At the system's front end. low-pass unity gain filter. linear representation of the light source. The simplistic approach in Figure 1 is not without its design challenges. A MFB single-stage (second-order filter) provides two poles while inverting the signal. and Asb = –35 dB. (Figure 1) The transfer function of this system is: where A OL (jΩ) is the open-loop gain of the amplifier over frequency. then press the "update" button. You should also evaluate the effects of amplifier slew rate. As the frequency increases beyond this point. Ron. the gain circuit is negative and equal to the resistor ratio of R2 and R1 . where CPD is the device's capacitance and CDIFF is the differential amplifier's capacitance.    gain accuracy is important. Calculate the noise contribution and the e3 region in the same manner with f P=1/[2π(RPD||RF ) (CPD+CCM +CDIFF +CF +CRF )] and f Z =1/[2π(RF ) (CF +CRF )]. The capacitors are hard-wired in. and C2 is the parallel combination of CF and CRF . A conservative calculation that allows variation in amplifier bandwidth. As you do this make sure that fs is ten times higher than fc. Your circuit also may require a low Q. a sharp pencil. Figure 1 shows the details of a single-supply. e6 . IC designers regularly cover their wafers under test to shield out extraneous light. and currents. the system's phase shift is –90°. In these systems. Basically. the required accuracy in these applications continues to increase. the amplifier contributes an approximately –90° phase shift. To further exacerbate the difficulty of the design. Although the approximation method does not impact or correct this unexpected behavior. Elsevier-Newnes. vs. The caveat is that low capacitor values can result in significant errors due to parasitic capacitance and the input capacitance of the amplifier. Baker. particularly at the 3 dB corner frequency. An alternative filter design solution is to have one programmable filter after the multiplexer (Figure 1). An appropriate amplifier for this circuit would have a FET. high-frequency noise on the analog signal before it reaches the ADC. you have learned to quickly produce an adjustable analog filter. replaces the two resistors in this circuit. "Photo-sensing circuits: The eyes of the electronic world are watching. Figure 2 shows the frequency response of the 1/β curve and the amplifier's open-loop-gain response: f P=1/(2π(R PD||RF )(CPD+CCM +CDIFF +CF +CRF )). low-noise performance. position equipment. The LSB for a 16-bit system with the same input-voltage range is 76. 2. a silicon sensor converts light into charge or an electrical current. This is because the op amp is in a unity-gain buffer configuration with a high open-loop gain. region is V 1/f:fB–fA =A N . Additionally. amplifier-feedback element. second-order programmable low-pass Sallen-key filter. A reprogramming of the dual digital potentiometer in Figure 1 changes the filter's frequency cut-off and approximation (Butterworth. the system's phase margin is 65°. in turn. to mention a few. To produce the remaining filters in Table 1. The obvious advantage is reduction in chip count. The A OL (jΩ) curve intersects the 1/β curve at an interesting point. Both LSBs are peak-to-peak numbers. step response. References 1. and BW is the bandwidth of interest. such as the charged digitizing ADC. WEBENCH Filter Designer program. Then. baby. Change to Capacitor seed value to 15 e-9 or 15 nF. if you would rather use an amplifier other than the suggested device. a wide variety of applications use silicon to sense the intensity and characteristics of light. Select a second-order Butterworth from the list and click "Open Design. In this configuration. and f Z +1/(2π(R F )(CF +CRF )). Selecting the correct operational amplifier for an active lowpass-filter circuit can appear overwhelming as you read an amplifier's data sheet and view all of the specifications. The photo sensor. A sixth-order filter consisting of three MFB stages inverts the signal three times. consider that a 12-bit system operating with a 5V input range has an LSB of 1. and 3. these filters use a Butterworth design. The MFB topology creates a second-order filter." EDN. it is possible to program second-order Bessel or Chebyshev filters with a programmable corner frequency range of 1:100. The difference in the frequency response at the point at which the three amplifiers change to a positive slope depends on the amplifier's output impedance. Generally. scientists quickly discovered the photo-sensing characteristics of silicon in the lab. represents the noise contribution of the feedback resistor. All I can say is filter design has come a long way. and closed-loop ac response. The gain of this circuit is 1+R 2 /R1 . second-order. 2008. Q < 3) At unity-gain. The voltage drop that the input-bias-current error causes appears as an additional input-offset voltage. Bishop. 2008. There are instances where the Sallen-Key topology is a better choice. V CMR limits the range of your input signal. operational amplifier. Mancini. T is temperature in Kelvin. you can use a multiple-feedback circuit. Doing so ensures that your filter does not create signal distortions due to slew limitations. but this is not the only potential advantage. This circuit places the photodiode across the amplifier's inverting input and ground of the operational amplifier. You then connect the noninverting input to ground (Figure 1). Consequently. and CF in the amplifier's feedback loop. and its margin is 90°. In Figure 1. "FilterPro MFB and Sallen-Key Low-Pass Filter Design Program. Further. and the square root of the region's bandwidth. As the amplifier's open-loop gain decreases. If the noise on the input signal is more than half the sampling frequency of the converter. In this manner. For Q i greater than 1. where Q i is the quality factor of the ith partial filter and ki is the ratio of the partial-filter-corner frequency to the overall. resulting in an inverted output after the third stage." The next page displays a list of filter response options that meet your requirements. analog filter using a dual digital potentiometer. In Figure 1. To this day. rail-to-rail op amp. Consequently. You can use a dual digital potentiometer. The first approach was purely analog. Consequently. particularly if you have differential input/output stages. With the MFB topology. low-pass unity gain filter. "RMS and peak-to-peak noise trade-off. Consequently. The most common topologies for active. and a single amplifier to configure a low-pass. References 1. using the transimpedance amplifier and following it with a lowpass filter. WEBENCH Filter Designer program. Photo-sensing circuits have changed over the years. enable the low-pass filter button and type in a filter bandwidth with Ao = 1 V/V. Figure 1: A second-order. A single. and RM Stitt." EDN. Doing so attenuates superimposed. replaces the two resistors in this circuit. which does not display this reversal in the gain response at higher frequencies and does not swing the input stage's common-mode voltage. In this view. and the values in this column are root-mean-square values. The traditional design topology of the transimpedance amplifier captures this low-level signal in a hybrid approach that starts with an amplifier and a high-value resistor in the feedback loop. however. The amplifier bandwidth calculation involves variations of the amplifier's bandwidth and variations of the passive components over time. Doing so ensures that your filter does not create signal distortions due to slew limitations. f CUT (or –3-dB frequency). The gain of this circuit is equal to 1V/V.com A second set of three curves in Figure 1 shows the frequency response of second-order. It is no coincidence that the "flattening" of the filter response occurs at this crossing. as well as RF . An alternative to this tedious design exercise is to determine the capacitor and resistor values using TI's WEBENCH Filter Designer software. The amplifier does not gain the contribution of noise from the feedback resistor: where K is Boltzmann's constant. You cannot use a digital filter to reduce in-band noise after digitizing the signal. equaling 1/(1+Z F /ZIN). the amplifier contributes an approximately –90° phase shift. must be at least 100×gain×f CUT ×k i . single-supply filter. the Sallen-Key topology is better if: 1. References 1. The dual TPL0102-100. Eventually. The filter implementation requires two resistors and two capacitors. With this circuit. and feedback-resistor value places the system's pole of 1/β at half the frequency where the two curves intersect: where f GBW is the gain-bandwidth product of the amplifier. From the classic transimpedance amplifier. The higher the Q. It is not simply a matter of picking an amplifier with a gain-bandwidth-product that is 100 times higher than the filter's corner frequency. and 3. Selecting the correct amplifier.29 µV. The first approach was purely analog. The unity-gain Sallen-Key topology also requires fewer components–two resistors versus three for the MFB (Figure 2). Table 1 summarizes the digital potentiometer programmed settings. or [CP–R1 ||2C CM ||CDIFF ]. The migration of the photo-sensing-application product has moved on to totally integrated systems. you need to select an amplifier such that the slew rate (πV OUT P-Pf CUT ). a sharp pencil. "Photo-sensing circuits: The eyes of the electronic world are watching.com/ww/en/analog/Amplifiers-eBook/[1/14/14 10:58:49 AM] Back to top . and BW is the bandwidth of interest. You can calculate the appropriate resistance and capacitance with a little research. A signal path requires this type of dedicated filter to match the signal's requirements. the cost is lower when a single filter serves many analog inputs. input capacitance. the filter gain begins to increase at a rate of 20 dB/decade. Your circuit also may require a low Q." Application Note (SBFA001A). predicts the stability. The obvious advantage is reduction in chip count. A good tool for determining stability is a Bode plot. system designers have exploited this transfer of light into electrical energy in various systems. which comprises a photodiode. where V CM =V +/2. Further. Select a second-order Butterworth from the list and click "Open Design.    a unity-gain filter is needed. The area under the noise-density curve in the e1 . Bishop. By adding the 1/β phase shift from the A OL (jΩ) phase shift. As a rule of thumb. More than likely. This will produce a second-order Butterworth filter. the designer optimizes the stability. Figure 3: Sallen-Key second-order. provide a list of the appropriate amplifiers for the related filter. Figure 1 shows the front-end circuit of this system. Change to Capacitor seed value to 15 e-9 or 15 nF. an operational amplifier. detect smoke. The Sallen-Key topology may be preferable for low-Q. References 1. ISBN-0-7506-7701-5. Aug 7. 12 Back to top Transimpedance-amplifier stability is key in light-sensing applications A variety of precision applications sense light and convert that information into a useful digital word. equaling RF ||jΩ(C RF +CF ). and Chebyshev filters with the same circuit using a 1:10 corner frequency range. All I can say is filter design has come a long way. be aware that this circuit has high-frequency feedthrough. 2008. In designs where an even number of stages is required. To further exacerbate the difficulty of the design. where V OUT P-P is the expected peak-to-peak output-voltage swing below your filter's cutoff frequency. you should initially consider only two important specifications when selecting an amplifier for your active lowpass filter: gain-bandwidth product and slew rate." SBOA060. As the frequency increases beyond this point. or perform chromatography. These silicon sensors are the "eyes" in the electronics world that users can employ to analyze blood. RF is the feedback resistor in ohms. You then connect the noninverting input to ground (Figure 1). the quality factor. low-noise performance. and the step function's overshoot is 5%. fs = 1000. each filter's response flattens at the 0-dB crossing frequency of the op amp's open-loop gain. CF . When an inverting filter is an acceptable alternative.38×10–23 . The caveat is that low capacitor values can result in significant errors due to parasitic capacitance and the input capacitance of the amplifier. Light excitement on the photo sensor generates charge. particularly at the 3 dB corner frequency. flicker-noise (1/f). As the amplifier's open-loop gain decreases. Baker. Figure 2 shows the frequency response of the 1/β curve and the amplifier's open-loop-gain response: f P=1/(2π(R PD||RF )(CPD+CCM +CDIFF +CF +CRF )). IC designers regularly cover their wafers under test to shield out extraneous light. In this design. a 100 kΩ 8-bit digital potentiometer. and C2 is the parallel combination of CF and CRF . To ensure that your active filter does not enter into a slew condition. The LSB for a 16-bit system with the same input-voltage range is 76. where A N is the amplifier's input-noise-density at 1 Hz and f B is the corner frequency where the flicker noise tapers off. Z IN is the distributed input impedance. The value required for C1 in an MFB design can be quite low while the designer is trying to use reasonable resistor values. a silicon sensor converts light into charge or an electrical current. amplifier-feedback element. such as the open-loop gain or input range (Figure 1). at some point. Baker. You would expect this response from a second-order lowpass filter. for Q i of less than 1. f AMP=100×gain×(f CUT /a i )× filter-transfer function.    gain accuracy is important. Texas Instruments. the gain circuit is negative and equal to the resistor ratio of R2 and R1 . You can use a dual digital potentiometer. May 15. for these circuits. CRF . analog filter using a dual digital potentiometer. Although the approximation method does not impact or correct this unexpected behavior. The operational amplifier must have relatively low-picoampere input-bias currents and low input capacitance. Selecting the correct operational amplifier for an active lowpass-filter circuit can appear overwhelming as you read an amplifier's data sheet and view all of the specifications. Subsequently. In the input screen. and its margin is 90°. the output polarity is the same as the input signal. dual-supply. Q < 3) At unity-gain. second-order. low-pass filter. A MFB single-stage (second-order filter) provides two poles while inverting the signal. For Q i greater than 1. you can cascade both of these topologies. high-frequency noise on the analog signal before it reaches the ADC. 9 Back to top Closer to real-world analog filters There are two basic filter topologies that everyone recommends when they start to design their signal chain: multiple-feedback (MFB) or Sallen-Key topologies. 13 Back to top Transimpedance-amplifier noise issues How much noise is too much noise in a photodiode-preamplifier circuit? You can derive the noise performance of a transimpedance amplifier (Figure 1a) with calculations or by using a Spice simulation. "Noise Analysis of FET Transimpedance Amplifiers. a preamplifier converts the photodiode's current-output signal to a usable voltage level. The final design method is not always intuitively obvious. Both LSBs are peak-to-peak numbers. The noise in regions e4 and e5 uses the higher-frequency gain of the closed-loop-gain curve with the value of C1 being the parallel combination of the input capacitors. The photo sensor. Texas Instruments . In these systems. The appropriate Bode plot for this design includes the amplifier's open-loop gain and the 1/β curve. B Trump. Figure 1: MFB second-order single-supply low-pass filter. This is one technique to design a programmable anti-aliasing filter. Also. lowpass filters are the noninverting Sallen-Key and the inverting multiple feedback. In Figure 1. you need to determine the filter-cutoff frequency. the gain-bandwidth product of the amplifier. T is temperature in Kelvin. Baker. Although the light sensitivity of silicon is an undesirable byproduct of the silicon. the flicker-noise region usually ranges from dc to 100 or 1000 Hz. In Figure 1." The Filter Designer Design Summary view (page 3) allows the user to adjust the capacitor seed value to the desired value of C1 and C2 on the left side of the screen. and a feedback network. such as Texas Instruments' WEBENCH Filter Designer Web software. the more easily the circuit oscillates. the software changes the resistors in the circuit to appropriate values. 8 Back to top Choosing antialiasing-filter amplifiers When you digitize an analog signal. consider six regions in the frequency spectrum (Figure 1b) and add each region with a root-sum-square equation or the following equation: The first five regions are equal to the multiple of the areas under the closed-loop-gain and amplifier-noise-density curves. consider that a 12-bit system operating with a 5V input range has an LSB of 1. you can realize a combination of Butterworth. The gain-bandwidth product of the operational amplifiers appears in the specification table of the respective productdata sheet. Aug 7. Sallen-Key lowpass filters for each amplifier. these filters use a Butterworth design. One way to correct circuit instability is to add a feedback capacitor. An appropriate amplifier for this circuit would have a FET. the magnitude of that noise stays the same. November 2001. the gain is independent of the resistors in the circuit. Elsevier-Newnes. After the cutoff frequency. Before selecting the amplifier. its closed-loop output resistance increases. (Figure 1) The transfer function of this system is: where A OL (jΩ) is the open-loop gain of the amplifier over frequency. Mancini. the closure rate of the two curves is 20 dB/decade. Ron. Typically this type of filter is used to reduce analog-to-digital converter (ADC) aliasing errors and noise outside the signal bandwidth. unity gain. Sometimes amplifier manufacturers. 2. In this configuration. you can also select the supply requirements of Single Supply = +5 V. and Asb = –35 dB. to name a few applications. second-order Sallen-Key stage also provides two poles but is a non-inverting circuit. and closed-loop ac response. the contributed noise in this region is usually relatively low because of its location in the lower frequency range. Bessel. 2008.    a low Q is required (for example. "Photo-sensing circuits: The eyes of the electronic world are watching. the system's phase shift is –90°.22 mV. As you do this make sure that fs is ten times higher than fc. Then. vs. This program provides the actual amplifier suggestion along with options. A conservative calculation that allows variation in amplifier bandwidth. The circuit design uses resistance to provide a real-time. Additionally. The MFB topology creates a second-order filter. J. If the noise on the input signal is more than half the sampling frequency of the converter. 14 Transimpedance-amplifier application: The pulse oximeter http://www. the MFB topology allows low sensitivity to component tolerances. which is 1. and CF in the amplifier's feedback loop. You should also evaluate the effects of amplifier slew rate. two capacitors. In the end. Texas Instruments . The caveat to this action is that the following filter may interfere with the phase response of your intended filter.Texas Instruments . you use a lowpass filter to prevent aliasing errors from out-of-band noise. and these parts' parasitics combine to create quite a rat's nest of formulas for consideration." EDN. When you send large signals through the amplifier. C2 = 15 nF). When asking how much noise is too much noise in this photodiode-preamp circuit. the Sallen-Key topology has excellent gain accuracy. position equipment. Chebyshev) of this second-order. System elements determining the noise-gain frequency response are the photodiode's parasitics and the operational amplifier's input capacitance. such as the charged digitizing ADC. Generally. Filter Designer also provides SPICE simulations of the chosen filter's sine wave response. a wide variety of applications use silicon to sense the intensity and characteristics of light. This inversion may or may not be a concern in the application circuit. the closed-loop dc-noise gain (1+R F /RPD). "FilterPro MFB and Sallen-Key Low-Pass Filter Design Program. You can use filterdesign programs to determine the filter's capacitor and resistor values. 2. The OPA314 is a singlesupply. two capacitors. and a good eraser. If you use a Sallen-Key lowpass filter. The relationship between Q and the damping factor is Q = 1/2 ζ. a variety of different filters and corner frequency requirements may be required in the circuit prior to the multiplexer.TI. and the square root of the region's bandwidth. This may be preferable over a MFB single stage. The simplistic approach in Figure 1 is not without its design challenges. multiply the broadband noise of the amplifier. β is the system-feedback factor. February 1994. These filters use independent operational amplifiers (op amps) in combination with fixed resistors and capacitors. it is possible to have a variety of signals that reach the ADC where each signal source has its own set of filter requirements. represents the noise contribution of the feedback resistor. The gain of this circuit is 1+R 2 /R1 . The gain of this circuit is equal to −R2 /R1 . Figure 2: A Sallen-Key second-order. and f Z +1/(2π(R F )(CF +CRF )). This arrangement allows a variety of negative gains. but the frequency changes as it aliases back onto your signal of interest. you can use a multiple-feedback circuit. 10 Back to top Simply an adjustable low-pass filter A low-pass filter is the most common filter found in data acquisition systems. and layout of this transimpedance-amplifier circuit. A sampling ADC digitizes the signal after the analog filter." EDN. presents an interesting challenge. equaling RPD||jΩ(C PD+CCM +CDIFF ).or eighth-order filter. and Z F is the distributed feedback impedance. bandwidth. Op Amps for Everyone. resulting in an inverted output after the third stage. The data indicates that the lowpass filters are performing as you would expect for a little more than a decade after the cutoff frequency of 1 kHz. The only path of escape for this charge is through the highvalue resistor in the amplifier's feedback loop. Consequently. such as a fourth. Alternative filters can solve the problem without adding an RC filter. To this day. A resistor with a value of 100 kΩ to 10 MΩ connects the inverting input of the amplifier to the output. system designers understand how to convert light into a current. second-order Butterworth response with a programmable corner frequency range of 1:100. Then press the green button. which may cause additional ringing in the time domain. and the feedback factor contributes an approximately 0° phase shift. Bessel. scientists quickly discovered the photo-sensing characteristics of silicon in the lab. The difference in the frequency response at the point at which the three amplifiers change to a positive slope depends on the amplifier's output impedance. Here. input-common-mode-voltage range (VCMR) and input bias current (I B) can also affect you. The capacitors are hard-wired in. Can you think of another way to do this? Send in your ideas and we can discuss them! 11 Back to top Photo-sensing circuits: The eyes of the electronic world are watching Silicon photo sensors have been in electronic circuits since the inception of the era of silicon electronics. search noninvasively for tumors. this action also creates a stage whose output is not low-impedance. where ζ is the damping factor. Bonnie. in turn. Figure 1 shows the details of a single-supply. The voltage drop that the input-bias-current error causes appears as an additional input-offset voltage. Basically. combining the input and filtering stages separates the signal of interest from the noise floor. You can reduce the impact of the upward trend in the filter's response by following the offending active filter with a passive RC lowpass filter. This is because the op amp is in a unity-gain buffer configuration with a high open-loop gain. Table 1 shows the digital potentiometer program setting for Butterworth filter with corner frequencies ranging from 100 to 10 kHz (C1 = 33 nF. and currents. the system's phase margin is 65°.or a CMOS-input stage with lowvoltage noise and microvolt-offset specifications. internal currents charge these internal capacitors. Bonnie. Calculate the noise contribution and the e3 region in the same manner with f P=1/[2π(RPD||RF ) (CPD+CCM +CDIFF +CF +CRF )] and f Z =1/[2π(RF ) (CF +CRF )]. the R2 /R1 resistor ratio and the resistor errors determine the gain. Again. some characterization is in order. the required accuracy in these applications continues to increase. fc = 100. then press the "update" button. second-order programmable low-pass Sallen-key filter. from multiple op amps to one single amplifier. step response. as they worked from the daylight hours into the evening. In this manner. This circuit places the photodiode across the amplifier's inverting input and ground of the operational amplifier. the circuit will oscillate or ring with a step-function input. and one operational amplifier reduces chip count. but this is not the only potential advantage. e6 . but the real challenge is determining how to convert the low-level currents from the photo sensor into a useful electrical representation. the switched integrator has gained favor. When using the Sallen-Key circuit. where Q i is the quality factor of the ith partial filter and ki is the ratio of the partial-filter-corner frequency to the overall. At the system's front end.ti. The A OL (jΩ) curve intersects the 1/β curve at an interesting point. under the "Filter Topology Specifications" section. return to page one and set the conditions for the next filter. When this capacitor seed is set. The switched integrator was the first step toward bringing the digital portion of the circuit closer to the signal source. The slew rate depends on internal IC currents and capacitances. The signal after the transimpedance amplifier requires a multipole analog filter. to mention a few. If the closure rate of these two curves is 40 dB/decade. high-frequency filter sections. If the circuit has a front-end multiplexer. the input bias current conducts through the external source resistance. April 2003. indicating a phase shift of –180° and a phase margin of 0°. To produce the remaining filters in Table 1. A reprogramming of the dual digital potentiometer in Figure 1 changes the filter's frequency cut-off and approximation (Butterworth. where CPD is the device's capacitance and CDIFF is the differential amplifier's capacitance. The sixth part of the noise equation. There are instances where the Sallen-Key topology is a better choice. A sixth-order filter consisting of three MFB stages inverts the signal three times. When calculating the noise performance of the circuit. linear representation of the light source. Bonnie. "Start Filter Design. But what are the differences and why would you choose one over the other? The MFB topology (sometimes called infinite gain or Rauch) is often preferred because it has low sensitivity to amplifier variations. if you would rather use an amplifier other than the suggested device." The next page displays a list of filter response options that meet your requirements. The speed of this charging process depends on the amplifier's internal resistances. f AMP. V CM =V +/2. The WEBENCH Filter Designer program seems to have broken through some filter design barriers. V CMR limits the range of your input signal. the amplifier's gain is less than 0 dB. you have learned to quickly produce an adjustable analog filter. all three of the filter's responses show a slope of –40 dB/decade. 2008. In the e2 region. it is possible to program second-order Bessel or Chebyshev filters with a programmable corner frequency range of 1:100. Next. For many CMOS or FET amplifiers. References 1. In this design. If you need a higher order filter. rail-to-rail op amp. operational amplifier. or to change the amplifier to have a different frequency response or different input capacitance. region is V 1/f:fB–fA =A N . and 2. baby. For instance. resulting in a stable system. The closure rate between the two curves suggests the system's phase margin and. Additionally. A calculation proves that the contribution to noise in this low-frequency region is relatively low: where RF is the feedback resistor and RPD is the device's parallel resistance. 3. An alternative filter design solution is to have one programmable filter after the multiplexer (Figure 1). where ai is the ith coefficient in the partialfilter-corner frequency. capacitances. (Figure 1) The transfer function of this system is: where A OL (jΩ) is the open-loop gain of the amplifier over frequency. and f Z +1/(2π(R F )(CF +CRF )). The appropriate Bode plot for this design includes the amplifier's open-loop gain and the 1/β curve. To this day.or a CMOS-input stage with lowvoltage noise and microvolt-offset specifications. amplifier-feedback element.ti. a silicon sensor converts light into charge or an electrical current. which comprises a photodiode." EDN. The only path of escape for this charge is through the highvalue resistor in the amplifier's feedback loop. This circuit places the photodiode across the amplifier's inverting input and ground of the operational amplifier. a wide variety of applications use silicon to sense the intensity and characteristics of light. At the system's front end. Bonnie. a preamplifier converts the photodiode's current-output signal to a usable voltage level. Although the light sensitivity of silicon is an undesirable byproduct of the silicon. Photo-sensing circuits have changed over the years. The A OL (jΩ) curve intersects the 1/β curve at an interesting point. the amplifier contributes an approximately –90° phase shift.TI. CF . and Z F is the distributed feedback impedance. input capacitance. as well as RF . system designers have exploited this transfer of light into electrical energy in various systems. and feedback-resistor value places the system's pole of 1/β at half the frequency where the two curves intersect: where f GBW is the gain-bandwidth product of the amplifier.com/ww/en/analog/Amplifiers-eBook/[1/14/14 10:48:43 AM] . CRF . indicating a phase shift of –180° and a phase margin of 0°. or to change the amplifier to have a different frequency response or different input capacitance. such as the charged digitizing ADC. predicts the stability. For instance. and CF in the amplifier's feedback loop. but the real challenge is determining how to convert the low-level currents from the photo sensor into a useful electrical representation. In this manner. the required accuracy in these applications continues to increase. The photo sensor. By adding the 1/β phase shift from the A OL (jΩ) phase shift. system designers understand how to convert light into a current. using the transimpedance amplifier and following it with a lowpass filter. the designer optimizes the stability. Light excitement on the photo sensor generates charge. the system's phase margin is 65°. to name a few applications. 12 Back to top Transimpedance-amplifier stability is key in light-sensing applications A variety of precision applications sense light and convert that information into a useful digital word. combining the input and filtering stages separates the signal of interest from the noise floor. the circuit will oscillate or ring with a step-function input. You then connect the noninverting input to ground (Figure 1). and a feedback network. equaling RPD||jΩ(C PD+CCM +CDIFF ). One way to correct circuit instability is to add a feedback capacitor. Here. and these parts' parasitics combine to create quite a rat's nest of formulas for consideration. To further exacerbate the difficulty of the design. in turn. A resistor with a value of 100 kΩ to 10 MΩ connects the inverting input of the amplifier to the output. operational amplifier. 2008. equaling 1/(1+Z F /ZIN). The simplistic approach in Figure 1 is not without its design challenges. an operational amplifier. Baker. the system's phase shift is –90°. In the end. "Photo-sensing circuits: The eyes of the electronic world are watching. or perform chromatography. The switched integrator was the first step toward bringing the digital portion of the circuit closer to the signal source. and its margin is 90°. detect smoke. Z IN is the distributed input impedance. and the feedback factor contributes an approximately 0° phase shift. IC designers regularly cover their wafers under test to shield out extraneous light. equaling RF ||jΩ(C RF +CF ). More than likely. If the closure rate of these two curves is 40 dB/decade. Consequently. Figure 1 shows the front-end circuit of this system. From the classic transimpedance amplifier. you have learned to quickly produce an adjustable analog filter. Aug 7. The operational amplifier must have relatively low-picoampere input-bias currents and low input capacitance. resulting in a stable system. the closure rate of the two curves is 20 dB/decade. scientists quickly discovered the photo-sensing characteristics of silicon in the lab. In this design.Texas Instruments . Can you think of another way to do this? Send in your ideas and we can discuss them! 11 Back to top Photo-sensing circuits: The eyes of the electronic world are watching Silicon photo sensors have been in electronic circuits since the inception of the era of silicon electronics. The traditional design topology of the transimpedance amplifier captures this low-level signal in a hybrid approach that starts with an amplifier and a high-value resistor in the feedback loop. References 1. A sampling ADC digitizes the signal after the analog filter. This is one technique to design a programmable anti-aliasing filter. The closure rate between the two curves suggests the system's phase margin and. A conservative calculation that allows variation in amplifier bandwidth. bandwidth. and the step function's overshoot is 5%. The signal after the transimpedance amplifier requires a multipole analog filter. Basically. The final design method is not always intuitively obvious. In these systems. the switched integrator has gained favor. low-noise performance. Figure 2 shows the frequency response of the 1/β curve and the amplifier's open-loop-gain response: f P=1/(2π(R PD||RF )(CPD+CCM +CDIFF +CF +CRF )). An appropriate amplifier for this circuit would have a FET. In this design. as they worked from the daylight hours into the evening. System elements determining the noise-gain frequency response are the photodiode's parasitics and the operational amplifier's input capacitance. The circuit design uses resistance to provide a real-time. A good tool for determining stability is a Bode plot. β is the system-feedback factor. position equipment. The migration of the photo-sensing-application product has moved on to totally integrated systems. C2 = 15 nF).com Table 1 shows the digital potentiometer program setting for Butterworth filter with corner frequencies ranging from 100 to 10 kHz (C1 = 33 nF. and layout of this transimpedance-amplifier circuit. http://www. linear representation of the light source. The first approach was purely analog. These silicon sensors are the "eyes" in the electronics world that users can employ to analyze blood. search noninvasively for tumors. The simplistic approach in Figure 1 is not without its design challenges. and one operational amplifier reduces chip count. The OPA314 is a singlesupply. and BW is the bandwidth of interest. 13 Back to top Transimpedance-amplifier noise issues How much noise is too much noise in a photodiode-preamplifier circuit? You can derive the noise performance of a transimpedance amplifier (Figure 1a) with calculations or by using a Spice simulation. Light excitement on the photo sensor generates charge. region is V 1/f:fB–fA =A N . then press the "update" button. A signal path requires this type of dedicated filter to match the signal's requirements. Figure 1: A second-order. Doing so ensures that your filter does not create signal distortions due to slew limitations. at some point. You can reduce the impact of the upward trend in the filter's response by following the offending active filter with a passive RC lowpass filter. C2 = 15 nF). This program provides the actual amplifier suggestion along with options. these filters use a Butterworth design. The higher the Q. resulting in an inverted output after the third stage. A reprogramming of the dual digital potentiometer in Figure 1 changes the filter's frequency cut-off and approximation (Butterworth. internal currents charge these internal capacitors.TI. however. Baker. low-pass unity gain filter. If you use a Sallen-Key lowpass filter. system designers have exploited this transfer of light into electrical energy in various systems. Figure 1 shows the details of a single-supply. Bessel. as well as RF . For many CMOS or FET amplifiers. In Figure 1. second-order programmable low-pass Sallen-key filter. and a good eraser. you can use a multiple-feedback circuit. to mention a few. As the amplifier's open-loop gain decreases. using the transimpedance amplifier and following it with a lowpass filter. May 15. Further. In the e2 region." The Filter Designer Design Summary view (page 3) allows the user to adjust the capacitor seed value to the desired value of C1 and C2 on the left side of the screen. 14 Transimpedance-amplifier application: The pulse oximeter http://www. The value required for C1 in an MFB design can be quite low while the designer is trying to use reasonable resistor values. The obvious advantage is reduction in chip count. When asking how much noise is too much noise in this photodiode-preamp circuit. Additionally. A calculation proves that the contribution to noise in this low-frequency region is relatively low: where RF is the feedback resistor and RPD is the device's parallel resistance. presents an interesting challenge. System elements determining the noise-gain frequency response are the photodiode's parasitics and the operational amplifier's input capacitance. Consequently. input-common-mode-voltage range (VCMR) and input bias current (I B) can also affect you. and the values in this column are root-mean-square values. This inversion may or may not be a concern in the application circuit. References 1. Texas Instruments. Mancini. Consequently. the MFB topology allows low sensitivity to component tolerances. In Figure 1. References 1. But what are the differences and why would you choose one over the other? The MFB topology (sometimes called infinite gain or Rauch) is often preferred because it has low sensitivity to amplifier variations. The A OL (jΩ) curve intersects the 1/β curve at an interesting point." Application Note (SBFA001A). amplifier-feedback element. which may cause additional ringing in the time domain. return to page one and set the conditions for the next filter." EDN. The caveat to this action is that the following filter may interfere with the phase response of your intended filter. and 3. Select a second-order Butterworth from the list and click "Open Design. input capacitance. be aware that this circuit has high-frequency feedthrough. 12 Back to top Transimpedance-amplifier stability is key in light-sensing applications A variety of precision applications sense light and convert that information into a useful digital word." The next page displays a list of filter response options that meet your requirements. the circuit will oscillate or ring with a step-function input. By adding the 1/β phase shift from the A OL (jΩ) phase shift. RF is the feedback resistor in ohms. This arrangement allows a variety of negative gains. The speed of this charging process depends on the amplifier's internal resistances. 8 Back to top Choosing antialiasing-filter amplifiers When you digitize an analog signal. The gain of this circuit is equal to −R2 /R1 . an operational amplifier. and currents. Both LSBs are peak-to-peak numbers. detect smoke. A conservative calculation that allows variation in amplifier bandwidth. An alternative filter design solution is to have one programmable filter after the multiplexer (Figure 1). predicts the stability. rail-to-rail op amp. two capacitors. which does not display this reversal in the gain response at higher frequencies and does not swing the input stage's common-mode voltage. "Noise Analysis of FET Transimpedance Amplifiers. Typically this type of filter is used to reduce analog-to-digital converter (ADC) aliasing errors and noise outside the signal bandwidth. replaces the two resistors in this circuit. β is the system-feedback factor. bandwidth. After the cutoff frequency. but this is not the only potential advantage. A sixth-order filter consisting of three MFB stages inverts the signal three times. A resistor with a value of 100 kΩ to 10 MΩ connects the inverting input of the amplifier to the output. where V CM =V +/2. More than likely. such as the open-loop gain or input range (Figure 1). When you send large signals through the amplifier. Bonnie. With this circuit. The most common topologies for active. represents the noise contribution of the feedback resistor. November 2001. you need to determine the filter-cutoff frequency. must be at least 100×gain×f CUT ×k i . Filter Designer also provides SPICE simulations of the chosen filter's sine wave response. Change to Capacitor seed value to 15 e-9 or 15 nF. 3. second-order. and Z F is the distributed feedback impedance. low-noise performance. Generally. The area under the noise-density curve in the e1 . such as a fourth. The first approach was purely analog. The switched integrator was the first step toward bringing the digital portion of the circuit closer to the signal source. dual-supply. The closure rate between the two curves suggests the system's phase margin and. and the step function's overshoot is 5%. (Figure 1) The transfer function of this system is: where A OL (jΩ) is the open-loop gain of the amplifier over frequency." EDN. A good tool for determining stability is a Bode plot. The noise in regions e4 and e5 uses the higher-frequency gain of the closed-loop-gain curve with the value of C1 being the parallel combination of the input capacitors. 9 Back to top Closer to real-world analog filters There are two basic filter topologies that everyone recommends when they start to design their signal chain: multiple-feedback (MFB) or Sallen-Key topologies. These filters use independent operational amplifiers (op amps) in combination with fixed resistors and capacitors. If the circuit has a front-end multiplexer. the Sallen-Key topology has excellent gain accuracy. lowpass filters are the noninverting Sallen-Key and the inverting multiple feedback. February 1994. capacitances. As the frequency increases beyond this point. but the frequency changes as it aliases back onto your signal of interest. indicating a phase shift of –180° and a phase margin of 0°. The data indicates that the lowpass filters are performing as you would expect for a little more than a decade after the cutoff frequency of 1 kHz. In this configuration. enable the low-pass filter button and type in a filter bandwidth with Ao = 1 V/V. "Start Filter Design.ti. Here. 2008. system designers understand how to convert light into a current. If the noise on the input signal is more than half the sampling frequency of the converter. The appropriate Bode plot for this design includes the amplifier's open-loop gain and the 1/β curve. The caveat is that low capacitor values can result in significant errors due to parasitic capacitance and the input capacitance of the amplifier. The final design method is not always intuitively obvious. e6 . second-order Butterworth response with a programmable corner frequency range of 1:100. Figure 1 shows the front-end circuit of this system. As a rule of thumb. One way to correct circuit instability is to add a feedback capacitor. The sixth part of the noise equation. you need to select an amplifier such that the slew rate (πV OUT P-Pf CUT ). Baker. The WEBENCH Filter Designer program seems to have broken through some filter design barriers. The capacitors are hard-wired in. For Q i greater than 1. You cannot use a digital filter to reduce in-band noise after digitizing the signal. The gain-bandwidth product of the operational amplifiers appears in the specification table of the respective productdata sheet. This circuit places the photodiode across the amplifier's inverting input and ground of the operational amplifier. Selecting the correct amplifier. the amplifier contributes an approximately –90° phase shift. The MFB topology creates a second-order filter. The amplifier bandwidth calculation involves variations of the amplifier's bandwidth and variations of the passive components over time. the output polarity is the same as the input signal. With the MFB topology. you have learned to quickly produce an adjustable analog filter. Calculate the noise contribution and the e3 region in the same manner with f P=1/[2π(RPD||RF ) (CPD+CCM +CDIFF +CF +CRF )] and f Z =1/[2π(RF ) (CF +CRF )]. where V OUT P-P is the expected peak-to-peak output-voltage swing below your filter's cutoff frequency." EDN. V CMR limits the range of your input signal.or eighth-order filter. References 1. such as Texas Instruments' WEBENCH Filter Designer Web software. CRF . Next. This is because the op amp is in a unity-gain buffer configuration with a high open-loop gain. Consequently. fc = 100. where CPD is the device's capacitance and CDIFF is the differential amplifier's capacitance. the closed-loop dc-noise gain (1+R F /RPD). resulting in a stable system. An appropriate amplifier for this circuit would have a FET. Op Amps for Everyone. operational amplifier. it is possible to program second-order Bessel or Chebyshev filters with a programmable corner frequency range of 1:100. and a feedback network. and f Z +1/(2π(R F )(CF +CRF )). its closed-loop output resistance increases. and 2. equaling RF ||jΩ(C RF +CF ). Then press the green button. Can you think of another way to do this? Send in your ideas and we can discuss them! 11 Back to top Photo-sensing circuits: The eyes of the electronic world are watching Silicon photo sensors have been in electronic circuits since the inception of the era of silicon electronics. this action also creates a stage whose output is not low-impedance. position equipment. the cost is lower when a single filter serves many analog inputs. two capacitors. You should also evaluate the effects of amplifier slew rate. the flicker-noise region usually ranges from dc to 100 or 1000 Hz. V CM =V +/2. Aug 7. Figure 2: A Sallen-Key second-order. the gain circuit is negative and equal to the resistor ratio of R2 and R1 . Bishop. or [CP–R1 ||2C CM ||CDIFF ]. Subsequently. from multiple op amps to one single amplifier. The difference in the frequency response at the point at which the three amplifiers change to a positive slope depends on the amplifier's output impedance. multiply the broadband noise of the amplifier. high-frequency noise on the analog signal before it reaches the ADC. the software changes the resistors in the circuit to appropriate values. In this design. or perform chromatography.    gain accuracy is important. The relationship between Q and the damping factor is Q = 1/2 ζ. From the classic transimpedance amplifier. CF . flicker-noise (1/f). the amplifier's gain is less than 0 dB. In the input screen. or to change the amplifier to have a different frequency response or different input capacitance. "Photo-sensing circuits: The eyes of the electronic world are watching. Table 1 shows the digital potentiometer program setting for Butterworth filter with corner frequencies ranging from 100 to 10 kHz (C1 = 33 nF. the system's phase shift is –90°. You can use filterdesign programs to determine the filter's capacitor and resistor values. unity gain. but the real challenge is determining how to convert the low-level currents from the photo sensor into a useful electrical representation. 2008. "FilterPro MFB and Sallen-Key Low-Pass Filter Design Program. Bonnie. April 2003. Texas Instruments . The only path of escape for this charge is through the highvalue resistor in the amplifier's feedback loop. The Sallen-Key topology may be preferable for low-Q. Although the approximation method does not impact or correct this unexpected behavior. In designs where an even number of stages is required. Photo-sensing circuits have changed over the years. The LSB for a 16-bit system with the same input-voltage range is 76. equaling RPD||jΩ(C PD+CCM +CDIFF ).    a unity-gain filter is needed. where ζ is the damping factor. consider six regions in the frequency spectrum (Figure 1b) and add each region with a root-sum-square equation or the following equation: The first five regions are equal to the multiple of the areas under the closed-loop-gain and amplifier-noise-density curves. The dual TPL0102-100. 2008. You would expect this response from a second-order lowpass filter. and feedback-resistor value places the system's pole of 1/β at half the frequency where the two curves intersect: where f GBW is the gain-bandwidth product of the amplifier. particularly if you have differential input/output stages. Additionally. It is no coincidence that the "flattening" of the filter response occurs at this crossing. You then connect the noninverting input to ground (Figure 1). There are instances where the Sallen-Key topology is a better choice. To produce the remaining filters in Table 1. and Asb = –35 dB. A single. the magnitude of that noise stays the same. In these systems. you can also select the supply requirements of Single Supply = +5 V. B Trump. low-pass filter. The traditional design topology of the transimpedance amplifier captures this low-level signal in a hybrid approach that starts with an amplifier and a high-value resistor in the feedback loop. for these circuits. step response. Chebyshev) of this second-order. As you do this make sure that fs is ten times higher than fc. Aug 7. you can realize a combination of Butterworth. the designer optimizes the stability. The circuit design uses resistance to provide a real-time. Again. These silicon sensors are the "eyes" in the electronics world that users can employ to analyze blood. all three of the filter's responses show a slope of –40 dB/decade. You can use a dual digital potentiometer. and RM Stitt. Texas Instruments . vs. All I can say is filter design has come a long way. Selecting the correct operational amplifier for an active lowpass-filter circuit can appear overwhelming as you read an amplifier's data sheet and view all of the specifications. particularly at the 3 dB corner frequency. a preamplifier converts the photodiode's current-output signal to a usable voltage level. and closed-loop ac response. provide a list of the appropriate amplifiers for the related filter. some characterization is in order. where A N is the amplifier's input-noise-density at 1 Hz and f B is the corner frequency where the flicker noise tapers off. the switched integrator has gained favor. and CF in the amplifier's feedback loop. and a single amplifier to configure a low-pass. and layout of this transimpedance-amplifier circuit. This will produce a second-order Butterworth filter. you can cascade both of these topologies. single-supply filter. If you need a higher order filter.22 mV. combining the input and filtering stages separates the signal of interest from the noise floor. "Photo-sensing circuits: The eyes of the electronic world are watching. For instance. The amplifier does not gain the contribution of noise from the feedback resistor: where K is Boltzmann's constant.    a low Q is required (for example. Baker. An alternative to this tedious design exercise is to determine the capacitor and resistor values using TI's WEBENCH Filter Designer software. you should initially consider only two important specifications when selecting an amplifier for your active lowpass filter: gain-bandwidth product and slew rate. The slew rate depends on internal IC currents and capacitances. At the system's front end.com A second set of three curves in Figure 1 shows the frequency response of second-order. high-frequency filter sections. the required accuracy in these applications continues to increase. Bonnie. Figure 3: Sallen-Key second-order. equaling 1/(1+Z F /ZIN). where Q i is the quality factor of the ith partial filter and ki is the ratio of the partial-filter-corner frequency to the overall. consider that a 12-bit system operating with a 5V input range has an LSB of 1.or a CMOS-input stage with lowvoltage noise and microvolt-offset specifications. Elsevier-Newnes. The voltage drop that the input-bias-current error causes appears as an additional input-offset voltage. the quality factor. and Chebyshev filters with the same circuit using a 1:10 corner frequency range. When calculating the noise performance of the circuit. Figure 2 shows the frequency response of the 1/β curve and the amplifier's open-loop-gain response: f P=1/(2π(R PD||RF )(CPD+CCM +CDIFF +CF +CRF )). Z IN is the distributed input impedance. linear representation of the light source. Basically. the gain-bandwidth product of the amplifier.com/ww/en/analog/Amplifiers-eBook/[1/14/14 10:58:49 AM] Back to top . J. In this manner. Bessel. and the square root of the region's bandwidth. To this day. 2. a sharp pencil. the R2 /R1 resistor ratio and the resistor errors determine the gain. to name a few applications. baby. it is possible to have a variety of signals that reach the ADC where each signal source has its own set of filter requirements. the gain is independent of the resistors in the circuit. scientists quickly discovered the photo-sensing characteristics of silicon in the lab. To further exacerbate the difficulty of the design. the more easily the circuit oscillates. which comprises a photodiode. It is not simply a matter of picking an amplifier with a gain-bandwidth-product that is 100 times higher than the filter's corner frequency. search noninvasively for tumors. Sallen-Key lowpass filters for each amplifier. second-order Sallen-Key stage also provides two poles but is a non-inverting circuit. f CUT (or –3-dB frequency). the contributed noise in this region is usually relatively low because of its location in the lower frequency range. analog filter using a dual digital potentiometer. ISBN-0-7506-7701-5. a silicon sensor converts light into charge or an electrical current. Figure 1: MFB second-order single-supply low-pass filter. Although the light sensitivity of silicon is an undesirable byproduct of the silicon. Sometimes amplifier manufacturers. To ensure that your active filter does not enter into a slew condition. f AMP. and C2 is the parallel combination of CF and CRF . This may be preferable over a MFB single stage. Alternative filters can solve the problem without adding an RC filter. the closure rate of the two curves is 20 dB/decade. Your circuit also may require a low Q. The filter implementation requires two resistors and two capacitors. Eventually. A MFB single-stage (second-order filter) provides two poles while inverting the signal. Ron. a variety of different filters and corner frequency requirements may be required in the circuit prior to the multiplexer. such as the charged digitizing ADC. and these parts' parasitics combine to create quite a rat's nest of formulas for consideration." SBOA060. If the closure rate of these two curves is 40 dB/decade. When using the Sallen-Key circuit. "RMS and peak-to-peak noise trade-off. for Q i of less than 1. Then. Doing so attenuates superimposed. The gain of this circuit is equal to 1V/V. and the feedback factor contributes an approximately 0° phase shift. the system's phase margin is 65°. IC designers regularly cover their wafers under test to shield out extraneous light. The signal after the transimpedance amplifier requires a multipole analog filter. the filter gain begins to increase at a rate of 20 dB/decade. The photo sensor. a 100 kΩ 8-bit digital potentiometer.38×10–23 . This is one technique to design a programmable anti-aliasing filter. In the end. 2. f AMP=100×gain×(f CUT /a i )× filter-transfer function. The operational amplifier must have relatively low-picoampere input-bias currents and low input capacitance. fs = 1000. the Sallen-Key topology is better if: 1. In this view. you use a lowpass filter to prevent aliasing errors from out-of-band noise. the input bias current conducts through the external source resistance. Also. under the "Filter Topology Specifications" section. where ai is the ith coefficient in the partialfilter-corner frequency.Texas Instruments . and its margin is 90°. in turn. each filter's response flattens at the 0-dB crossing frequency of the op amp's open-loop gain. 10 Back to top Simply an adjustable low-pass filter A low-pass filter is the most common filter found in data acquisition systems. The unity-gain Sallen-Key topology also requires fewer components–two resistors versus three for the MFB (Figure 2). The gain of this circuit is 1+R 2 /R1 . You can calculate the appropriate resistance and capacitance with a little research. as they worked from the daylight hours into the evening. A sampling ADC digitizes the signal after the analog filter. When this capacitor seed is set. Q < 3) At unity-gain. if you would rather use an amplifier other than the suggested device. When an inverting filter is an acceptable alternative. WEBENCH Filter Designer program. References 1. The migration of the photo-sensing-application product has moved on to totally integrated systems. In this design. Before selecting the amplifier. which is 1. a wide variety of applications use silicon to sense the intensity and characteristics of light. Table 1 summarizes the digital potentiometer programmed settings.29 µV. T is temperature in Kelvin. Neil.or CMOS-amplifier input devices usually meet this requirement. In the circuit in Figure 1. This condition could affect you if you are a pilot. Again. Townsend. flicker-noise (1/f).TI. Figure 1 shows a simplified block diagram of a pulse oximeter. The alarms on the pulse oximeter usually sound when the SpO 2 level drops below 90%. position and proximity sensors. The signal also travels through a lowpass filter to regulate the driver power to the LEDs.com 13 Back to top Transimpedance-amplifier noise issues How much noise is too much noise in a photodiode-preamplifier circuit? You can derive the noise performance of a transimpedance amplifier (Figure 1a) with calculations or by using a Spice simulation. 3. or [CP–R1 ||2C CM ||CDIFF ].com/ww/en/analog/Amplifiers-eBook/[1/14/14 10:48:43 AM] . The sixth part of the noise equation. For many CMOS or FET amplifiers." EDN. 2. represents the noise contribution of the feedback resistor. May 15. Both LSBs are peak-to-peak numbers. A second consideration is that the low-frequency voltage noise of your amplifier must be very low. T is temperature in Kelvin. "RMS and peak-to-peak noise trade-off. or even undergoing surgery. Texas Instruments. "Photo-sensing circuits: The eyes of the electronic world are watching. When asking how much noise is too much noise in this photodiode-preamp circuit. When calculating the noise performance of the circuit. where A N is the amplifier's input-noise-density at 1 Hz and f B is the corner frequency where the flicker noise tapers off. a bandpass filter eliminates the noise above 5 Hz.ti. both LEDs are off for 450 µsec. converts the photodiode current generated by the LEDs to a voltage at the output. Aug 7.22 mV. taking the ratio between the intensities from a photodiode. In the e2 region. the closed-loop dc-noise gain (1+R F /RPD). "Pulse Oximetry. "Noise Analysis of FET Transimpedance Amplifiers. and then both LEDs are off for 450 µsec. multiply the broadband noise of the amplifier. The signal then travels through a bandpass filter and gain stage to the 12-bit ADC. you will primarily find transimpedance amplifiers in the CT (computedtomography)-scanner front end and the pulse oximeter. Baker. The area under the noise-density curve in the e1 . and comparing that ratio with an SpO 2 look-up table in the microcontroller. ranging from 95 to 100%.and NIR-LED signals." Medical Electronics. 14 Back to top Transimpedance-amplifier application: The pulse oximeter Have you ever been in need of a second opinion when your sanity is at stake? The pulse oximeter may be able to provide that second opinion if your brain is oxygen-deprived. The system repeats this cycle continuously. or perhaps you just dance to the beat of a different drummer. hiking in the high altitude of a mountain range. MD. The noise in regions e4 and e5 uses the higher-frequency gain of the closed-loop-gain curve with the value of C1 being the parallel combination of the input capacitors. After the transimpedance amplifier. region is V 1/f:fB–fA =A N . The amplifier's input-bias current creates an output-voltage error by conducting through the high-impedance resistor. photographic analyzers. References 1. and C2 is the parallel combination of CF and CRF . Michaelmas 2001. Baker. e6 . which is 1. and the square root of the region's bandwidth. The LCD shows a percentage of oxygenated hemoglobin versus nonoxygenated hemoglobin and your heart rate. computes the ratio of the red. A normal output for the pulse oximeter is approximately 97%±2%. the NIR LED is on for 50 µsec.Texas Instruments . you need to make sure that the amplifier's input-bias current is very low or in a picoamp region at 25°C. Texas Instruments. bar-code scanners. http://www. the red LED is on for 50 µsec. The transimpedance amplifier appears in medical and laboratory instrumentation. The transimpedance amplifier. the amplifier's initial offset error and overtemperature should be in the microvolt region if you want to minimize linearity errors. the contributed noise in this region is usually relatively low because of its location in the lower frequency range. A calculation proves that the contribution to noise in this low-frequency region is relatively low: where RF is the feedback resistor and RPD is the device's parallel resistance.29 µV. you may want to explore other diagnostic avenues. where CPD is the device's capacitance and CDIFF is the differential amplifier's capacitance. The pulse oximeter is a noninvasive instrument that monitors SpO 2 (saturation of hemoglobin with oxygen) in your blood. You measure the oxygen in the blood by alternating the on-times of a red LED with a 650-mm wavelength and an NIR (near-infrared) LED with a 940-mm wavelength. and BW is the bandwidth of interest. The LSB for a 16-bit system with the same input-voltage range is 76. you may experience poor judgment or loss of motor function. Bonnie. and even smoke detectors. FET. A 1 . February 1994. and compares the results with a look-up table. 2008. RF is the feedback resistor in ohms." EDN. When you consider the input-voltage noise of the amplifier. consider that a 12-bit system operating with a 5V input range has an LSB of 1. In the medical field. If there is a shortage of oxygen in your system. It may be worthwhile to use an autozero amplifier. The amplifier does not gain the contribution of noise from the feedback resistor: where K is Boltzmann's constant. 2010. Calculate the noise contribution and the e3 region in the same manner with f P=1/[2π(RPD||RF ) (CPD+CCM +CDIFF +CF +CRF )] and f Z =1/[2π(RF ) (CF +CRF )]. consider six regions in the frequency spectrum (Figure 1b) and add each region with a root-sum-square equation or the following equation: The first five regions are equal to the multiple of the areas under the closed-loop-gain and amplifier-noise-density curves. in the amplifier's feedback loop." SBOA060. When you choose your device for the pulse-oximeter transimpedance-amplifier circuit. 2. Good luck! References 1. scrutinize the impact of the flicker noise. The microcontroller acquires the signals from the 12-bit ADC. and the values in this column are root-mean-square values. Medical Instruments Applications Guide.38×10–23 . the flicker-noise region usually ranges from dc to 100 or 1000 Hz. If a pulse oximeter indicates that your oxygen levels are stable. 2008. RF . Finally. Bonnie. are the highest performance analog products in front of the ADC appropriate? How do you determine which products are good enough for your system? Avoiding production-floor notifications or field failures may be your definition of "good enough. the more easily the circuit oscillates. Again. as well as RF . One criterion is to keep the signal within the dynamic range of the full-scale range of the ADC. single-supply filter. you must understand the impact of your PCB (printed-circuit board) on the picoampere levels of current. You measure the oxygen in the blood by alternating the on-times of a red LED with a 650-mm wavelength and an NIR (near-infrared) LED with a 940-mm wavelength. which makes the manipulation of the photodiode signal manageable. Further. and these parts' parasitics combine to create quite a rat's nest of formulas for consideration. With this circuit. I ran around and asked for advice. This arrangement allows a variety of negative gains. The difference in the frequency response at the point at which the three amplifiers change to a positive slope depends on the amplifier's output impedance. the dctransfer function of this circuit is 1V/V. and RM Stitt. The WEBENCH Filter Designer program seems to have broken through some filter design barriers. the configuration for all three amplifiers is a dc noise gain of 1000V/V. the NMOS transistors are on." Application Note (SBFA001A). The amplifier's closed-loop gain is 10V/V. The obvious advantage is reduction in chip count. Figure 2: A Sallen-Key second-order. The end result is that the input offset voltage changes between the stages. The closure rate between the two curves suggests the system's phase margin and. The OPA314 is a singlesupply. If you are not on top of the explanation. A CL_DC is the closed-loop gain at dc. at the hands of several individuals including Harold S. and closed-loop ac response. but this is not the only potential advantage. analog filter using a dual digital potentiometer. In this design. The signal after the transimpedance amplifier requires a multipole analog filter. After the transimpedance amplifier. the PMOS transistors are on. and layout of this transimpedance-amplifier circuit. an operational amplifier. Change to Capacitor seed value to 15 e-9 or 15 nF. or 20 dB. high closed-loop gains. A normal output for the pulse oximeter is approximately 97%±2%. and the step function's overshoot is 5%. an integrator." EDN. there is still a lot to learn about the amplifier and their appropriate placements in your application circuits. we have a 16-bit converter. We also find that the amplifier bandwidth needs to be at least 100 times higher to maintain ADC integrity. f AMP=100×gain×(f CUT /a i )× filter-transfer function. lowpass filters are the noninverting Sallen-Key and the inverting multiple feedback. It is no coincidence that the "flattening" of the filter response occurs at this crossing. A good approach is to make the noise of A 2 less than or equal to that of A 1 . making A 2 's bandwidth much greater than that of A 1 .9 mV maximum at full scale. and C are 38 MHz. If the resistors around A 3 are not equal. internal currents charge these internal capacitors. However. During this test. rail-to-rail op amp. Generally. you exchange one noise problem. CRF . second-order. computes the ratio of the red. But what are the differences and why would you choose one over the other? The MFB topology (sometimes called infinite gain or Rauch) is often preferred because it has low sensitivity to amplifier variations. or perhaps you just dance to the beat of a different drummer. the circuit will oscillate or ring with a step-function input. Bonnie. Single-Supply.29 µV. As long as the resistors surrounding A 3 are equal." SBOA060. If there is a shortage of oxygen in your system. Consequently. This intersection has a simple. If the THD+N tests include the amplifier's input crossover distortion. Subsequently. resulting in unexpected noise. Rail-to-Rail Operational Amplifiers. For the most part. If you comprehend the general guidelines. however. These silicon sensors are the "eyes" in the electronics world that users can employ to analyze blood. With these types of amplifiers. A 1 and A 2 require low-inputcurrent CMOS or FET op amps. As a rule of thumb. If you make A CL1 equal to 9. "RMS and peak-to-peak noise trade-off. If you are using your amplifier as the key component in a transimpedance amplifier. Photo-sensing circuits have changed over the years. Additionally. the gain is independent of the resistors in the circuit. input capacitance. Figure 1: MFB second-order single-supply low-pass filter. One problem I ran into involved the stability of an amplifier circuit. is ±500 µV×10. but you will never see ESD-leakage current in bipolar amplifiers. Bonnie.or CMOS-amplifier input devices usually meet this requirement. In fact. 16 Back to top Single-supply amplifier outputs don't swing rail to rail Single-supply amplifiers do not truly swing rail to rail at the output. Medical Instruments Applications Guide. McGraw-Hill. This will produce a second-order Butterworth filter. you need to make sure that the amplifier's input-bias current is very low or in a picoamp region at 25°C. The Sallen-Key topology may be preferable for low-Q. the amplifier's gain is less than 0 dB. The microcontroller acquires the signals from the 12-bit ADC. A second set of three curves in Figure 1 shows the frequency response of second-order. These beasts were just coming alive in the 1930s – 1940s. The amplifier does not gain the contribution of noise from the feedback resistor: where K is Boltzmann's constant. CF . but. 3 Back to top Where did all that racket come from? Since arriving in the market. 15 Back to top Remote photo sensing Photodiodes transform a basic physical occurrence. In the input region near ground. 7 Back to top Sallen-Key lowpass-filter stopband limitations When you design an analog. In our circuit. The switched integrator was the first step toward bringing the digital portion of the circuit closer to the signal source. the circuit still oscillated. From the classic transimpedance amplifier. However. If you use a Sallen-Key lowpass filter. The result of this added capacitance increases the circuit's noise gain unless you increase the amplifier's feedback capacitor. The GBWP of this amplifier is 20 MHz. Although the approximation method does not impact or correct this unexpected behavior. A high level of input noise. This condition could affect you if you are a pilot. The LCD shows a percentage of oxygenated hemoglobin versus nonoxygenated hemoglobin and your heart rate. Sept 4. With this design. you have learned to quickly produce an adjustable analog filter. With the RSS formula.    a unity-gain filter is needed. with a capacitive load. an operational amplifier. e6 . 2. At the end of the signal chain. Miro. However. f CUT (or –3-dB frequency)." So. The MFB topology creates a second-order filter. where Q i is the quality factor of the ith partial filter and ki is the ratio of the partial-filter-corner frequency to the overall. For many CMOS or FET amplifiers. The system repeats this cycle continuously. photographic analyzers. After the cutoff frequency. Next. I returned to the engineer who gave me the first bit of great advice. the closure rate of the two curves is 20 dB/decade. 3. In designs where an even number of stages is required. however. or even undergoing surgery. input-common-mode-voltage range (VCMR) and input bias current (I B) can also affect you. such as the open-loop gain or input range (Figure 1). a bandpass filter eliminates the noise above 5 Hz. sboa035. unless you expanded your design task to using two photodiodes. bandwidth. The product characteristics that influence the extent of the dynamic range are the system's cumulative offset and gain errors. Then. The faster signals have the potential to produce EMI signals that other devices or traces on the board can receive. Figure 2 shows the frequency response of the 1/β curve and the amplifier's open-loop-gain response: f P=1/(2π(R PD||RF )(CPD+CCM +CDIFF +CF +CRF )). 2010. the output of single-supply amplifiers can come within only 50 to 300 mV of each rail (Figure 1). The input-bias current swamps out the picoampere leakage current from the ESD cells. A signal path requires this type of dedicated filter to match the signal's requirements. Texas Instruments . Baker. where CA1 and CA2 represent the sum of their input differential and common-mode capacitance. Texas Instruments. flicker-noise (1/f). a variety of different filters and corner frequency requirements may be required in the circuit prior to the multiplexer.or eighth-order filter. V CM =V +/2. the bandwidth of the closed-loop system is approximately 100 times lower than f 3 dB . when the amplifier's inputs are near the negative rail. A classic photo-sensing system circuit has a photodiode. Oct 16. first-order attenuation. I was wide-eyed and excited–confronting new problems left and right. make sure you use the same evaluation technique in your manufacturing process to quantify the effects of the processes–such as solder reflow–that you impose on these devices and the end-of-life effects due to environmental exposures. For instance. Aug 7. Select a second-order Butterworth from the list and click "Open Design. multiply the broadband noise of the amplifier. and currents. The term "input-bias current" loses its meaning when you look at JFET or CMOS input amplifiers. When the PMOS and NMOS transistors are both on. The migration of the photo-sensing-application product has moved on to totally integrated systems. So. In this circuit. 14 Back to top Transimpedance-amplifier application: The pulse oximeter Have you ever been in need of a second opinion when your sanity is at stake? The pulse oximeter may be able to provide that second opinion if your brain is oxygen-deprived. The gain-bandwidth product of the operational amplifiers appears in the specification table of the respective productdata sheet. It may be worthwhile to use an autozero amplifier. the amplifier's initial offset error and overtemperature should be in the microvolt region if you want to minimize linearity errors. the amplifier bandwidth needs to be at least 10 times higher than the signal. resulting in a stable system. Finally. Texas Instruments. but I have a different opinion. you would think that we have finally come to terms with understanding this five-terminal device. and H. Because the leakage current is from the reverse-biased ESD diodes. and feedback-capacitance elements limit the bandwidth of the circuit. Texas Instruments.speed amplifiers. second-order Sallen-Key stage also provides two poles but is a non-inverting circuit. January 2005. To this day. You define the common-mode voltage of the input signals as (VA1OUT +V A2OUT )/2. the output polarity is the same as the input signal. you will be OK. The configurations of A 1 and A 2 act as traditional current-to-voltage converters or transimpedance amps. The difference between the input signal and the output signal in this system falls across the cable/diode capacitance. Consequently. the cable capacitance. If the open-loop-gain and closed-loop-gain curves cross with a 40-dB/decade closure rate. the MFB topology allows low sensitivity to component tolerances. taking the ratio between the intensities from a photodiode.004%. the incident light on the photodiode causes current to flow through the diode from cathode to anode. you will see greater accuracy in proximity and difference (Figure 1). Can you think of another way to do this? Send in your ideas and we can discuss them! 11 Back to top Photo-sensing circuits: The eyes of the electronic world are watching Silicon photo sensors have been in electronic circuits since the inception of the era of silicon electronics. this assumption is a safe one. and 300 kHz. baby. Once you take this first step. A bipolar amplifier's input-bias current is the same as the base current of the NPN or PNP transistors at the input of the amplifier. August 1993. sang like a bird. in this circuit. You can calculate the appropriate resistance and capacitance with a little research. You then connect the noninverting input to ground (Figure 1). If you configure the complementary-input amplifier in a noninverting configuration. An equal illumination on the two photodiodes makes the output voltage 0V. The circuit design uses resistance to provide a real-time. given my work load. replaces the two resistors in this circuit. This may be preferable over a MFB single stage. The output stage of a single-supply amplifier usually has an AB topology. The key performance parameters for A 1 and A 2 are input capacitance. Adding a matched photodiode and monitoring the difference between the two diode outputs removes the equal offsets the two diodes produce. V CMR limits the range of your input signal. a capacitance transducer.Texas Instruments . Consequently. the gain circuit is negative and equal to the resistor ratio of R2 and R1 . 8 Back to top Choosing antialiasing-filter amplifiers When you digitize an analog signal. an analog filter. Recalling a circuit from another column. The amplifier's input-bias current creates an output-voltage error by conducting through the high-impedance resistor. the red LED is on for 50 µsec. offset. Additionally. You may say that it will become obsolete soon. the system's phase shift is –90°." When I asked why. second-order programmable low-pass Sallen-key filter. Photodiodes with a relatively low diode capacitance do not benefit from this circuit. Then press the green button. A 1 . Given this scenario. The calculated CMR (common-moderejection) error due to resistor mismatches is 100×(1+R 2 /R1 )/(% of mismatch error). you need higher-speed amplifiers in the circuit for two basic reasons. A single photodiode provides some measure of a light's intensity through the magnitude of the diode's output signal. Figure 3: Sallen-Key second-order. Given the long time span between these beginning years and the present. f AMP. in the amplifier's feedback loop. Elsevier-Newnes. The gain of this circuit is equal to 1V/V. Input-bias and input-leakage current can change over temperature. The higher the Q. a wide variety of applications use silicon to sense the intensity and characteristics of light. 6 Back to top Don't be fooled by your amplifier's bandwidth As we design our SAR-converter analog circuits. consider that a 12-bit system operating with a 5V input range has an LSB of 1. OPA2350. References 1. dual-supply. you might find that the amplifier's input-bias current creates an offset-voltage error through the resistors in your circuit. However. I built a new circuit as suggested. The gain of this circuit is 1+R 2 /R1 . and a feedback resistor/capacitor pair at the front end.38×10–23 . it has come back to haunt me.0112% error at the full-scale frequency. fc = 100. Figure 1 shows the details of a single-supply. Baker. the Sallen-Key topology is better if: 1. Single-Supply. Texas Instruments . IC designers regularly cover their wafers under test to shield out extraneous light. In these systems. This specification is independent of the common-mode voltage and the magnitude-amplifier power. Figure 1 illustrates the behavior of three Sallen-Key lowpass filters using signal-supply amplifiers. A sixth-order filter consisting of three MFB stages inverts the signal three times. low-pass filter. scrutinize the impact of the flicker noise. because faster amplifiers on the board potentially can produce layout headaches. The value required for C1 in an MFB design can be quite low while the designer is trying to use reasonable resistor values. In the bipolar-amplifier days. if you combine all of these terms. To fix this problem. "Start Filter Design. Light excitement on the photo sensor generates charge.098% or equals 4. and Chebyshev filters with the same circuit using a 1:10 corner frequency range. the bandwidth of the circuit decreases. and the PMOS transistors are off. Baker. the input stage's impact on THD+N complicates the nature of this specification with single-supply amplifiers. 5. In the input screen. The speed of this charging process depends on the amplifier's internal resistances. And. respectively. at some point. "OPA363. If you use three op amps in a differential-photodiode-amp configuration. and the values in this column are root-mean-square values. His recommendation: "Change the 100Ω resistor to a 500Ω resistor.5 mV. CL . You can keep this difference low selecting A 2 with wider bandwidth than A 1 and keeping A 2 's output impedance low. 10 Back to top Simply an adjustable low-pass filter A low-pass filter is the most common filter found in data acquisition systems.    a low Q is required (for example." EDN. Good luck! References 1. The appropriate Bode plot for this design includes the amplifier's open-loop gain and the 1/β curve. ISBN-0-7506-7701-5. The contribution of the amplifier-gain stage. So. The THD+N performance of a single-supply amplifier also originates in the amplifier's input and output stages. In Figure 1. RF ." Instead of choosing the best products. bar-code scanners. Precision Unity Gain Differential Amplifier. The combination of the load capacitor. 4. or [CP–R1 ||2C CM ||CDIFF ]. a sharp pencil. a wider bandwidth than A 1 . In ensuring that the input-leakage current remains low with JFET and CMOS amplifiers. You would expect this response from a second-order lowpass filter. you need to determine the filter-cutoff frequency. you would expect its gain amplitude to continuously decrease beyond the filter's cutoff frequency. OPA2364. depending on the operational-amplifier design. where ζ is the damping factor. 12 Back to top Transimpedance-amplifier stability is key in light-sensing applications A variety of precision applications sense light and convert that information into a useful digital word. and. "Photo-sensing circuits: The eyes of the electronic world are watching. you could use a single photodiode to determine the brightness of your bulbs. which makes the magnitude of the output voltage a direct measurement of the difference between the direct light impinging on D1 and D2 . a higher level of quiescent current through the output stage reduces the amplifier's THD. One way to correct circuit instability is to add a feedback capacitor. Also. will oscillate. for Q i of less than 1. These calculations can assist you in making logical and economical product decisions. George Philbrick. be aware that this circuit has high-frequency feedthrough." EDN. As it was my first job. Doing so ensures that your filter does not create signal distortions due to slew limitations. predicts the stability. The alarms on the pulse oximeter usually sound when the SpO 2 level drops below 90%. With a signal gain of 10V/V. An appropriate amplifier for this circuit would have a FET. In Figure 1. you can use the RSS (root-sum-square) algebraic approach. At the system's front end. B. the output stage displays a crossover distortion similar to the input-stage crossover distortion." The magnitude of leakage current with JFET or CMOS amplifiers is less than 1 pA at 25°C. The most common topologies for active. the term "input-bias current" was an accurate descriptor. One sticky point with this circuit is that the amplifier's closed-loop gain is not equal to 20 dB all the way up to 2 MHz. Table 1 summarizes the digital potentiometer programmed settings. the filter-stage (IC2 ) offset error is ±500 µV. and CF in the amplifier's feedback loop. ISBN: 0-07-024247. When you choose your device for the pulse-oximeter transimpedance-amplifier circuit. It is not simply a matter of picking an amplifier with a gain-bandwidth-product that is 100 times higher than the filter's corner frequency. 5 Back to top A good holiday-season project Which of the bulbs on a Christmas tree is the brightest? If you had the time and desire to answer this question. and Z F is the distributed feedback impedance. or perform chromatography. This is down 3 dB. assume that the maximum offset error of IC1 and IC2 is 0. particularly at the 3 dB corner frequency. I need to know what is really going on! What I did not understand then but understand now is that a capacitor and a resistor that hang on the output of an amplifier change the amplifier's open-loop-gain curve. A single. and Asb = –35 dB. 2008. Dec 15. 2. More important. second-order Butterworth response with a programmable corner frequency range of 1:100. to mention a few. References 1. 2." EDN. The JFET and CMOS input amplifiers may not be. which does not display this reversal in the gain response at higher frequencies and does not swing the input stage's common-mode voltage. which may cause additional ringing in the time domain. RO. but one issue came to mind as I read the comments in the Talkback section for a recent column. April 2003. or both can increase the amplifier's overall THD+N level. 9 Back to top Closer to real-world analog filters There are two basic filter topologies that everyone recommends when they start to design their signal chain: multiple-feedback (MFB) or Sallen-Key topologies." EDN. worse yet. This new circuit was still singing. using the transimpedance amplifier and following it with a lowpass filter. The input capacitance of the buffer replaces the summation of the input capacitance of A 1 . Because the inverting input of A 1 and A 2 has high impedance.22 mV. The area under the noise-density curve in the e1 . put a 100Ω load resistor between the amplifier output and the load capacitor. The capacitors are hard-wired in. 2 MHz. Background light adds only an offset to the photodiode's outputs. McGraw-Hill. 2008. 1978. vs. particularly if you have differential input/output stages. but the frequency changes as it aliases back onto your signal of interest. Near the rail. The dual TPL0102-100. As you do this make sure that fs is ten times higher than fc. When the amplifier's inputs are closer to the positive rail. The cumulative possible offset error at the =940 mV. MicroAmplifier Series. such as Texas Instruments' WEBENCH Filter Designer Web software. in turn. Bonnie. Instead. you should initially consider only two important specifications when selecting an amplifier for your active lowpass filter: gain-bandwidth product and slew rate. and f Z +1/(2π(R F )(CF +CRF )). Selecting the correct operational amplifier for an active lowpass-filter circuit can appear overwhelming as you read an amplifier's data sheet and view all of the specifications. "Photo-sensing circuits: The eyes of the electronic world are watching. The two photodiodes let you find a light's position by monitoring the difference between their output signals. There are instances where the Sallen-Key topology is a better choice. A 2 's input capacitance is the only capacitance that plays a role in the ac-transfer function of the transimpedance system. A sampling ADC digitizes the signal after the analog filter. by Bonnie Baker Amplifiers have been around for a long time. The operational amplifier must have relatively low-picoampere input-bias currents and low input capacitance. you would use the RSS formula to determine gain-error contribution that limits the dynamic range from the four stages in this circuit. Almost all amplifiers have ESD cells for protection from an ESD event. you take the square root of the sum of the squares of several terms that are statistically independent. then press the "update" button. Graeme. In this design. with another. you would calculate the combined RSS value of offset and gain. the amplifier circuit will be marginally unstable or. however.ti. bias current. hiking in the high altitude of a mountain range. a noticeable gain error can occur between the two input signals. On the contrary. as they worked from the daylight hours into the evening. Now. you'll find that the type of amplifier you select for A 2 is somewhat easy. The primary reason is to accommodate the lost bandwidth in amplifier circuits that have a closed-loop noise gain greater than one. light. Another major THD+N contributor can be the operational amplifier's output stage. OPA2363. The transimpedance amplifier appears in medical and laboratory instrumentation. Photodiode Amplifiers: Op Amp Solutions. and the feedback factor contributes an approximately 0° phase shift. where V CM =V +/2. Aug 7." Texas Instruments. A good rule of thumb is to have CA2 <<(CA1 +CCA+CPD).or a CMOS-input stage with lowvoltage noise and microvolt-offset specifications. This is because the op amp is in a unity-gain buffer configuration with a high open-loop gain. Sometimes amplifier manufacturers.8V. noise. Calculate the noise contribution and the e3 region in the same manner with f P=1/[2π(RPD||RF ) (CPD+CCM +CDIFF +CF +CRF )] and f Z =1/[2π(RF ) (CF +CRF )]. and a good eraser. equaling RPD||jΩ(C PD+CCM +CDIFF ). The most effective way you can reduce or minimize the effects of input-bias or input-leakage current is to check your circuit configurations. A 1 . The simplistic approach in Figure 1 is not without its design challenges. We find that a 20-kHz amplifier does not work for our circuit. the NMOS-transistor pair dominates the offset error. Both pairs are on as the amplifier's inputs pass between these two regions. equaling RF ||jΩ(C RF +CF ). two capacitors. This program provides the actual amplifier suggestion along with options. the diagram captures the open-loop gain of each amplifier as the response crosses 0 dB. but rather they are permanent fixtures in the electronics world. For instance.    gain accuracy is important. from multiple op amps to one single amplifier. isn't there something else out there that has successfully obsoleted these devices? I have worked with people who are surprised when I tell them that the operational amplifier is not dying. The amplifier characteristics shown in Figure 1 appear to be a perfect fit. however. β is the system-feedback factor. subtracting the output voltages of A 1 and A 2 . the closed-loop gain at 2 MHz is 17 dB. Basically. The noise in regions e4 and e5 uses the higher-frequency gain of the closed-loop-gain curve with the value of C1 being the parallel combination of the input capacitors. and one operational amplifier reduces chip count. Generally. this type of mismatch can introduce nonlinearities in the system when the common-mode voltage of the two inputs changes. If you need a higher order filter." The Filter Designer Design Summary view (page 3) allows the user to adjust the capacitor seed value to the desired value of C1 and C2 on the left side of the screen. With a maximum signal frequency input from dc to 20 kHz. This circuit places the photodiode across the amplifier's inverting input and ground of the operational amplifier. The filter implementation requires two resistors and two capacitors. and then the response turns around and starts to increase in gain with frequency. In the end. the current sinking or sourcing from the amplifier's input pins is actually the leakage current from the input-ESD (electrostatic-discharge) cells (Figure 1). With the MFB topology. The only important performance specifications are low input capacitance. When you send large signals through the amplifier. If the circuit has a front-end multiplexer. where ai is the ith coefficient in the partialfilter-corner frequency. References 1. You can use a dual digital potentiometer. The following excerpts from the EDN Baker's Best column represent the best of the amplifier short articles. 3." Texas Instruments. the load resistor. The caveat is that low capacitor values can result in significant errors due to parasitic capacitance and the input capacitance of the amplifier.) The gain error of both the sensor cell and the IC1 amplifier configuration depends on the ±1% maximum resistor tolerances as well as on a maximum sensor-resistor tolerance of ±2%.7% lower than the closed-loop gain. February 2003. the circuit's closed-loop gain does not instantaneously change from 20 dB to 17 dB at 2 MHz. one would think that the required amplifiers would have very low unity-gain bandwidths or a gain-bandwidth product (GBWP). its closed-loop output resistance increases. They have been complied for your convenience. low noise. WEBENCH Filter Designer program. Figure 1 shows a simplified block diagram of a pulse oximeter. capacitances. and feedback-resistor value places the system's pole of 1/β at half the frequency where the two curves intersect: where f GBW is the gain-bandwidth product of the amplifier. As the frequency increases beyond this point. FET. and it still is. Kurz. The photo sensor. When an inverting filter is an acceptable alternative. SBOS145. ranging from 95 to 100%. The ADC's contributed gain error is 0. or a 29. 2008. bringing these errors to the ADC's input. Is this statement true or false? Using this type of logic may give you a confident feeling that your circuit will work correctly the first time. The unity-gain buffer removes the noise effect from A 1 .com/ww/en/analog/Amplifiers-eBook/[1/14/14 1:13:55 PM] . in Figure 2 the THD+N is 0. With our amplifier circuit. in that the output stage switches from transistor to transistor. In Figure 1. system designers have exploited this transfer of light into electrical energy in various systems. the worst-case sensor-resistive offset error would be ±94 mV×10V/V. a sample-and-hold circuit. and the NMOS transistors are off (Figure 1). and C2 is the parallel combination of CF and CRF . The other reason is to make sure the bits in your system at the end of the signal path can reliably convert the signal throughout the entire system's frequency range. which produces a ˜0. B Trump. The voltage drop that the input-bias-current error causes appears as an additional input-offset voltage. In this manner. the common-mode voltage region is approximately 400 mV. Ideally. the system's phase margin is 65°. including instrumentation amplifiers (INA) and programmable gain amplifiers (PGA). Black. If a pulse oximeter indicates that your oxygen levels are stable. Dov. An alternative to this tedious design exercise is to determine the capacitor and resistor values using TI's WEBENCH Filter Designer software. The voltage at the inverting input of the amp tracks the voltage at the amp's noninverting input. provide a list of the appropriate amplifiers for the related filter. Creating this pole and zero does not disrupt the amplifier's stability as long as they cancel each other out before the open-loop-gain curve crosses the closed-loop-gain curve. IC1 . This is one technique to design a programmable anti-aliasing filter." EDN. a preamplifier converts the photodiode's current-output signal to a usable voltage level. it is possible to program second-order Bessel or Chebyshev filters with a programmable corner frequency range of 1:100. The final design method is not always intuitively obvious. In this configuration. A 3 rejects the common-mode signal and delivers the differential-voltage signal to the circuit output at V OUT . to take it a step further. High-Speed. The offset error of the ADC is ±1 LSB or ±1. a small amount of dust. or any other circuit with high-impedance components around your amplifier. Doing so attenuates superimposed. the designer optimizes the stability. November 2001.22 mV. Yet another column details the stability of this circuit. The LSB for a 16-bit system with the same input-voltage range is 76. This is particularly true as long as the real-world remains in the analog domain. which is 1. Bessel. Texas Instruments. A 2 's gain roll-off introduces an upper limit for the bandwidth improvement. C2 = 15 nF). two capacitors. The words of wisdom that came down to me were. you may want to explore other diagnostic avenues. In the complementary-differentialinput-stage topology. the bipolar input-bias current can be fairly stable. May 15. the gain-bandwidth product of the amplifier. 2. increases. The sixth part of the noise equation. lo and behold. and 3. you will primarily find transimpedance amplifiers in the CT (computedtomography)-scanner front end and the pulse oximeter. and BW is the bandwidth of interest. In the region near the positive power supply. it will work. "FilterPro MFB and Sallen-Key Low-Pass Filter Design Program." The next page displays a list of filter response options that meet your requirements. look at the voltage characteristics of each node and make sure that you understand the impact of all of the current paths in your circuit. but it was producing a new frequency. and a single amplifier to configure a low-pass. the input of the amplifier has a large capacitance across its input. You can see this by comparing the intersect of the closed-loop gain (f 3 dB ) with the amplifier GBWP frequency (f GBWP ). and the amplifier's open-loop resistance. Figure 1: A second-order. Several types of single-supply amplifier topologies can accept input signals across the power supplies. 2 Back to top Just use a 100Ω resistor: Understanding a rule of thumb for oscillating amplifiers So there I was. In the diagram. As you examine your circuits. A more accurate descriptor for this current error is "input-leakage current. "A Comparison of Tolerance Analysis Methods. the filter gain begins to increase at a rate of 20 dB/decade. amplifier. the quality factor. In the top three curves. February 1994. The A OL (jΩ) curve intersects the 1/β curve at an interesting point. If the feedback capacitor. a buffer-amplifier circuit. This design topology has significant variations in the amplifier's offset voltage across the common-mode input range. In the e2 region. combining the input and filtering stages separates the signal of interest from the noise floor. All I can say is filter design has come a long way. you use a lowpass filter to prevent aliasing errors from out-of-band noise. T is temperature in Kelvin. are here to stay as standalone devices and as integrated functions inside complex ICs. must be at least 100×gain×f CUT ×k i . Bessel. introduces a pole to the open-loop-gain curve. RF is the feedback resistor in ohms. some characterization is in order. A good tool for determining stability is a Bode plot. equaling 1/(1+Z F /ZIN). the magnitude of that noise stays the same. This circuit requires stability optimization as you balance CF and the input capacitance of A 2 . such as the temperature-sensor circuit in Figure 1. (Figure 1) The transfer function of this system is: where A OL (jΩ) is the open-loop gain of the amplifier over frequency. and compares the results with a look-up table. Q < 3) At unity-gain. the THD+N is 0.0006% if you avoid using the input transition. the cost is lower when a single filter serves many analog inputs. 2010. both LEDs are off for 450 µsec. but it's not necessarily true with the classic Sallen-Key lowpass-filter design. or water molecules can increase leakage current and masquerade as input-bias current. To ensure that your active filter does not enter into a slew condition. You can implement a differential amp discretely or with an off-the-shelf product. Table 1 shows the digital potentiometer program setting for Butterworth filter with corner frequencies ranging from 100 to 10 kHz (C1 = 33 nF. you can cascade both of these topologies. Before selecting the amplifier. the software changes the resistors in the circuit to appropriate values. where A N is the amplifier's input-noise-density at 1 Hz and f B is the corner frequency where the flicker noise tapers off. "A good holiday-season project. We keep the amplifier bandwidths as low as possible.com Follow Us   |    Table of Contents Introduction Chapter 1: Operational amplifiers When is good enough good enough? Just use a 100Ω resistor: Understanding a rule of thumb for oscillating amplifiers Where did all that racket come from? Is your amplifier offset way out of whack? A good holiday-season project Don't be fooled by your amplifier's bandwidth Sallen-Key lowpass-filter stopband limitations Choosing antialiasing-filter amplifiers Closer to real-world analog filters Simply an adjustable low-pass filter Photo-sensing circuits: The eyes of the electronic world are watching Transimpedance-amplifier stability is key in light-sensing applications Transimpedance-amplifier noise issues Transimpedance-amplifier application: The pulse oximeter Remote photo sensing Single-supply amplifier outputs don't swing rail to rail The non-negotiable single-supply operational amplifier What does "rail to rail" input operation really mean? Rail-to-rail input amplifier application solutions Voltage. an amplifier with a singledifferential input stage and charge pump is a more appropriate choice. OPA4350 High-Speed. References 1. For instance. A 1 . scientists quickly discovered the photo-sensing characteristics of silicon in the lab. to name a few applications. 1996. When asking how much noise is too much noise in this photodiode-preamp circuit. system designers understand how to convert light into a current. or 60 dB. backgroundlight conditions can influence this magnitude. OPA364. the differential amp rejects common-mode-voltage changes. CMOS single-supply amplifiers have been beneficial to single-supply-system designers worldwide. Bishop. Figure 1: The system bandwidth becomes lower due to the closed-loop (ACL) intercept point with the amplifier's open-loop gain (AOL ) and the need to accommodate bit accuracy across the entire frequency range at f 1 . Z IN is the distributed input impedance. The first approach was purely analog. Op Amps for Everyone. Photodiode Amplifiers: Op Amp Solutions. where V OUT P-P is the expected peak-to-peak output-voltage swing below your filter's cutoff frequency. step response. March 1999. The slew rate depends on internal IC currents and capacitances. You can easily compensate for this type of error by replacing any of the four resistors with a potentiometer. In this design. The pulse oximeter is a noninvasive instrument that monitors SpO 2 (saturation of hemoglobin with oxygen) in your blood. Finding that brightest bulb among the many and the background light would be a laborious task. "Just do it.9988752. oil." EDN. Steven M. under the "Filter Topology Specifications" section. and a feedback network. 2006. Figure 1 shows an example of how the amplifier's closed-loop gain affects the system's overall bandwidth. the flicker-noise region usually ranges from dc to 100 or 1000 Hz. Design engineers methodically convert the photodetector's current to a usable voltage. a new hire for a leading-edge analog-electronics company. operational amplifier." Medical Electronics. You should also evaluate the effects of amplifier slew rate." EDN. represents the noise contribution of the feedback resistor. it makes sense to keep the bandwidth of the circuit's amplifiers as low as possible. When sensing with a photodiode with a large parasitic capacitance or from a remote site. Sandler. You can avoid this type of amplifier crossover distortion by usingg an inverting configuration. position equipment. the PMOS transistor's offset error is dominant. A 2 . high-frequency filter sections. if you would rather use an amplifier other than the suggested device. Key players affecting the THD+N (total-harmonic-distortion-plus-noise) characteristics of dual-supply amplifiers are input noise and output-stage crossover distortion. such as a fourth. high-frequency noise on the analog signal before it reaches the ADC. Furthermore. converts the photodiode current generated by the LEDs to a voltage at the output. The goal is to select amps in which these parameters are as low as possible. note that a 12-bit converter is at the end of the signal chain. OPA4364. A resistor with a value of 100 kΩ to 10 MΩ connects the inverting input of the amplifier to the output. When this capacitor seed is set. following such logic goes only so far when you try to justify the cost-versus-performance factors of the products you are using. Sallen-Key lowpass filters for each amplifier. Bonnie. 1. the Sallen-Key topology has excellent gain accuracy. To determine the dynamic-range limitations of the circuit. lowpass. position and proximity sensors. and f 3 dB is the corner 3-dB frequency of this amplifier system. however. The amplifier's input noise is another contributor to the THD+N specification. You can find the pole and zero locations in this circuit in the following equations: What have I learned from this situation? It pays to understand why an engineer's rule of thumb works. consider six regions in the frequency spectrum (Figure 1b) and add each region with a root-sum-square equation or the following equation: The first five regions are equal to the multiple of the areas under the closed-loop-gain and amplifier-noise-density curves. removes the cable's capacitance and the photodiode's parasitic capacitance from the input of the transimpedance amplifier. enable the low-pass filter button and type in a filter bandwidth with Ao = 1 V/V. As an example. The unity-gain Sallen-Key topology also requires fewer components–two resistors versus three for the MFB (Figure 2). For instance. "Noise Analysis of FET Transimpedance Amplifiers. 90dB CMRR. you can realize a combination of Butterworth. "Start with the right op amp when driving SAR ADCs. By adding the 1/β phase shift from the A OL (jΩ) phase shift. such as the charged digitizing ADC. the photodiode's currents flow through the R1 feedback resistors. 2008. the engineer said. If the noise on the input signal is more than half the sampling frequency of the converter. 4 Back to top Is your amplifier offset way out of whack? Have you ever spent a great deal of time selecting the perfect operational amplifier for your circuit. linear representation of the light source. A reprogramming of the dual digital potentiometer in Figure 1 changes the filter's frequency cut-off and approximation (Butterworth. the switched integrator has gained favor. search noninvasively for tumors. So much for lower. When calculating the noise performance of the circuit. As the amplifier's open-loop gain decreases. We have a 20-kHz signal that we need to increase by 10 times. A conservative calculation that allows variation in amplifier bandwidth. Please enjoy your read through and let me know what you think! 1 Back to top Chapter 1: Operational amplifiers When is good enough good enough? If you are having difficulty making product-selection decisions in a consumer circuit. indicating a phase shift of –180° and a phase margin of 0°.TI. region is V 1/f:fB–fA =A N . and its margin is 90°. detect smoke. and the parasitic capacitance of the photodiode. References 1. Here. we are not so lucky. and the differential amp removes this effect. In this application. RL ." still with no explanation. March 20. Filter Designer also provides SPICE simulations of the chosen filter's sine wave response. There are many ways to approach the photosensing-circuit problem. The correct formula to determine the attenuation back from the 3-dB point toward 0 Hz where A CL1 is the target closed-loop gain. low-noise performance. The transimpedance amplifier. The good news is that. the amp's outputs change in voltage along with the IR drop across the R1 resistors. Trust me. Ron. Baker. "Bootstrapping reduces amplifier input capacitance. the bandwidth of op amps A. The signal then travels through a bandpass filter and gain stage to the 12-bit ADC. 3. Bode. The gain of this circuit is equal to −R2 /R1 . These filters use independent operational amplifiers (op amps) in combination with fixed resistors and capacitors. Typically this type of filter is used to reduce analog-to-digital converter (ADC) aliasing errors and noise outside the signal bandwidth. A 2 . A reader requested a circuit that reduces the noise impact of a photodiode with a large parasitic capacitance. Baker. it gradually gets closer to the closed-loop gain curve starting about a decade before 2 MHz. An alternative filter design solution is to have one programmable filter after the multiplexer (Figure 1). Using the same logic. A 3 and R2 form a difference amp. In the medical field. I suggest that you try to explain the offset error by re-examining your amplifier's specifications. Because I had a huge community of experts around me. this action also creates a stage whose output is not low-impedance. and the square root of the region's bandwidth. requiring calibrated and impractical measurement conditions. D1 and D2 respond linearly to illumination intensity. Jerald G.and current-feedback amps are almost the same Chapter 2: INAs and PGAs Understanding CMR and instrumentation amplifiers Common-mode range can bite hard Instrumentation amplifier input-circuit strategies Wringing out thermistor nonlinearities A good holiday-season project Author's biography   Introduction The Best of Baker's Best – Amplifiers Introduction. unity gain. you may experience poor judgment or loss of motor function. When you consider the input-voltage noise of the amplifier. the required accuracy in these applications continues to increase. Bonnie. The Sallen-Key filter attenuates any signals in the frequency range above the cutoff frequency to a point. References 1. The only path of escape for this charge is through the highvalue resistor in the amplifier's feedback loop. This inversion may or may not be a concern in the application circuit. In this view. For Q i greater than 1. you can quickly solve this problem by choosing the absolute best performing parts for each socket. I did not return to his suggestion for several years. and CL and RL then introduce a zero to the open-loop-gain curve (Figure 1). When using the Sallen-Key circuit. Neil. or to change the amplifier to have a different frequency response or different input capacitance. Eventually. A unity-gain buffer. The caveat to this action is that the following filter may interfere with the phase response of your intended filter. You can reduce the impact of the upward trend in the filter's response by following the offending active filter with a passive RC lowpass filter. The relationship between Q and the damping factor is Q = 1/2 ζ. "Photo-sensing circuits: The eyes of the electronic world are watching. and low output impedance. To optimize a complementary-input-single-supply amplifier's THD+N performance. This calculation illustrates that the sensor cell contributes the most error with little ADC's input is impact from the amplifiers or the ADC. Mancini. If the system requires the amplifier to be configured as a noninverting buffer. The data indicates that the lowpass filters are performing as you would expect for a little more than a decade after the cutoff frequency of 1 kHz. Consequently. and Avner Cohen. the R2 /R1 resistor ratio and the resistor errors determine the gain. To further exacerbate the difficulty of the design. Figure 1 shows the front-end circuit of this system. fs = 1000. you can also select the supply requirements of Single Supply = +5 V. or 20 dB. Amplifiers. Alternative filters can solve the problem without adding an RC filter. W. Grame. the NIR LED is on for 50 µsec. References 1. for these circuits. If the closure rate of these two curves is 40 dB/decade. The traditional design topology of the transimpedance amplifier captures this low-level signal in a hybrid approach that starts with an amplifier and a high-value resistor in the feedback loop. "OPA350. these filters use a Butterworth design. resulting in an inverted output after the third stage." AEi Systems LLC. and then both LEDs are off for 450 µsec. if you exercise special care. you can use a bootstrap circuit (Figure 1). current. "Transimpedance-amplifier stability is key in light-sensing applications. Rail-to-Rail I/O Operational Amplifier. Texas Instruments. MD. The magnitude of the bipolar amplifier's input-bias current ranges from a few nanoamperes for low-power devices to hundreds of nanoamperes for higher-power devices. You cannot use a digital filter to reduce in-band noise after digitizing the signal. 2. The output voltages of A 1 and A 2 contain both difference and common-mode signals.22 mV. you can use a multiple-feedback circuit. we already require an amplifier that has a bandwidth that is 10 times higher than our input signal. The amplifier bandwidth calculation involves variations of the amplifier's bandwidth and variations of the passive components over time. Baker. the photodiode. A MFB single-stage (second-order filter) provides two poles while inverting the signal. f 1 is the target is f 1 =f 3 dB × frequency. The signal also travels through a lowpass filter to regulate the driver power to the LEDs. Selecting the correct amplifier. Townsend. during your first consumer-product-selection attempt. Of course. Chebyshev) of this second-order. and 2. Jerald. each filter's response flattens at the 0-dB crossing frequency of the op amp's open-loop gain. References 1. Rail-to-Rail OPERATIONAL AMPLIFIERS MicroAmplifier ™ Series. you can use RSS calculations. You cannot use an RSS formula with entities that have correlated variations that are not statistically independent. System elements determining the noise-gain frequency response are the photodiode's parasitics and the operational amplifier's input capacitance. Your circuit also may require a low Q. and even smoke detectors. 2008. This crossover-distortion phenomenon affects the amplifier's THD. A 20-MHz bandwidth for an amplifier should be good enough. it will come back to bite you. the contributed noise in this region is usually relatively low because of its location in the lower frequency range. the leakage current increases approximately two times per 10°C change. Aug 7. When designing this circuit. Oljaca. As the output signal sweeps from rail to rail. More than likely.and NIR-LED signals. the closed-loop dc-noise gain (1+R F /RPD). antialiasing filter. into an electrical form. 13 Back to top Transimpedance-amplifier noise issues How much noise is too much noise in a photodiode-preamplifier circuit? You can derive the noise performance of a transimpedance amplifier (Figure 1a) with calculations or by using a Spice simulation. it is possible to have a variety of signals that reach the ADC where each signal source has its own set of filter requirements. but let's do the math. the input crossover distortion can affect the amplifier's THD+N performance. Although the light sensitivity of silicon is an undesirable byproduct of the silicon. Photodiode Monitoring with Op Amps.3% increase is needed to get up to that curve. In the circuit in Figure 1. 7MHz. Both LSBs are peak-to-peak numbers. you can build a PCB that will adhere to a 1-pA performance specification. It solved my problem. CF . but we are looking for the correct amplifier for a 16-bit system. "Pulse Oximetry. Bonnie. For linear operation. you need to select an amplifier such that the slew rate (πV OUT P-Pf CUT ). You can use filterdesign programs to determine the filter's capacitor and resistor values. only to find that the offset voltage is wrong at the manufacturer's bench-specified input? What if you find that it is more than 10 times higher than specification in your application circuit? Do you send the chip in for failure analysis or just toss the chip out and have another look at your list of amplifiers? As an alternative. presents an interesting challenge. but the real challenge is determining how to convert the low-level currents from the photo sensor into a useful electrical representation. Michaelmas 2001. place the amplifier in an inverting-gain configuration and keep the closed-loop gain low. January 1995. References 1. and the ADC (IC3 ) offset error is ±1. A second consideration is that the low-frequency voltage noise of your amplifier must be very low. http://www. This situation places our amplifier unity-gain bandwidth at 20 MHz. 2008. References 1. and Bonnie Baker. 1998. return to page one and set the conditions for the next filter. the input bias current conducts through the external source resistance. a silicon sensor converts light into charge or an electrical current. and comparing that ratio with an SpO 2 look-up table in the microcontroller. low-pass unity gain filter. which comprises a photodiode. 2. which is approximately 70. the amplifier contributes an approximately –90° phase shift. and temperature drift. a 100 kΩ 8-bit digital potentiometer. all three of the filter's responses show a slope of –40 dB/decade. J. ISBN 0-07-024247-X. where CPD is the device's capacitance and CDIFF is the differential amplifier's capacitance. To produce the remaining filters in Table 1. A calculation proves that the contribution to noise in this low-frequency region is relatively low: where RF is the feedback resistor and RPD is the device's parallel resistance. amplifier-feedback element. Single-Supply. (The full-scale range of the ADC is 5V. the amplifier is nonlinear. These layout headaches come from putting higher-speed amplifiers in the circuit that are producing fast rise and fall times. we may be tempted to match the voltage-feedback amplifier's data-sheet bandwidth to the bandwidth of our analog signal source. "Oh. the transfer function for this circuit is expressed as The V B voltage can be anywhere between ground and V S. differential-input stage. The single differential-input stage with a charge pump buys you a 20. noninverting amplifier. "A good holiday-season project. The difference between these two topologies in this figure is obvious. the CMRR is equal to 20*log (ΔV CM /V OS ). Sept 4. For proper operation. There are circuit-design tricks at your disposal for minimizing this input-stage crossover effect. taking the ratio between the intensities from a photodiode. V H is the maximum voltage level at the output in the dc-open-loop-gain measurement. V H is less than V OH. They began to use the all-too-common charge pump to push a single differential-input stage of the amplifier above the positive-power supply (Figure 1). 3. the input of the amplifier has a large capacitance across its input. 50MHz. Figure 3: The inverting-amplifier circuit keeps the amplifier's common-mode voltage at a constant voltage. "Where did all the racket come from?" EDN.004%. an op amp with a composite input topology may limit the input range of the buffer. the amplifier is nonlinear. if they have the proper input circuitry to support this function. This disparity in specification only points out that the amplifier's input stage has a composite input topology. and the NMOS transistors are completely off. Single-Supply Rail-to-Rail Operational Amplifiers. The amplifier output voltage swings from 140 mV to 4. In many applications. and the PMOS transistors are off. Figure 1 shows a simplified block diagram of a pulse oximeter. the system THD is –88 dB. "Bootstrapping reduces amplifier input capacitance." Texas Instruments. 17 Back to top The non-negotiable single-supply operational amplifier Fundamental analog devices that serve applications such as high-resolution delta-sigma or SAR (successive-approximation-register) converter systems are feeling the crunch from amplifiers that have difficulty with achieving good rail-to-rail input performance. In the circuit in Figure 1. The composite input stage is one example. The op-amp topology in Figure 1 provides superior common-mode performance over the entire input range. photographic analyzers. At lower common-mode input voltages. the THD of an ADC driven by an op amp in a buffer configuration is the root-sum square of distortion contributions of the ADC and op amp. A 1 . I'll talk about the rail-to-rail amp's output stage and its ability to achieve rail-to-rail performance. "Photo-sensing circuits: The eyes of the electronic world are watching. When using composite input amplifiers. The noninverting amplifier circuit gains the input signal with a voltage level-shift implemented with V B. A 2 's gain roll-off introduces an upper limit for the bandwidth improvement. For linear operation. "Transimpedance-amplifier stability is key in light-sensing applications. References 1. This increase has a positive effect on amplifiers in buffer configurations. you may experience poor judgment or loss of motor function." EDN. you will see what I mean. CF . the headroom between the signal and rails is 140 mV. The microcontroller acquires the signals from the 12-bit ADC. or even undergoing surgery. to keep the output range at V O between the power-supply rails. Texas Instruments. In this configuration. Near the rail. Since the V B voltage is static. Jerald G. and feedback-capacitance elements limit the bandwidth of the circuit. If a pulse oximeter indicates that your oxygen levels are stable. When you bring V IN+ toward the negative rail. "OPA365. It may be worthwhile to use an autozero amplifier. and compares the results with a look-up table. A good approach is to make the noise of A 2 less than or equal to that of A 1 . the NIR LED is on for 50 µsec. The result of this added capacitance increases the circuit's noise gain unless you increase the amplifier's feedback capacitor. however. current. The V OH maximum specification is V DD –20 mV. engineers continue to demand lower system power supplies and insist on better signal integrity.0004%. A common single-supply amplifier input topology has parallel PMOS and NMOS differential input stages. A 2 . Single-supply amplifiers continue to keep pace with high-resolution converters because engineers implement innovative amplifier-circuit topologies. As the common-mode input voltage increases. In the region near ground. position and proximity sensors. V B. A typical common-mode rejection specification for an amplifier with a composite input stage has two or more CMRR specifications (Table 1). This specification encompasses the full amplifier input range. 3. The trend toward designing single-supply op amps started in the 1970s with a single differential-input stage that spanned a portion of the common-mode input range. 2010. the photodiode. Photodiodes with a relatively low diode capacitance do not benefit from this circuit. This circuit requires stability optimization as you balance CF and the input capacitance of A 2 .com/ww/en/analog/Amplifiers-eBook/[1/14/14 10:58:49 AM] . Later. Recalling a circuit from another column. into an electrical form. The transimpedance amplifier appears in medical and laboratory instrumentation. If the circuit designer wants a distortion-free output from this amplifier circuit. where V OUT is the output voltage and V OS is the input offset voltage. Medical Instruments Applications Guide." Single-supply amplifier manufacturers also claim they have devices that will swing rail-to-rail on the output.8V below the positive power-supply rail as 92 dB (typ). In this circuit. V OH is the absolute maximum voltage level with respect to V DD (drain-to-drain voltage) that the output can reach.com Have you ever been in need of a second opinion when your sanity is at stake? The pulse oximeter may be able to provide that second opinion if your brain is oxygen-deprived.to 30-dB increase in the amplifier's CMMR (common-mode-rejection ratio). The signal also travels through a lowpass filter to regulate the driver power to the LEDs. The amplifier's linearity starts to degrade long before reaching the output-swing maximums. The offset voltage of an op amp with a composite input varies across changes in the common-mode voltage. Table 1: The single-supply op-amp data sheet describes the CMRR test conditions as well as the actual specified values. To achieve optimum performance. June 2006. you can read about them in "Rail-to-rail input amplifier application solutions. the PMOS transistors are completely on. There are more ways to tackle this input topology problem with the IC design. your system's performance will improve. In fact. OPA344 data sheet.1 to 2V. When designing this circuit. circuit designers need to consider the behavior of the single-supply op-amp input stage. the rail-to-rail input operation across the complete amplifier's rail-to-rail common-mode range (Reference 1). the PMOS offset-error portion of the input stage is dominant. light. the V OL minimum specification is 15 mV above ground. and 4 kHz and so on. A normal output for the pulse oximeter is approximately 97%±2%. in this circuit. Figure 2 compares the response of the charge pump to composite amplifier inputs. Bottom line: noninverting-amplifier circuit Composite input: the input signal (VIN) must not travel into the transition region of the PMOS and CMOS differential input stages Charge-pump input: performs without distortion in reaction to common-mode voltage or V IN changes With a unity-gain buffer (Figure 5). combining the advantages of those stages to achieve actual rail-to-rail input operation (Figure 1). Baker. However. Alternatively. In this 5V-supply system. Line 3 specifies the CMRR across the entire input range as 70 dB (typ. ranging from 95 to 100%. but one issue came to mind as I read the comments in the Talkback section for a recent column. If the input signal always remains less or more than the composite input-stage’s transition voltage. you exchange one noise problem. 2008.TI. Texas Instruments. Low-Noise. the NMOS transistors start to turn on. You can also expect almost a tenfold decrease in the amplifier's THD (total-harmonic-distortion) performance. You can keep this difference low selecting A 2 with wider bandwidth than A 1 and keeping A 2 's output impedance low. A reader requested a circuit that reduces the noise impact of a photodiode with a large parasitic capacitance. Finally. and the parasitic capacitance of the photodiode. 1978.ti. making A 2 's bandwidth much greater than that of A 1 . The circuit requires a bias voltage. Application solutions Circuit designers can use rail-to-rail input op amps in virtually any op-amp configuration. a regulated charge pump lifts the top of the differential input as well as its biasing current source to 1. VB can be assigned so that the input signal is not required to go across the composite input stage’s crossover region. or complementary. The alarms on the pulse oximeter usually sound when the SpO 2 level drops below 90%. as long the voltage at V O remains between the power-supply rails. In Figure 1. With this design. This is true for the inverting-amplifier circuit found in Figure 3. Design engineers methodically convert the photodetector's current to a usable voltage. 19 Back to top Rail-to-rail input amplifier application solutions Many single-supply operational amplifiers are capable of rail-to-rail input operation. The unity-gain buffer removes the noise effect from A 1 . the composite amplifier exhibits some limitations in linearity due to the input-stage topology. If the input signal always remains less than the composite input stage’s transition voltage.and NIR-LED signals. with the PMOS transistors completely off. the input signal (VIN) can traverse from one rail to the other rail. if an operational amplifier with a complementary input stage has a THD specification of 0. Figure 1: This composite input stage of the op amp uses PMOS and NMOS differential pairs so the input-voltage range can extend from above the positive rail to below the negative rail." EDN. In the medical field.1V. and comparing that ratio with an SpO 2 look-up table in the microcontroller. an operational amplifier. Townsend. In the region near the positive power supply. There are many ways to approach the photosensing-circuit problem. A 1 . LMP7701 data sheet. V IN. but it can get close. Figure 2a illustrates the nonlinearity-output-stage effects with a single-supply CMOS amplifier by showing distortion at 2. Vendors of single-supply op amps claim their amplifiers have rail-to-rail input capability. hiking in the high altitude of a mountain range.8V. which makes the manipulation of the photodiode signal manageable. you can see an op amp's output limitations to swinging rail to rail when the op amp is driving an ADC. When sensing with a photodiode with a large parasitic capacitance or from a remote site. From approximately 1. Figure 2: As the amplifier's common-mode voltage changes from ground to the positive supply. Revisiting single-supply input topologies The CMRR (common-mode rejection ratio) specification describes changes in the amplifier's offset-voltage versus common-mode input changes. At approximately 1. 2. 3. and the amplifier output never reaches either rail. allowing the amplifier's output swing to go to and well beyond the power-supply rails.8V above the powersupply voltage. 2. When you choose your device for the pulse-oximeter transimpedance-amplifier circuit. The transfer function for the circuit is expressed as The input common-mode voltage (VIN) can vary between ground and V S. Bottom line: inverting-amplifier circuit Composite input: performs without distortion with V B outside the PMOS/NMOS transition region Charge-pump input: performs without distortion The noninverting amplifier’s common-mode voltage (Figure 4) is equal to the input voltage. Texas Instruments. When you drive the output high. 1996. Figure 2: The op-amp offset voltage of an op amp with a charge-pump input remains constant across changes in the common-mode voltage. V B. Amplifier designers place the switching mechanism's frequency above the amplifier's bandwidth and keep the switching noise lower than the amplifier's thermal noise floor. Single-supply-amplifier. scrutinize the impact of the flicker noise. If V IN travels across this entire range. crossover phenomena. Figure 1 shows a typical single-supply amplifier's output swing as you drive the output to the rails. Figure 4 The noninverting gained amplifier configuration allows the amplifier’s commonmode voltage to change with input signals. the red LED is on for 50 µsec. Bonnie. Bonnie. OPA2365 2. Using these equations. Good luck! References 1. the bandwidth of the circuit decreases. R2 . McGraw-Hill. The 4. 2. the input stage of the CMOS amplifier completely changes from its PMOS input pair to its NMOS input pair at -2V below the 3V positive supply rail. the output of single-supply amplifiers can come within only 50 to 300 mV of each rail (Figure 1). ISBN 0-07-024247-X. if you use an amplifier that has a charge pump in its input stage to drive highprecision SAR or delta-sigma converters. the data looks perfect with only the ADC distortion (Figure 2b). The conditions of the dc-open-loop-gain specification define the amplifier's linear operating output range. the amplifier's common-mode voltage remains constant. Alternatively. You measure the oxygen in the blood by alternating the on-times of a red LED with a 650-mm wavelength and an NIR (near-infrared) LED with a 940-mm wavelength. April 23. Baker. where CA1 and CA2 represent the sum of their input differential and common-mode capacitance. to keep the output range at V O between the powersupply rails. You may recall that this topology exhibited an offsetvoltage. These specification tables provide evidence of the characteristics of the input-stage topology. If there is a shortage of oxygen in your system. In Table 1. Baker. the NMOS section of the circuit finally takes over. the input range extends at least 100 mV beyond both power-supply rails. The best way to view the input stage's behavior is to look at the offset voltage versus the common-mode input voltage (Figure 2). The system repeats this cycle continuously. March 2006. V B. 2010. Neil. From a signal-chain perspective. The transfer function of the circuit in Figure 5 is shown here: The input common-mode voltage (VIN) can vary between power-supply rails. you need to make sure that the amplifier's input-bias current is very low or in a picoamp region at 25°C. 2009. the PMOS transistors are in operation. computes the ratio of the red. Neither of these approaches produced an amplifier adequate for the high-precision systems to span the amplifier's full common-mode input range. crossover distortion as the input common-mode voltage traveled across the full rail-to-rail input voltage range. if the op amp's input stage has a charge pump with a THD specification that is 0. This increased headroom allows for a single differential PMOS input stage to replace the composite differential input stage. amplifier." Medical Electronics. The charge pump is a good stopgap. Graeme. When V B establishes the commonmode voltage of the amplifier. The pulse oximeter is a noninvasive instrument that monitors SpO 2 (saturation of hemoglobin with oxygen) in your blood. choose the V B voltage to be below or above the PMOS/NMOS transition region. read your data sheet and refer to the conditions on the open-loop-gain test. and V OL is the absolute minimum voltage level that the output can reach. When using a single-supply amplifier. Line 2 specifies the CMRR from ground to 1. FET. the NMOS offset error dominates. there are performance compromises that the circuit designer must address. the output cannot go all the way to the rails. "Pulse Oximetry. converts the photodiode current generated by the LEDs to a voltage at the output. When you consider the input-voltage noise of the amplifier.4V. A 1 . A good rule of thumb is to have CA2 <<(CA1 +CCA+CPD). The LCD shows a percentage of oxygenated hemoglobin versus nonoxygenated hemoglobin and your heart rate. a wider bandwidth than A 1 . The dc open loop gain in decibels is 20 log(ΔV OUT /ΔV OS ). Next time. however. the circuit does not create distortion from the composite’s input stage. Aug 7.6MHz. The signal then travels through a bandpass filter and gain stage to the 12-bit ADC. as V IN changes. V IN establishes the amplifier’s common-mode voltage. the system THD is: where THDOPA =20log(THD OPA–% ×100) and THDOPA–% is the THD specification in the operational amplifier's data sheet in units of percentage. The design topology in Figure 1 can have wide variations in offset voltage across the amplifier's common-mode input range. rail-to-rail-output ads can give a false sense of security. If the circuit designer uses the amplifier in a voltage follower or buffer configuration. Photodiode Amplifiers: Op Amp Solutions. such as an input stage with a charge pump. with some distortion. both LEDs are off for 450 µsec. low noise. Baker. or perhaps you just dance to the beat of a different drummer. 16 Back to top Single-supply amplifier outputs don't swing rail to rail Single-supply amplifiers do not truly swing rail to rail at the output. 2008. To fix this problem. V O. This condition could affect you if you are a pilot. V S=5V). and the 16-bit SAR ADC has a THD specification of –99 dB. increases. V IN. The simple rail-to-rail operational amplifier must have a transistor design that spans the power supply with minimal distortion. bar-code scanners. In this design. the charge-pump design can provide an output ripple voltage that is low enough so as to not produce undesirable noise or distortion at the output of the op amp. the noninverting amplifier circuit may require a bias voltage. Here V B establishes the common-mode voltage of the amplifier. March 20." Texas Instruments. and V L is greater than V OL . The FFT plot in Figure 2a shows the amplifier/ADC-combination response to a 1-kHz signal in a 5V system. with the NMOS transistors turned off. the NMOS transistors are in use. A classic photo-sensing system circuit has a photodiode. MD. crossover phenomena. The two stages shared. 2. Bonnie.Texas Instruments . With those types of amplifiers. Composite input op amps can create distortion at the output. the circuit will not create distortion from the composite’s input stage. in the amplifier's feedback loop. Bonnie. The inverting-amplifier circuit gains the input signal as well as changes the signal polarity. with another. Let's consider the behavior of the composite and chargepump amplifier inputs in a single-supply inverting amplifier. removes the cable's capacitance and the photodiode's parasitic capacitance from the input of the transimpedance amplifier. For this amplifier. A typical common-mode rejection specification for an amplifier with a charge-pump input stage has one CMRR specification." EDN. the amplifier's common-mode input voltage can remain at a static voltage.or CMOS-amplifier input devices usually meet this requirement. the cable capacitance. A 2 . Eventually. and low output impedance. OPA2333 1. and then both LEDs are off for 450 µsec. 4. V O. Although the precision. 15 Back to top Remote photo sensing Photodiodes transform a basic physical occurrence. The amplifier's typical closed-loop bandwidth is about 3 MHz with a typical slew rate of 2. you will primarily find transimpedance amplifiers in the CT (computedtomography)-scanner front end and the pulse oximeter. and even smoke detectors. 5. R1 . both the PMOS and NMOS transistors are operating. and V IN) keeps the output (VO) between V S and ground. References 1. After the transimpedance amplifier. low-power OPA344 input stage in Figure 1 has rail-to-rail input operation. So. When you bring the input terminals to the positive rail. 18 Back to top What does "rail to rail" input operation really mean? A hot discussion topic with single-supply operational amplifiers is whether they are capable of rail-to-rail input or output operation. The only important performance specifications are low input capacitance. the system THD becomes –98 dB. References 1. IC designers borrowed a technology from other devices to solve this problem. The specification conditions can subtly describe the amplifier's rail-to-rail input topology. microPower CMOS Operational Amplifiers. Line 1 verifies that the amplifier has true rail-to-rail input capability. the amplifier's initial offset error and overtemperature should be in the microvolt region if you want to minimize linearity errors. By reducing the amplifier's output signal to 272 mV from each rail. "OPA333. As a second ICdesign strategy. This article is an expansion of the blog post "What does 'rail to rail' input operation really mean?" By examining an additional inputstage topology and implementing these op amps into some common op-amp circuits.3V/µsec. Yet another column details the stability of this circuit. Bottom line: buffer circuit Composite input: the op amp’s input range (VIN) must not travel into the transition region between the PMOS and CMOS differential http://www. an amplifier with a charge-pump input stage does not create this unwanted distortion across the entire input range. For example. V L is the minimum voltage level at the output in the dc-open-loop-gain measurement. a bandpass filter eliminates the noise above 5 Hz. The difference between the input signal and the output signal in this system falls across the cable/diode capacitance. the composite input amplifier produces distortion at the circuit output. Kurz. of the buffer circuit changes the amplifier’s common-mode voltage. which in this case is an amplifier with a composite input stage. and Avner Cohen. a unique zero-crossover input topology provides superior common-mode performance over the entire input range. A second consideration is that the low-frequency voltage noise of your amplifier must be very low. you'll find that the type of amplifier you select for A 2 is somewhat easy. designers added a second. Note that this specification does not describe the range specified in Line 1.66V. you can use a bootstrap circuit (Figure 1). RF . but the chip designer has to make some compromises to achieve this type of performance. Switching from one differential input stage to the other causes this distortion. If the feedback capacitor. read the fine print! Some single-supply amps have output-stage charge pumps. Zero-Drift Series. as long as the combination of the elements in this circuit (VB. The transimpedance amplifier. Dov. Figure 1: This single PMOS differential input stage in conjunction with a high-side chargepump eliminates the common-mode crossover distortion found in composite input stages. The input capacitance of the buffer replaces the summation of the input capacitance of A 1 . Dec 15. You may think that the charge pump's output ripple voltage will become a noise problem. The article discusses a composite input-stage topology for single-supply op amps. and a feedback resistor/capacitor pair at the front end." EDN. Figure 5 The input signal. rail-to-rail input/output LMP7701 CMOS amplifier in Figure 2 exhibits the offset-voltage-error crossover behavior around 1. you may want to explore other diagnostic avenues. and buffer circuits. V S. A unity-gain buffer. with an input voltage of 4V p-p.2V. The amplifier's input-bias current creates an output-voltage error by conducting through the high-impedance resistor. A 2 's input capacitance is the only capacitance that plays a role in the ac-transfer function of the transimpedance system. In every case. Michaelmas 2001. When using composite input amplifiers. the NMOS section of the circuit finally takes over. The amplifier's input-bias current creates an output-voltage error by conducting through the high-impedance resistor. Single-supply amplifiers continue to keep pace with high-resolution converters because engineers implement innovative amplifier-circuit topologies. The V OH maximum specification is V DD –20 mV." EDN. IC designers borrowed a technology from other devices to solve this problem. It may be worthwhile to use an autozero amplifier. however. The system repeats this cycle continuously. A normal output for the pulse oximeter is approximately 97%±2%. There are more ways to tackle this input topology problem with the IC design. The unity-gain buffer removes the noise effect from A 1 . At lower common-mode input voltages. This increased headroom allows for a single differential PMOS input stage to replace the composite differential input stage. The simple rail-to-rail operational amplifier must have a transistor design that spans the power supply with minimal distortion. the data looks perfect with only the ADC distortion (Figure 2b). At approximately 1. The single differential-input stage with a charge pump buys you a 20. the system THD becomes –98 dB.Texas Instruments . A typical common-mode rejection specification for an amplifier with a charge-pump input stage has one CMRR specification. "OPA365. The inverting-amplifier circuit gains the input signal as well as changes the signal polarity. the bandwidth of the circuit decreases. but one issue came to mind as I read the comments in the Talkback section for a recent column. The 4. Neither of these approaches produced an amplifier adequate for the high-precision systems to span the amplifier's full common-mode input range. Figure 1 shows a typical single-supply amplifier's output swing as you drive the output to the rails. OPA2365 2. In the region near the positive power supply. References 1. current. When sensing with a photodiode with a large parasitic capacitance or from a remote site. as long the voltage at V O remains between the power-supply rails. Design engineers methodically convert the photodetector's current to a usable voltage. computes the ratio of the red. The transimpedance amplifier. In this design. with another. 2008. The dc open loop gain in decibels is 20 log(ΔV OUT /ΔV OS ). I'll talk about the rail-to-rail amp's output stage and its ability to achieve rail-to-rail performance. OPA2333 1. The difference between the input signal and the output signal in this system falls across the cable/diode capacitance. The noninverting amplifier circuit gains the input signal with a voltage level-shift implemented with V B. Michaelmas 2001. an op amp with a composite input topology may limit the input range of the buffer. 5. Recalling a circuit from another column. For linear operation. Figure 3: The inverting-amplifier circuit keeps the amplifier's common-mode voltage at a constant voltage. taking the ratio between the intensities from a photodiode. A 1 . the amplifier is nonlinear. and low output impedance. 2010. A 2 . Graeme. the red LED is on for 50 µsec. 3." Texas Instruments.com Have you ever been in need of a second opinion when your sanity is at stake? The pulse oximeter may be able to provide that second opinion if your brain is oxygen-deprived. LMP7701 data sheet. In the circuit in Figure 1. The signal also travels through a lowpass filter to regulate the driver power to the LEDs. V S=5V). This disparity in specification only points out that the amplifier's input stage has a composite input topology. "Photo-sensing circuits: The eyes of the electronic world are watching. you may want to explore other diagnostic avenues. a regulated charge pump lifts the top of the differential input as well as its biasing current source to 1. combining the advantages of those stages to achieve actual rail-to-rail input operation (Figure 1). Kurz. References 1. to keep the output range at V O between the power-supply rails. 2. the amplifier's common-mode voltage remains constant. MD. engineers continue to demand lower system power supplies and insist on better signal integrity. V B. In the region near ground. If there is a shortage of oxygen in your system. the headroom between the signal and rails is 140 mV. microPower CMOS Operational Amplifiers. V H is less than V OH. Photodiodes with a relatively low diode capacitance do not benefit from this circuit.8V above the powersupply voltage. "OPA333. When V B establishes the commonmode voltage of the amplifier. and the parasitic capacitance of the photodiode. "Transimpedance-amplifier stability is key in light-sensing applications. choose the V B voltage to be below or above the PMOS/NMOS transition region. your system's performance will improve. In the medical field. and then both LEDs are off for 450 µsec." Medical Electronics. the transfer function for this circuit is expressed as The V B voltage can be anywhere between ground and V S. 1978. To achieve optimum performance. if an operational amplifier with a complementary input stage has a THD specification of 0. the PMOS offset-error portion of the input stage is dominant. Vendors of single-supply op amps claim their amplifiers have rail-to-rail input capability. V IN. with an input voltage of 4V p-p. Figure 2a illustrates the nonlinearity-output-stage effects with a single-supply CMOS amplifier by showing distortion at 2. Bottom line: inverting-amplifier circuit Composite input: performs without distortion with V B outside the PMOS/NMOS transition region Charge-pump input: performs without distortion The noninverting amplifier’s common-mode voltage (Figure 4) is equal to the input voltage. The op-amp topology in Figure 1 provides superior common-mode performance over the entire input range.1V.TI. Sept 4. V H is the maximum voltage level at the output in the dc-open-loop-gain measurement. McGraw-Hill. You can also expect almost a tenfold decrease in the amplifier's THD (total-harmonic-distortion) performance. such as an input stage with a charge pump. The input capacitance of the buffer replaces the summation of the input capacitance of A 1 . V S. an operational amplifier. Figure 1: This single PMOS differential input stage in conjunction with a high-side chargepump eliminates the common-mode crossover distortion found in composite input stages. and V L is greater than V OL . These specification tables provide evidence of the characteristics of the input-stage topology. 18 Back to top What does "rail to rail" input operation really mean? A hot discussion topic with single-supply operational amplifiers is whether they are capable of rail-to-rail input or output operation. rail-to-rail input/output LMP7701 CMOS amplifier in Figure 2 exhibits the offset-voltage-error crossover behavior around 1. 2009. A 1 . The microcontroller acquires the signals from the 12-bit ADC. scrutinize the impact of the flicker noise. R2 . March 20. A 1 . When you bring the input terminals to the positive rail. the PMOS transistors are completely on. a bandpass filter eliminates the noise above 5 Hz. However. an amplifier with a charge-pump input stage does not create this unwanted distortion across the entire input range. Single-supply-amplifier. 2010. Figure 2: As the amplifier's common-mode voltage changes from ground to the positive supply. The article discusses a composite input-stage topology for single-supply op amps. Alternatively. To fix this problem. ISBN 0-07-024247-X. Amplifier designers place the switching mechanism's frequency above the amplifier's bandwidth and keep the switching noise lower than the amplifier's thermal noise floor. Bonnie. and Avner Cohen. CF . and the amplifier output never reaches either rail. The design topology in Figure 1 can have wide variations in offset voltage across the amplifier's common-mode input range. In Figure 1. crossover phenomena. This circuit requires stability optimization as you balance CF and the input capacitance of A 2 . as V IN changes. RF . you can read about them in "Rail-to-rail input amplifier application solutions. If the feedback capacitor. of the buffer circuit changes the amplifier’s common-mode voltage. FET. the V OL minimum specification is 15 mV above ground. the input signal (VIN) can traverse from one rail to the other rail. 2. A classic photo-sensing system circuit has a photodiode. The trend toward designing single-supply op amps started in the 1970s with a single differential-input stage that spanned a portion of the common-mode input range. You can keep this difference low selecting A 2 with wider bandwidth than A 1 and keeping A 2 's output impedance low. as long as the combination of the elements in this circuit (VB. "A good holiday-season project. Note that this specification does not describe the range specified in Line 1. "Pulse Oximetry. with the PMOS transistors completely off. and the NMOS transistors are completely off.3V/µsec. the input stage of the CMOS amplifier completely changes from its PMOS input pair to its NMOS input pair at -2V below the 3V positive supply rail. V B.2V. Baker. you may experience poor judgment or loss of motor function. you need to make sure that the amplifier's input-bias current is very low or in a picoamp region at 25°C. Yet another column details the stability of this circuit.8V. Table 1: The single-supply op-amp data sheet describes the CMRR test conditions as well as the actual specified values. the NIR LED is on for 50 µsec. both LEDs are off for 450 µsec. V IN. in the amplifier's feedback loop. Single-Supply Rail-to-Rail Operational Amplifiers. If the circuit designer wants a distortion-free output from this amplifier circuit. circuit designers need to consider the behavior of the single-supply op-amp input stage. 4. a wider bandwidth than A 1 . 17 Back to top The non-negotiable single-supply operational amplifier Fundamental analog devices that serve applications such as high-resolution delta-sigma or SAR (successive-approximation-register) converter systems are feeling the crunch from amplifiers that have difficulty with achieving good rail-to-rail input performance. 1996. April 23. A reader requested a circuit that reduces the noise impact of a photodiode with a large parasitic capacitance. ranging from 95 to 100%. if they have the proper input circuitry to support this function. Switching from one differential input stage to the other causes this distortion. Zero-Drift Series. Bonnie. you can see an op amp's output limitations to swinging rail to rail when the op amp is driving an ADC. and even smoke detectors. In fact. with some distortion. you will primarily find transimpedance amplifiers in the CT (computedtomography)-scanner front end and the pulse oximeter. and feedback-capacitance elements limit the bandwidth of the circuit." Texas Instruments. The alarms on the pulse oximeter usually sound when the SpO 2 level drops below 90%. "Where did all the racket come from?" EDN. 16 Back to top Single-supply amplifier outputs don't swing rail to rail Single-supply amplifiers do not truly swing rail to rail at the output. which in this case is an amplifier with a composite input stage. The transimpedance amplifier appears in medical and laboratory instrumentation. V IN establishes the amplifier’s common-mode voltage. Texas Instruments. From a signal-chain perspective. A 2 's input capacitance is the only capacitance that plays a role in the ac-transfer function of the transimpedance system. You may think that the charge pump's output ripple voltage will become a noise problem. The transfer function for the circuit is expressed as The input common-mode voltage (VIN) can vary between ground and V S. but it can get close. The FFT plot in Figure 2a shows the amplifier/ADC-combination response to a 1-kHz signal in a 5V system. the THD of an ADC driven by an op amp in a buffer configuration is the root-sum square of distortion contributions of the ADC and op amp." EDN.ti. differential-input stage. Good luck! References 1. the input range extends at least 100 mV beyond both power-supply rails. bar-code scanners. The amplifier's linearity starts to degrade long before reaching the output-swing maximums. The signal then travels through a bandpass filter and gain stage to the 12-bit ADC. Near the rail. For example. A 2 . Finally. Figure 2: The op-amp offset voltage of an op amp with a charge-pump input remains constant across changes in the common-mode voltage. The conditions of the dc-open-loop-gain specification define the amplifier's linear operating output range. A 2 's gain roll-off introduces an upper limit for the bandwidth improvement. 19 Back to top Rail-to-rail input amplifier application solutions Many single-supply operational amplifiers are capable of rail-to-rail input operation.and NIR-LED signals. The best way to view the input stage's behavior is to look at the offset voltage versus the common-mode input voltage (Figure 2). the photodiode. By reducing the amplifier's output signal to 272 mV from each rail. Texas Instruments. This article is an expansion of the blog post "What does 'rail to rail' input operation really mean?" By examining an additional inputstage topology and implementing these op amps into some common op-amp circuits. VB can be assigned so that the input signal is not required to go across the composite input stage’s crossover region. and buffer circuits. As a second ICdesign strategy.6MHz. you exchange one noise problem. noninverting amplifier. The result of this added capacitance increases the circuit's noise gain unless you increase the amplifier's feedback capacitor. This condition could affect you if you are a pilot. For this amplifier." Single-supply amplifier manufacturers also claim they have devices that will swing rail-to-rail on the output. If the input signal always remains less or more than the composite input-stage’s transition voltage. A common single-supply amplifier input topology has parallel PMOS and NMOS differential input stages. In every case. The pulse oximeter is a noninvasive instrument that monitors SpO 2 (saturation of hemoglobin with oxygen) in your blood. allowing the amplifier's output swing to go to and well beyond the power-supply rails. the NMOS offset error dominates.1 to 2V. the NMOS transistors are in use. This is true for the inverting-amplifier circuit found in Figure 3. A second consideration is that the low-frequency voltage noise of your amplifier must be very low. hiking in the high altitude of a mountain range. Line 2 specifies the CMRR from ground to 1. The difference between these two topologies in this figure is obvious. rail-to-rail-output ads can give a false sense of security. in this circuit. however. When you choose your device for the pulse-oximeter transimpedance-amplifier circuit. 2. and a feedback resistor/capacitor pair at the front end. You measure the oxygen in the blood by alternating the on-times of a red LED with a 650-mm wavelength and an NIR (near-infrared) LED with a 940-mm wavelength. the amplifier's initial offset error and overtemperature should be in the microvolt region if you want to minimize linearity errors. low-power OPA344 input stage in Figure 1 has rail-to-rail input operation. The amplifier output voltage swings from 140 mV to 4.0004%. Composite input op amps can create distortion at the output. Figure 2 compares the response of the charge pump to composite amplifier inputs. both the PMOS and NMOS transistors are operating. Later. Although the precision. After the transimpedance amplifier. you will see what I mean. designers added a second. In Table 1. "Bootstrapping reduces amplifier input capacitance. the output cannot go all the way to the rails. Aug 7. Low-Noise. where V OUT is the output voltage and V OS is the input offset voltage. You may recall that this topology exhibited an offsetvoltage. and V IN) keeps the output (VO) between V S and ground. If a pulse oximeter indicates that your oxygen levels are stable. Photodiode Amplifiers: Op Amp Solutions. The circuit requires a bias voltage. The only important performance specifications are low input capacitance. Using these equations. If the circuit designer uses the amplifier in a voltage follower or buffer configuration. you can use a bootstrap circuit (Figure 1). When using a single-supply amplifier. This specification encompasses the full amplifier input range. but the chip designer has to make some compromises to achieve this type of performance. Figure 1 shows a simplified block diagram of a pulse oximeter. and comparing that ratio with an SpO 2 look-up table in the microcontroller. and V OL is the absolute minimum voltage level that the output can reach. References 1. the composite input amplifier produces distortion at the circuit output. They began to use the all-too-common charge pump to push a single differential-input stage of the amplifier above the positive-power supply (Figure 1). V L is the minimum voltage level at the output in the dc-open-loop-gain measurement. When you drive the output high. the noninverting amplifier circuit may require a bias voltage. The amplifier's typical closed-loop bandwidth is about 3 MHz with a typical slew rate of 2. The charge pump is a good stopgap. if you use an amplifier that has a charge pump in its input stage to drive highprecision SAR or delta-sigma converters. Jerald G. A typical common-mode rejection specification for an amplifier with a composite input stage has two or more CMRR specifications (Table 1). The composite input stage is one example. Line 1 verifies that the amplifier has true rail-to-rail input capability. Dov. making A 2 's bandwidth much greater than that of A 1 . which makes the manipulation of the photodiode signal manageable. the rail-to-rail input operation across the complete amplifier's rail-to-rail common-mode range (Reference 1). converts the photodiode current generated by the LEDs to a voltage at the output. There are many ways to approach the photosensing-circuit problem. Baker. to keep the output range at V O between the powersupply rails. the NMOS transistors start to turn on. where CA1 and CA2 represent the sum of their input differential and common-mode capacitance. 3. the CMRR is equal to 20*log (ΔV CM /V OS ). into an electrical form. R1 . the PMOS transistors are in operation. In many applications. This increase has a positive effect on amplifiers in buffer configurations. crossover distortion as the input common-mode voltage traveled across the full rail-to-rail input voltage range. June 2006. Here V B establishes the common-mode voltage of the amplifier. 50MHz. The transfer function of the circuit in Figure 5 is shown here: The input common-mode voltage (VIN) can vary between power-supply rails." EDN. Revisiting single-supply input topologies The CMRR (common-mode rejection ratio) specification describes changes in the amplifier's offset-voltage versus common-mode input changes. When you bring V IN+ toward the negative rail. the cable capacitance. A unity-gain buffer. position and proximity sensors. March 2006. Bonnie. Texas Instruments. Since the V B voltage is static. 15 Back to top Remote photo sensing Photodiodes transform a basic physical occurrence. Bonnie.004%. Dec 15.to 30-dB increase in the amplifier's CMMR (common-mode-rejection ratio). V O. and 4 kHz and so on. Bottom line: noninverting-amplifier circuit Composite input: the input signal (VIN) must not travel into the transition region of the PMOS and CMOS differential input stages Charge-pump input: performs without distortion in reaction to common-mode voltage or V IN changes With a unity-gain buffer (Figure 5). the circuit will not create distortion from the composite’s input stage. there are performance compromises that the circuit designer must address. the system THD is: where THDOPA =20log(THD OPA–% ×100) and THDOPA–% is the THD specification in the operational amplifier's data sheet in units of percentage. For proper operation. Bottom line: buffer circuit Composite input: the op amp’s input range (VIN) must not travel into the transition region between the PMOS and CMOS differential http://www. Alternatively.or CMOS-amplifier input devices usually meet this requirement. the circuit does not create distortion from the composite’s input stage. Medical Instruments Applications Guide. the output of single-supply amplifiers can come within only 50 to 300 mV of each rail (Figure 1). Figure 5 The input signal. the amplifier's common-mode input voltage can remain at a static voltage. As the common-mode input voltage increases. In this circuit. Figure 4 The noninverting gained amplifier configuration allows the amplifier’s commonmode voltage to change with input signals. crossover phenomena. 3. photographic analyzers. Next time. Baker. Let's consider the behavior of the composite and chargepump amplifier inputs in a single-supply inverting amplifier. if the op amp's input stage has a charge pump with a THD specification that is 0. 2. The specification conditions can subtly describe the amplifier's rail-to-rail input topology. the charge-pump design can provide an output ripple voltage that is low enough so as to not produce undesirable noise or distortion at the output of the op amp. The two stages shared. amplifier. With those types of amplifiers. V O. A good approach is to make the noise of A 2 less than or equal to that of A 1 . light. The LCD shows a percentage of oxygenated hemoglobin versus nonoxygenated hemoglobin and your heart rate. So. or complementary. From approximately 1. With this design. removes the cable's capacitance and the photodiode's parasitic capacitance from the input of the transimpedance amplifier. OPA344 data sheet. the input of the amplifier has a large capacitance across its input. and the 16-bit SAR ADC has a THD specification of –99 dB. 2008. the system THD is –88 dB. the composite amplifier exhibits some limitations in linearity due to the input-stage topology. you'll find that the type of amplifier you select for A 2 is somewhat easy. Application solutions Circuit designers can use rail-to-rail input op amps in virtually any op-amp configuration. When you consider the input-voltage noise of the amplifier. and the PMOS transistors are off. with the NMOS transistors turned off. read your data sheet and refer to the conditions on the open-loop-gain test. a unique zero-crossover input topology provides superior common-mode performance over the entire input range. Baker. V B.com/ww/en/analog/Amplifiers-eBook/[1/14/14 10:58:49 AM] . read the fine print! Some single-supply amps have output-stage charge pumps. Line 3 specifies the CMRR across the entire input range as 70 dB (typ. There are circuit-design tricks at your disposal for minimizing this input-stage crossover effect. Townsend.8V below the positive power-supply rail as 92 dB (typ). and compares the results with a look-up table. If the input signal always remains less than the composite input stage’s transition voltage.66V. A good rule of thumb is to have CA2 <<(CA1 +CCA+CPD). or even undergoing surgery. In this configuration. Figure 1: This composite input stage of the op amp uses PMOS and NMOS differential pairs so the input-voltage range can extend from above the positive rail to below the negative rail. When designing this circuit." EDN.4V. or perhaps you just dance to the beat of a different drummer. low noise. In this 5V-supply system. The offset voltage of an op amp with a composite input varies across changes in the common-mode voltage. increases. Eventually. If V IN travels across this entire range. Neil. V OH is the absolute maximum voltage level with respect to V DD (drain-to-drain voltage) that the output can reach. and even smoke detectors. References 1. So. Bottom line: buffer circuit Composite input: the op amp’s input range (VIN) must not travel into the transition region between the PMOS and CMOS differential http://www. The unity-gain buffer removes the noise effect from A 1 . the input of the amplifier has a large capacitance across its input. ranging from 95 to 100%. 1978. You may think that the charge pump's output ripple voltage will become a noise problem. This condition could affect you if you are a pilot. the circuit will not create distortion from the composite’s input stage. 3. Figure 5 The input signal. "Pulse Oximetry. the red LED is on for 50 µsec. making A 2 's bandwidth much greater than that of A 1 . V S=5V). an amplifier with a charge-pump input stage does not create this unwanted distortion across the entire input range. the bandwidth of the circuit decreases. V IN. combining the advantages of those stages to achieve actual rail-to-rail input operation (Figure 1). These specification tables provide evidence of the characteristics of the input-stage topology. with another. crossover distortion as the input common-mode voltage traveled across the full rail-to-rail input voltage range. Revisiting single-supply input topologies The CMRR (common-mode rejection ratio) specification describes changes in the amplifier's offset-voltage versus common-mode input changes. Figure 2: The op-amp offset voltage of an op amp with a charge-pump input remains constant across changes in the common-mode voltage. The alarms on the pulse oximeter usually sound when the SpO 2 level drops below 90%. with an input voltage of 4V p-p. I'll talk about the rail-to-rail amp's output stage and its ability to achieve rail-to-rail performance. This increased headroom allows for a single differential PMOS input stage to replace the composite differential input stage. The pulse oximeter is a noninvasive instrument that monitors SpO 2 (saturation of hemoglobin with oxygen) in your blood. For example. When using a single-supply amplifier. 2010. A 2 's input capacitance is the only capacitance that plays a role in the ac-transfer function of the transimpedance system. A 2 's gain roll-off introduces an upper limit for the bandwidth improvement. Figure 1 shows a simplified block diagram of a pulse oximeter. a regulated charge pump lifts the top of the differential input as well as its biasing current source to 1. You can also expect almost a tenfold decrease in the amplifier's THD (total-harmonic-distortion) performance. and comparing that ratio with an SpO 2 look-up table in the microcontroller. the transfer function for this circuit is expressed as The V B voltage can be anywhere between ground and V S. After the transimpedance amplifier. the PMOS transistors are in operation. hiking in the high altitude of a mountain range. The single differential-input stage with a charge pump buys you a 20. which in this case is an amplifier with a composite input stage. In Table 1. and compares the results with a look-up table. The article discusses a composite input-stage topology for single-supply op amps. the output cannot go all the way to the rails. and the NMOS transistors are completely off." Texas Instruments. Photodiode Amplifiers: Op Amp Solutions. Bonnie. Later. Amplifier designers place the switching mechanism's frequency above the amplifier's bandwidth and keep the switching noise lower than the amplifier's thermal noise floor.004%. circuit designers need to consider the behavior of the single-supply op-amp input stage. 2009. the rail-to-rail input operation across the complete amplifier's rail-to-rail common-mode range (Reference 1). a wider bandwidth than A 1 . Bonnie. the output of single-supply amplifiers can come within only 50 to 300 mV of each rail (Figure 1). R1 . McGraw-Hill. the system THD is: where THDOPA =20log(THD OPA–% ×100) and THDOPA–% is the THD specification in the operational amplifier's data sheet in units of percentage. and V IN) keeps the output (VO) between V S and ground. There are more ways to tackle this input topology problem with the IC design. From approximately 1. bar-code scanners. the CMRR is equal to 20*log (ΔV CM /V OS ). To achieve optimum performance. Recalling a circuit from another column. the composite input amplifier produces distortion at the circuit output.to 30-dB increase in the amplifier's CMMR (common-mode-rejection ratio).1V." Single-supply amplifier manufacturers also claim they have devices that will swing rail-to-rail on the output. read your data sheet and refer to the conditions on the open-loop-gain test. removes the cable's capacitance and the photodiode's parasitic capacitance from the input of the transimpedance amplifier. Texas Instruments. When you choose your device for the pulse-oximeter transimpedance-amplifier circuit. converts the photodiode current generated by the LEDs to a voltage at the output. increases. There are circuit-design tricks at your disposal for minimizing this input-stage crossover effect. in this circuit.and NIR-LED signals. The transimpedance amplifier. Note that this specification does not describe the range specified in Line 1. This circuit requires stability optimization as you balance CF and the input capacitance of A 2 . FET. "OPA333. The system repeats this cycle continuously. In this design.4V. the amplifier's common-mode voltage remains constant. References 1. your system's performance will improve. Single-supply-amplifier. Design engineers methodically convert the photodetector's current to a usable voltage. Figure 2: As the amplifier's common-mode voltage changes from ground to the positive supply. IC designers borrowed a technology from other devices to solve this problem. both the PMOS and NMOS transistors are operating. but one issue came to mind as I read the comments in the Talkback section for a recent column. Figure 2a illustrates the nonlinearity-output-stage effects with a single-supply CMOS amplifier by showing distortion at 2. an operational amplifier. In this configuration. Figure 1: This composite input stage of the op amp uses PMOS and NMOS differential pairs so the input-voltage range can extend from above the positive rail to below the negative rail. This article is an expansion of the blog post "What does 'rail to rail' input operation really mean?" By examining an additional inputstage topology and implementing these op amps into some common op-amp circuits. you can use a bootstrap circuit (Figure 1).ti. if you use an amplifier that has a charge pump in its input stage to drive highprecision SAR or delta-sigma converters. The 4. When V B establishes the commonmode voltage of the amplifier. Alternatively. 17 Back to top The non-negotiable single-supply operational amplifier Fundamental analog devices that serve applications such as high-resolution delta-sigma or SAR (successive-approximation-register) converter systems are feeling the crunch from amplifiers that have difficulty with achieving good rail-to-rail input performance. Dov. "Where did all the racket come from?" EDN. The V OH maximum specification is V DD –20 mV. Let's consider the behavior of the composite and chargepump amplifier inputs in a single-supply inverting amplifier.1 to 2V. to keep the output range at V O between the power-supply rails. The difference between these two topologies in this figure is obvious. You measure the oxygen in the blood by alternating the on-times of a red LED with a 650-mm wavelength and an NIR (near-infrared) LED with a 940-mm wavelength. Aug 7. V B. A reader requested a circuit that reduces the noise impact of a photodiode with a large parasitic capacitance. you exchange one noise problem. If the circuit designer uses the amplifier in a voltage follower or buffer configuration. For proper operation.com Have you ever been in need of a second opinion when your sanity is at stake? The pulse oximeter may be able to provide that second opinion if your brain is oxygen-deprived. When you bring the input terminals to the positive rail. the NMOS section of the circuit finally takes over. noninverting amplifier. which makes the manipulation of the photodiode signal manageable. VB can be assigned so that the input signal is not required to go across the composite input stage’s crossover region. The offset voltage of an op amp with a composite input varies across changes in the common-mode voltage. The charge pump is a good stopgap. The inverting-amplifier circuit gains the input signal as well as changes the signal polarity. The circuit requires a bias voltage. "Transimpedance-amplifier stability is key in light-sensing applications. the input range extends at least 100 mV beyond both power-supply rails. V IN. Alternatively. A good approach is to make the noise of A 2 less than or equal to that of A 1 . V IN establishes the amplifier’s common-mode voltage. Baker. "OPA365. March 2006. March 20. and 4 kHz and so on. however. When you bring V IN+ toward the negative rail. engineers continue to demand lower system power supplies and insist on better signal integrity. with some distortion. 5. The dc open loop gain in decibels is 20 log(ΔV OUT /ΔV OS ). It may be worthwhile to use an autozero amplifier. microPower CMOS Operational Amplifiers." EDN. The conditions of the dc-open-loop-gain specification define the amplifier's linear operating output range. if the op amp's input stage has a charge pump with a THD specification that is 0.0004%. If V IN travels across this entire range. Bottom line: inverting-amplifier circuit Composite input: performs without distortion with V B outside the PMOS/NMOS transition region Charge-pump input: performs without distortion The noninverting amplifier’s common-mode voltage (Figure 4) is equal to the input voltage. the V OL minimum specification is 15 mV above ground. If the input signal always remains less than the composite input stage’s transition voltage. V H is less than V OH. Near the rail. and Avner Cohen. Single-Supply Rail-to-Rail Operational Amplifiers. Since the V B voltage is static. RF . the circuit does not create distortion from the composite’s input stage.8V above the powersupply voltage.6MHz. an op amp with a composite input topology may limit the input range of the buffer. There are many ways to approach the photosensing-circuit problem. Jerald G. Michaelmas 2001. Yet another column details the stability of this circuit.3V/µsec. The amplifier's input-bias current creates an output-voltage error by conducting through the high-impedance resistor. Figure 4 The noninverting gained amplifier configuration allows the amplifier’s commonmode voltage to change with input signals. A 2 . Table 1: The single-supply op-amp data sheet describes the CMRR test conditions as well as the actual specified values. A 2 . Using these equations. low-power OPA344 input stage in Figure 1 has rail-to-rail input operation. and a feedback resistor/capacitor pair at the front end. and feedback-capacitance elements limit the bandwidth of the circuit. where V OUT is the output voltage and V OS is the input offset voltage. In this circuit. 18 Back to top What does "rail to rail" input operation really mean? A hot discussion topic with single-supply operational amplifiers is whether they are capable of rail-to-rail input or output operation. In fact. as long as the combination of the elements in this circuit (VB. you need to make sure that the amplifier's input-bias current is very low or in a picoamp region at 25°C. choose the V B voltage to be below or above the PMOS/NMOS transition region. For linear operation. Line 3 specifies the CMRR across the entire input range as 70 dB (typ. into an electrical form. Good luck! References 1. They began to use the all-too-common charge pump to push a single differential-input stage of the amplifier above the positive-power supply (Figure 1). When you consider the input-voltage noise of the amplifier. position and proximity sensors. or complementary. the system THD becomes –98 dB. Here V B establishes the common-mode voltage of the amplifier. a unique zero-crossover input topology provides superior common-mode performance over the entire input range. A 1 . With this design. and then both LEDs are off for 450 µsec. Next time. 2. In many applications. The two stages shared. LMP7701 data sheet. light. and buffer circuits. but the chip designer has to make some compromises to achieve this type of performance." EDN. the headroom between the signal and rails is 140 mV. and the amplifier output never reaches either rail." Medical Electronics.8V below the positive power-supply rail as 92 dB (typ). Neil. however. the data looks perfect with only the ADC distortion (Figure 2b). you may want to explore other diagnostic avenues. In the region near ground. If the feedback capacitor. Vendors of single-supply op amps claim their amplifiers have rail-to-rail input capability. This increase has a positive effect on amplifiers in buffer configurations. Composite input op amps can create distortion at the output. and the 16-bit SAR ADC has a THD specification of –99 dB. The specification conditions can subtly describe the amplifier's rail-to-rail input topology. A typical common-mode rejection specification for an amplifier with a charge-pump input stage has one CMRR specification." EDN. the NMOS offset error dominates. As a second ICdesign strategy. the NIR LED is on for 50 µsec.66V. allowing the amplifier's output swing to go to and well beyond the power-supply rails. A 1 . Baker. When sensing with a photodiode with a large parasitic capacitance or from a remote site. the system THD is –88 dB. where CA1 and CA2 represent the sum of their input differential and common-mode capacitance. The LCD shows a percentage of oxygenated hemoglobin versus nonoxygenated hemoglobin and your heart rate. the composite amplifier exhibits some limitations in linearity due to the input-stage topology. June 2006. "A good holiday-season project. and the parasitic capacitance of the photodiode. V S. you will see what I mean. you'll find that the type of amplifier you select for A 2 is somewhat easy." Texas Instruments. the amplifier's common-mode input voltage can remain at a static voltage.or CMOS-amplifier input devices usually meet this requirement. Baker. crossover phenomena. both LEDs are off for 450 µsec. Bonnie. the charge-pump design can provide an output ripple voltage that is low enough so as to not produce undesirable noise or distortion at the output of the op amp. If there is a shortage of oxygen in your system. OPA2365 2. 16 Back to top Single-supply amplifier outputs don't swing rail to rail Single-supply amplifiers do not truly swing rail to rail at the output. there are performance compromises that the circuit designer must address. 2. the amplifier is nonlinear. The FFT plot in Figure 2a shows the amplifier/ADC-combination response to a 1-kHz signal in a 5V system. rail-to-rail input/output LMP7701 CMOS amplifier in Figure 2 exhibits the offset-voltage-error crossover behavior around 1. in the amplifier's feedback loop. A good rule of thumb is to have CA2 <<(CA1 +CCA+CPD). Figure 3: The inverting-amplifier circuit keeps the amplifier's common-mode voltage at a constant voltage. the photodiode. Townsend. The best way to view the input stage's behavior is to look at the offset voltage versus the common-mode input voltage (Figure 2). 2010. The op-amp topology in Figure 1 provides superior common-mode performance over the entire input range. 2. Although the precision. as long the voltage at V O remains between the power-supply rails. The only important performance specifications are low input capacitance. crossover phenomena. V B.2V. or perhaps you just dance to the beat of a different drummer. Line 1 verifies that the amplifier has true rail-to-rail input capability. the PMOS offset-error portion of the input stage is dominant. The result of this added capacitance increases the circuit's noise gain unless you increase the amplifier's feedback capacitor. Dec 15. 50MHz. Medical Instruments Applications Guide. Single-supply amplifiers continue to keep pace with high-resolution converters because engineers implement innovative amplifier-circuit topologies. and the PMOS transistors are off. you can see an op amp's output limitations to swinging rail to rail when the op amp is driving an ADC. OPA344 data sheet. the noninverting amplifier circuit may require a bias voltage. When using composite input amplifiers. When designing this circuit. 3. A second consideration is that the low-frequency voltage noise of your amplifier must be very low. 2008. 2008. a bandpass filter eliminates the noise above 5 Hz. read the fine print! Some single-supply amps have output-stage charge pumps. 19 Back to top Rail-to-rail input amplifier application solutions Many single-supply operational amplifiers are capable of rail-to-rail input operation. 15 Back to top Remote photo sensing Photodiodes transform a basic physical occurrence. with the NMOS transistors turned off. Bottom line: noninverting-amplifier circuit Composite input: the input signal (VIN) must not travel into the transition region of the PMOS and CMOS differential input stages Charge-pump input: performs without distortion in reaction to common-mode voltage or V IN changes With a unity-gain buffer (Figure 5). the cable capacitance. "Photo-sensing circuits: The eyes of the electronic world are watching. If the circuit designer wants a distortion-free output from this amplifier circuit. Texas Instruments. When you drive the output high. if an operational amplifier with a complementary input stage has a THD specification of 0. Texas Instruments. V B. and low output impedance. From a signal-chain perspective. the input stage of the CMOS amplifier completely changes from its PMOS input pair to its NMOS input pair at -2V below the 3V positive supply rail. V O. The signal then travels through a bandpass filter and gain stage to the 12-bit ADC. Baker. With those types of amplifiers. 3. R2 . A unity-gain buffer. Figure 2 compares the response of the charge pump to composite amplifier inputs. Finally. The noninverting amplifier circuit gains the input signal with a voltage level-shift implemented with V B. MD. you may experience poor judgment or loss of motor function.TI. as V IN changes. Application solutions Circuit designers can use rail-to-rail input op amps in virtually any op-amp configuration. This specification encompasses the full amplifier input range. the NMOS transistors start to turn on. ISBN 0-07-024247-X. Low-Noise. the input signal (VIN) can traverse from one rail to the other rail. To fix this problem.8V. The composite input stage is one example. References 1. Graeme. You may recall that this topology exhibited an offsetvoltage. In the circuit in Figure 1. such as an input stage with a charge pump. differential-input stage. For this amplifier. Line 2 specifies the CMRR from ground to 1. you will primarily find transimpedance amplifiers in the CT (computedtomography)-scanner front end and the pulse oximeter. Neither of these approaches produced an amplifier adequate for the high-precision systems to span the amplifier's full common-mode input range. Figure 1 shows a typical single-supply amplifier's output swing as you drive the output to the rails. Kurz. if they have the proper input circuitry to support this function. The transfer function of the circuit in Figure 5 is shown here: The input common-mode voltage (VIN) can vary between power-supply rails. the NMOS transistors are in use. If a pulse oximeter indicates that your oxygen levels are stable. The simple rail-to-rail operational amplifier must have a transistor design that spans the power supply with minimal distortion. This disparity in specification only points out that the amplifier's input stage has a composite input topology. However. rail-to-rail-output ads can give a false sense of security. 4. At approximately 1. computes the ratio of the red. By reducing the amplifier's output signal to 272 mV from each rail. In this 5V-supply system. 1996. The microcontroller acquires the signals from the 12-bit ADC. A 1 . taking the ratio between the intensities from a photodiode. If the input signal always remains less or more than the composite input-stage’s transition voltage. Bonnie. Sept 4. CF . The input capacitance of the buffer replaces the summation of the input capacitance of A 1 . This is true for the inverting-amplifier circuit found in Figure 3. A common single-supply amplifier input topology has parallel PMOS and NMOS differential input stages. In the medical field. The amplifier's typical closed-loop bandwidth is about 3 MHz with a typical slew rate of 2. Eventually. In Figure 1. The difference between the input signal and the output signal in this system falls across the cable/diode capacitance. Photodiodes with a relatively low diode capacitance do not benefit from this circuit. the THD of an ADC driven by an op amp in a buffer configuration is the root-sum square of distortion contributions of the ADC and op amp. A classic photo-sensing system circuit has a photodiode. At lower common-mode input voltages. April 23. with the PMOS transistors completely off. of the buffer circuit changes the amplifier’s common-mode voltage. The transimpedance amplifier appears in medical and laboratory instrumentation. photographic analyzers. Zero-Drift Series. The amplifier output voltage swings from 140 mV to 4. In the region near the positive power supply. The trend toward designing single-supply op amps started in the 1970s with a single differential-input stage that spanned a portion of the common-mode input range. V L is the minimum voltage level at the output in the dc-open-loop-gain measurement. to keep the output range at V O between the powersupply rails. amplifier. The signal also travels through a lowpass filter to regulate the driver power to the LEDs. The amplifier's linearity starts to degrade long before reaching the output-swing maximums. V H is the maximum voltage level at the output in the dc-open-loop-gain measurement. current. the amplifier's initial offset error and overtemperature should be in the microvolt region if you want to minimize linearity errors. or even undergoing surgery. 2.Texas Instruments . you can read about them in "Rail-to-rail input amplifier application solutions. Figure 1: This single PMOS differential input stage in conjunction with a high-side chargepump eliminates the common-mode crossover distortion found in composite input stages. V O. V OH is the absolute maximum voltage level with respect to V DD (drain-to-drain voltage) that the output can reach. and V L is greater than V OL . A typical common-mode rejection specification for an amplifier with a composite input stage has two or more CMRR specifications (Table 1). The design topology in Figure 1 can have wide variations in offset voltage across the amplifier's common-mode input range.com/ww/en/analog/Amplifiers-eBook/[1/14/14 10:58:49 AM] . scrutinize the impact of the flicker noise." EDN. OPA2333 1. You can keep this difference low selecting A 2 with wider bandwidth than A 1 and keeping A 2 's output impedance low. and V OL is the absolute minimum voltage level that the output can reach. In every case. low noise. A normal output for the pulse oximeter is approximately 97%±2%. "Bootstrapping reduces amplifier input capacitance. but it can get close. As the common-mode input voltage increases. The transfer function for the circuit is expressed as The input common-mode voltage (VIN) can vary between ground and V S. designers added a second. the PMOS transistors are completely on. Switching from one differential input stage to the other causes this distortion. " Single-supply amplifier manufacturers also claim they have devices that will swing rail-to-rail on the output. both the PMOS and NMOS transistors are operating. the NMOS transistors are in use. and the NMOS transistors are completely off. There are circuit-design tricks at your disposal for minimizing this input-stage crossover effect. you will see what I mean. LMP7701 data sheet. but the chip designer has to make some compromises to achieve this type of performance. the NMOS section of the circuit finally takes over.1V. combining the advantages of those stages to achieve actual rail-to-rail input operation (Figure 1).TI. OPA344 data sheet. Although the precision. there are performance compromises that the circuit designer must address. A common single-supply amplifier input topology has parallel PMOS and NMOS differential input stages. Figure 2: As the amplifier's common-mode voltage changes from ground to the positive supply. The 4. OPA2333 1. 19 Back to top Rail-to-rail input amplifier application solutions Many single-supply operational amplifiers are capable of rail-to-rail input operation. the PMOS offset-error portion of the input stage is dominant.4V. if they have the proper input circuitry to support this function. Vendors of single-supply op amps claim their amplifiers have rail-to-rail input capability. From approximately 1. the NMOS offset error dominates.com/ww/en/analog/Amplifiers-eBook/[1/14/14 10:43:25 AM] . the PMOS transistors are in operation.Texas Instruments . This article is an expansion of the blog post "What does 'rail to rail' input operation really mean?" By examining an additional inputstage topology and implementing these op amps into some common op-amp circuits.ti. the NMOS transistors start to turn on. At lower common-mode input voltages. rail-to-rail input/output LMP7701 CMOS amplifier in Figure 2 exhibits the offset-voltage-error crossover behavior around 1. With those types of amplifiers. you can read about them in "Rail-to-rail input amplifier application solutions. 18 Back to top What does "rail to rail" input operation really mean? A hot discussion topic with single-supply operational amplifiers is whether they are capable of rail-to-rail input or output operation. "OPA333. http://www. In the region near ground. microPower CMOS Operational Amplifiers.6MHz. with the PMOS transistors completely off." Texas Instruments. As the common-mode input voltage increases. Figure 1: This composite input stage of the op amp uses PMOS and NMOS differential pairs so the input-voltage range can extend from above the positive rail to below the negative rail.1 to 2V. I'll talk about the rail-to-rail amp's output stage and its ability to achieve rail-to-rail performance. When you bring V IN+ toward the negative rail. References 1. At approximately 1. with the NMOS transistors turned off. Next time. low-power OPA344 input stage in Figure 1 has rail-to-rail input operation. and the PMOS transistors are off. the input stage of the CMOS amplifier completely changes from its PMOS input pair to its NMOS input pair at -2V below the 3V positive supply rail. The best way to view the input stage's behavior is to look at the offset voltage versus the common-mode input voltage (Figure 2). the output cannot go all the way to the rails. the PMOS transistors are completely on. Zero-Drift Series. The design topology in Figure 1 can have wide variations in offset voltage across the amplifier's common-mode input range.com 3.8V. but it can get close. When you bring the input terminals to the positive rail. Texas Instruments. 2. In the region near the positive power supply. Texas Instruments. March 2006. the input stage has two op amps in an adjustable-gain configuration and provides high input impedance on both the inverting (VIN−) and noninverting (VIN+) inputs. note that resolution is lost at high temperatures. This circuit's bandwidth. It is true that circuit designers can use rail-to-rail input amplifiers in virtually any op-amp configuration. 2005. Figure 2 compares the response of the charge pump to composite amplifier inputs. the most misunderstood is the common-mode-range requirement. the offset errors from the amplifier's open-loop gain also increase. rail-to-rail input/output LMP7701 CMOS amplifier in Figure 2 exhibits the offset-voltage-error crossover behavior around 1. This story goes deeper. 3.to 30-dB increase in the amplifier's CMMR (common-mode-rejection ratio). 3.1V. Revisiting single-supply input topologies The CMRR (common-mode rejection ratio) specification describes changes in the amplifier's offset-voltage versus common-mode input changes. At higher instrumentation-amplifier gains. The noninverting amplifier circuit gains the input signal with a voltage level-shift implemented with V B. In fact. go back and refine your feedback-resistor selection. a 10-bit ADC in this circuit can sense a limited temperature range (approximately ±25°C). Figure 2a illustrates the nonlinearity-output-stage effects with a single-supply CMOS amplifier by showing distortion at 2. and temperature drift. this 0. Application solutions Circuit designers can use rail-to-rail input op amps in virtually any op-amp configuration. A typical common-mode rejection specification for an amplifier with a composite input stage has two or more CMRR specifications (Table 1). and buffer circuits. D1 and D2 respond linearly to illumination intensity. The design topology in Figure 1 can have wide variations in offset voltage across the amplifier's common-mode input range. 22 Back to top Common-mode range can bite hard My last column provided a glimpse inside the CMR (common-mode rejection) of a three-op-amp INA (instrumentation amplifier) and revealed the main contributors to total CMR error (see "Understanding CMR and instrumentation amplifiers. R2 . You may think that the charge pump's output ripple voltage will become a noise problem. both inputs have a path to ground–in this case. Bottom line: inverting-amplifier circuit Composite input: performs without distortion with V B outside the PMOS/NMOS transition region Charge-pump input: performs without distortion The noninverting amplifier’s common-mode voltage (Figure 4) is equal to the input voltage. The output stage has four matched resistors around a single op amp. V CM . Background light adds only an offset to the photodiode's outputs. If you use three op amps in a differential-photodiode-amp configuration. The invalid voltage can appear to be legitimate. The input stages of A 1 and A 2 limit the range of these two input voltages. Thus. So how do designers calculate the INA's common-mode range? Consider the exposure to an input/gain-overload condition on the INA as a possibility. OPA2333 1. The A 3 output restrictions are similar to those of any other amplifier. 23 Back to top Instrumentation amplifier input-circuit strategies Many industrial and medical applications use instrumentation amplifiers to condition small signals in the presence of large common-mode voltages. The charge-pump input amplifier does not produce this same distortion.004%. An amplifier's open-loop gain is 20log(ΔV OUT /ΔV OS ). but you can reduce the feedback-resistor value with the current-feedback-amp circuit and get an increase in circuit bandwidth. A 1 and A 2 require low-inputcurrent CMOS or FET op amps. the photodiode's currents flow through the R1 feedback resistors." EDN. When you bring the input terminals to the positive rail. 2005. Single-Supply Rail-to-Rail Operational Amplifiers. the input signal (VIN) can traverse from one rail to the other rail. An example of this type of input/gain-overload condition occurs when the in-range voltages on the INA's inputs drive A 1 . The inverting-amplifier circuit gains the input signal as well as changes the signal polarity. The microcontroller firmware's temperature-sensing algorithm reads the 10-bit-ADC digital value and passes it to a PGA hysteresis-software routine. Zero-Drift Series. V H is the maximum voltage level at the output in the dc-open-loop-gain measurement. Amplifiers and Linear. the headroom between the signal and rails is 140 mV. and A 3 are constants that the thermistor manufacturer provides. the circuit's closed-loop bandwidth is 25. As with many INAs. Three basic kinds of nodes in the INA can cause input/gain-overload problems (Figure 1). The applied input voltages to the INA are equivalent to the common-mode input voltage plus or minus the differential input signal. three-op-amp INA (Figure 1). Because the inverting input of A 1 and A 2 has high impedance. . LMP7701 data sheet. subtracting the output voltages of A 1 and A 2 . References 1. the output cannot go all the way to the rails. You can now use those graphs to your advantage.or a voltage-feedback amp. Texas Instruments. As the gain of A 1 and A 2 increases. and 4 kHz and so on. From this point. Thomas. Of all the performance characteristics of an INA. allowing the amplifier's output swing to go to and well beyond the power-supply rails. you will see what I mean. if R1 . References 1. This situation is not so mysterious when you think about the inside of this device. the charge-pump design can provide an output ripple voltage that is low enough so as to not produce undesirable noise or distortion at the output of the op amp. read your data sheet and refer to the conditions on the open-loop-gain test. if you add channels to the circuit. Low-Noise. backgroundlight conditions can influence this magnitude. and R4 to the instrumentation amplifier's CMR–specifically. "Instrumentation amplifiers—they're not op amps butwhat are they?" TI E2E Community. In this condition. April 1. the V OA1 and V OA2 output levels of A 1 and A 2 . McGraw-Hill." The two basic types of thermistors are negative. the input range extends at least 100 mV beyond both power-supply rails. The composite input amplifier produces a crossover distortion. of the buffer circuit changes the amplifier’s common-mode voltage. Precision Unity Gain Differential Amplifier. In every case. Lower value resistors reduce the current-feedback amp's stability. The amplifier's typical closed-loop bandwidth is about 3 MHz with a typical slew rate of 2. A 3 measures an erroneous difference voltage. Single-Supply Operational Amplifier (Rev.ti. the output swing never spans all the way to the rails. however. Figure 2b illustrates a correct thermocouple connection to an INA. There are more ways to tackle this input topology problem with the IC design. Switching from one differential input stage to the other causes this distortion. "The right way to use instrumentation amplifiers. In Table 1. High CMRR. the inner workings of the op amp are straightforward. Figure 1 shows a circuit that is appropriate for either a current." Texas Instruments. where she navigates the ins and outs of signal chain designs. By reducing the amplifier's output signal to 272 mV from each rail. The dc open loop gain in decibels is 20 log(ΔV OUT /ΔV OS ). RTHERM . the THD of an ADC driven by an op amp in a buffer configuration is the root-sum square of distortion contributions of the ADC and op amp. the data looks perfect with only the ADC distortion (Figure 2b). The PGA gain and ADC value then pass to a microcontroller piecewise linear-interpolation routine. These specification tables provide evidence of the characteristics of the input-stage topology. From a signal-chain perspective. the system THD is: where THDOPA =20log(THD OPA–% ×100) and THDOPA–% is the THD specification in the operational amplifier's data sheet in units of percentage. Bonnie C. References 1. V O. crossover phenomena. Bruce. For instance. If the input signal always remains less or more than the composite input-stage’s transition voltage. When you drive the output high. engineers continue to demand lower system power supplies and insist on better signal integrity. A typical common-mode rejection specification for an amplifier with a charge-pump input stage has one CMRR specification. For instance. a regulated charge pump lifts the top of the differential input as well as its biasing current source to 1. "Advances in measuring with nonlinear sensors. The conditions of the dc-open-loop-gain specification define the amplifier's linear operating output range. you will see greater accuracy in proximity and difference (Figure 1). The circuit requires a bias voltage. But wait–there may be a problem with the output values of A 1 and A 2 (Figure 1). the amp's outputs change in voltage along with the IR drop across the R1 resistors. Alternatively. the noninverting amplifier circuit may require a bias voltage. In many applications.2V. as long as the combination of the elements in this circuit (VB. IC designers borrowed a technology from other devices to solve this problem. At a gain of one. CMR A3 is equivalent to the CMR of the entire instrumentation amplifier. designers added a second. or both to their positive or negative output-swing limit. The V OH maximum specification is V DD –20 mV. V B. 2. The FFT plot in Figure 2a shows the amplifier/ADC-combination response to a 1-kHz signal in a 5V system.1 to 2V. T is the temperature in degrees Kelvin. March 27.com. An equal illumination on the two photodiodes makes the output voltage 0V. however. Alternatively. noninverting amplifier. however. The output stage has a reference voltage that level-shifts the output with respect to ground. Figure 2: The op-amp offset voltage of an op amp with a charge-pump input remains constant across changes in the common-mode voltage. R3 .ti. Pay attention to the voltage levels of the INA's V IN+ and V IN– input pins. Composite input op amps can create distortion at the output.TI. R2 . Kitchin. How would you solve this problem? Comment below. In this configuration. The single differential-input stage with a charge pump buys you a 20. References 1. References 1. Adding a matched photodiode and monitoring the difference between the two diode outputs removes the equal offsets the two diodes produce. She has written hundreds of articles. Consequently. At first glance. or ground. There are circuit-design tricks at your disposal for minimizing this input-stage crossover effect. ISBN: 0-07-024247. For both current. the NMOS section of the circuit finally takes over. the circuit does not create distortion from the composite’s input stage. Note that the A 1 and A 2 stages do not gain the input common-mode voltage. Bottom line: noninverting-amplifier circuit Composite input: the input signal (VIN) must not travel into the transition region of the PMOS and CMOS differential input stages Charge-pump input: performs without distortion in reaction to common-mode voltage or V IN changes With a unity-gain buffer (Figure 5). The common-mode behavior of the three-op-amp INA may surprise you if you ignore the nuances of the internal A 1 and A 2 output stages. With CMR. A 2 ." Analog Applications Journal. your system's performance will improve. SBOS145. noise. the input-current leakages are not dissipated. For example. and Lew Counts. design and application notes. an op amp with a composite input topology may limit the input range of the buffer. The offset voltage of an op amp with a composite input varies across changes in the common-mode voltage.8V above the powersupply voltage. In Figure 1. the PMOS transistors are completely on. The final place to look for an input/gain-overload condition is at the output. A 2 's output voltage. This amplifier's drive capability limits the resistor's minimum value. Even though you can run this type of algorithm in your microcontroller firmware.and current-feedback amps are almost the same Current-feedback amplifiers have a higher slew rate than do voltage-feedback amplifiers.1% mismatch will cause a degradation of the instrumentation amplifier's CMR from ideal to a 66-dB level. Charles. crossover phenomena. The first and most dominant factor is the balance of the resistor ratios across A 3 ." Ax-09. if you choose to use a rail-to-rail input amplifier. the V OL minimum specification is 15 mV above ground. and A 0 . All inputs to the INA require a current-return path to ground and a bias-voltage reference. The amplifier's linearity starts to degrade long before reaching the output-swing maximums. If V IN travels across this entire range. the instrumentation amplifier's CMR performance values tend to reach a maximum value at higher gains. The op-amp topology in Figure 1 provides superior common-mode performance over the entire input range. When measuring temperature. Additional channels reduce the closed-loop bandwidth. V S. However. with the PMOS transistors completely off. from the CMR perspective. In this circuit.4V. 3. AMSC. Single-supply amplifiers continue to keep pace with high-resolution converters because engineers implement innovative amplifier-circuit topologies. This disparity in specification only points out that the amplifier's input stage has a composite input topology. The best way to view the input stage's behavior is to look at the offset voltage versus the common-mode input voltage (Figure 2). The maximum and minimum input-voltage limits vary from device to device. Figure 5 The input signal. As you separate the parts. Consequently. both the PMOS and NMOS transistors are operating. the transfer function for this circuit is expressed as The V B voltage can be anywhere between ground and V S. combining the advantages of those stages to achieve actual rail-to-rail input operation (Figure 1). The level shift is convenient in singlesupply applications. Equation 2 gives the contribution to the CMR error with respect to the relationship of these resistors: Equation 2 For instance. which in this case is an amplifier with a composite input stage. Trump. These four resistors combine with A 3 to subtract and gain the signals from the outputs of A 1 and A 2 . Texas Instruments. you can program the gain found in Figure 1 with a single resistor. If the circuit designer uses the amplifier in a voltage follower or buffer configuration. Under normal conditions. You can find the prescribed feedback-resistor value in the current-feedback amp's product data sheet. At a real-world level. of the INA. RF . and the NMOS transistors are completely off. All of the internal amplifier's output voltages relate to the linear common-mode input range of the complete INA.0004%. This increase has a positive effect on amplifiers in buffer configurations. If you design this circuit with such an amp. The specification conditions can subtly describe the amplifier's rail-to-rail input topology. but the chip designer has to make some compromises to achieve this type of performance. the closed-loop gain of each channel is –2V/V.6MHz. The trend toward designing single-supply op amps started in the 1970s with a single differential-input stage that spanned a portion of the common-mode input range. The feedback resistor's higher values reduce the current-feedback amp's bandwidth. Texas Instruments. the PMOS offset-error portion of the input stage is dominant. it pays to understand the CMRR impact on your circuit. 2009. Baker. in series with the thermistor. Low-Distortion. Typically. 25 Back to top A good holiday-season project Which of the bulbs on a Christmas tree is the brightest? If you had the time and desire to answer this question. Table 1: The single-supply op-amp data sheet describes the CMRR test conditions as well as the actual specified values. V IN. TAMU. The two stages shared. "Where did all the racket come from?" EDN. For instance. and the overall circuit noise limits the maximum value. FET gate. usually in the middle of the INA's output range. When the circuit designer exercises proper precautions with the input pins. the noise gain is: (1) where N is the number of input channels. The PGA hysteresis routine checks the PGA gain setting and compares the ADC digital value with the trip points that Figure 1 indicates. biased to a voltage within the INA's input range. V H is less than V OH. 2009." Single-supply amplifier manufacturers also claim they have devices that will swing rail-to-rail on the output. the circuit will not create distortion from the composite’s input stage. unless you expanded your design task to using two photodiodes. however. However. the instrumentation amplifier's CMR increases as the system's gain increases–a nice feature. and V IN) keeps the output (VO) between V S and ground. The simple rail-to-rail operational amplifier must have a transistor design that spans the power supply with minimal distortion. OPA2365 2. with the NMOS transistors turned off. this configuration rejects external common-mode voltage and noise. The definition of an op amp's openloop gain has not changed. Amplifier designers place the switching mechanism's frequency above the amplifier's bandwidth and keep the switching noise lower than the amplifier's thermal noise floor. crossover distortion as the input common-mode voltage traveled across the full rail-to-rail input voltage range. Ideally. The name "current-feedback amp" carries some mystique. When V B establishes the commonmode voltage of the amplifier. With those types of amplifiers. a microcontroller. the system THD becomes –98 dB. but A 1 and A 2 's open-loop gain error places a limit on this strategy." BC Components data sheet. In a classic. the dctransfer function of this circuit is 1V/V. Note that this specification does not describe the range specified in Line 1. 18 Back to top What does "rail to rail" input operation really mean? A hot discussion topic with single-supply operational amplifiers is whether they are capable of rail-to-rail input or output operation. and the V OUT output-swing capability of A 3 . 27 Content provided courtesy of EDN. to keep the output range at V O between the power-supply rails. as the Steinhart-Hart equation describes. Neither of these approaches produced an amplifier adequate for the high-precision systems to span the amplifier's full common-mode input range. As a second ICdesign strategy. where V OS is the offset voltage." Texas Instruments. and authored a book: "A Baker's Dozen: Real Analog Solutions for Digital Designers. You can determine the surrounding thermistor temperature by using the Steinhart-Hart equation: T=1/(A 0 +A 1 (lnR T )+A 3 (lnR T 3 )). To achieve optimum performance. offset. A single photodiode provides some measure of a light's intensity through the magnitude of the diode's output signal. the noise gain always equals the result of Equation 1. Either condition creates an invalid output voltage. An input/gain-overload condition can occur if A 1 's output voltage. When using a single-supply amplifier. and the 16-bit SAR ADC has a THD specification of –99 dB. The difference between these two topologies in this figure is obvious." EDN. The op amp's input offset-voltage error is easy to understand. A 3 rejects the common-mode signal and delivers the differential-voltage signal to the circuit output at V OUT . Baker. So what is the hang-up? Equation 1 yields the common CMR for a single op amp and instrumentation amplifier: Equation 1 where G is the system gain. as long as you pay attention to the input stage and the output of the first stage. the rail-to-rail input operation across the complete amplifier's rail-to-rail common-mode range (Reference 1). In addition to her fascination with circuit design. and you can be confident that the INA's output voltage is representative of the thermocouple's voltage. the NMOS transistors are in use. 19 Back to top Rail-to-rail input amplifier application solutions Many single-supply operational amplifiers are capable of rail-to-rail input operation. Next time. April 23. drive a reference current through the thermistor to create an equivalent voltage. rail-to-rail-output ads can give a false sense of security.and voltage-feedback amps. V IN+ and V IN− connect to a bipolar transistor base. R3 . The composite input stage is one example. On Board with Bonnie. the change of offset voltage is the only concern. If the feedback resistance. the composite input amplifier produces distortion at the circuit output. The output voltages of A 1 and A 2 contain both difference and common-mode signals. circuit designers need to consider the behavior of the single-supply op-amp input stage. Figure 4 The noninverting gained amplifier configuration allows the amplifier’s commonmode voltage to change with input signals. in turn. the CMR of the three-op-amp instrumentation amplifier is ideally infinite. you can read about them in "Rail-to-rail input amplifier application solutions. You can try to compensate for the thermistor's nonlinear response with a look-up table in your microcontroller. but it can get close. You can reach Bonnie at ti_bonniebaker@list. RG. Line 3 specifies the CMRR across the entire input range as 70 dB (typ. 21 Back to top Chapter 2: INAs and PGAs Understanding CMR and instrumentation amplifiers The three-op-amp instrumentation amplifier in Figure 1 is seemingly a simple configuration in that it uses a basic. Bonnie. The voltage-feedback-amp's feedback-resistance value is more forgiving. pg 69. "Rail-to-Rail Op Amps. Jerald. a 10-bit ADC.8V below the positive power-supply rail as 92 dB (typ). 2001. where V OUT is the output voltage and V OS is the input offset voltage. a small variation in the signal bandwidth and gain peaking in circuit may occur. however. The transfer function of the circuit in Figure 5 is shown here: The input common-mode voltage (VIN) can vary between power-supply rails. I'll talk about the rail-to-rail amp's output stage and its ability to achieve rail-to-rail performance. The floating thermocouple circuit in Figure 2a does not provide a current path to ground or a bias-voltage reference for the INA's input pins. From approximately 1. You can also expect almost a tenfold decrease in the amplifier's THD (total-harmonic-distortion) performance. the CMRR is equal to 20*log (ΔV CM /V OS ). © Copyright 2014 EDN. per the manufacturer's specification and the circuit's noise gain. the open-loop gain error of the op amps dominates. you could use a single photodiode to determine the brightness of your bulbs. Finding that brightest bulb among the many and the background light would be a laborious task.001 of R1 /R2 . Texas Instruments. the system THD is –88 dB. This article is an expansion of the blog post "What does 'rail to rail' input operation really mean?" By examining an additional inputstage topology and implementing these op amps into some common op-amp circuits. or both violate the internal output-swing restrictions. the amplifier's common-mode input voltage can remain at a static voltage. differential-input stage. The 4. "CMRR not created equal for all INAs. In the region near the positive power supply. As long as the resistors surrounding A 3 are equal. Bonnie has a drive to share her knowledge and experience. March 27. When using composite input amplifiers. Baker. You can implement a differential amp discretely or with an off-the-shelf product. read the fine print! Some single-supply amps have output-stage charge pumps. A common single-supply amplifier input topology has parallel PMOS and NMOS differential input stages. the relationship of R1 . current-feedback amps can better solve high-speed problems than their voltage-feedback counterparts. 20 Back to top Voltage. The configurations of A 1 and A 2 act as traditional current-to-voltage converters or transimpedance amps. Increasing the PGA's gain at these temperatures brings the output signal of the PGA back into a range at which the ADC can reliably provide conversions that identify the thermistor temperature. in a single-supply environment. If the resistors around A 3 are not equal. the input stage of the CMOS amplifier completely changes from its PMOS input pair to its NMOS input pair at -2V below the 3V positive supply rail. "Introduction to NTCs: NTC Thermistors. such as an input stage with a charge pump. you can see an op amp's output limitations to swinging rail to rail when the op amp is driving an ADC. That situation. instrumentation amplifiers are systems in which various parts contribute to the CMR error at different system gains. Obtaining data from a nonlinear thermistor sometimes can seem like an impossible task. The key performance parameters for A 1 and A 2 are input capacitance. or CMOS gate. The article discusses a composite input-stage topology for single-supply op amps. which makes the magnitude of the output voltage a direct measurement of the difference between the direct light impinging on D1 and D2 . V IN establishes the amplifier’s common-mode voltage. When you bring V IN+ toward the negative rail. VB can be assigned so that the input signal is not required to go across the composite input stage’s crossover region. Line 1 verifies that the amplifier has true rail-to-rail input capability. pg 14). Later. So. You define the common-mode voltage of the input signals as (VA1OUT +V A2OUT )/2. INAs have complete closed-loop operational amplifiers with feedback components included. choose the V B voltage to be below or above the PMOS/NMOS transition region. 2.8V. Heed those considerations for the INA's input stage. The thermistor-resistance change over temperature is nonlinear. A 1 . This erroneous voltage plus the voltage reference to the INA is incorrect and may be inside the output range of A 3 . RSER . Bottom line: buffer circuit Composite input: the op amp’s input range (VIN) must not travel into the transition region between the PMOS and CMOS differential input stages Charge-pump input: performs without distortion in reaction to common-mode voltage changes Conclusion Manufacturers use several input topologies in their designs of rail-to-rail input amplifiers. Line 2 specifies the CMRR from ground to 1. 50 MHz. however. equals 2R IN. requiring special considerations in some classes of circuits. At lower common-mode input voltages. You would then select the appropriate value for RIN. and the two inputs can float to any undefined voltage. Photodiode Monitoring with Op Amps. 2012. equals the gain-bandwidth product divided by the noise gain. V O. They began to use the all-too-common charge pump to push a single differential-input stage of the amplifier above the positive-power supply (Figure 1). This condition is impossible to directly measure. 2. So. Edgar Sánchez-Sinencio. If the input signal always remains less than the composite input stage’s transition voltage. Here V B establishes the common-mode voltage of the amplifier. except for a few key points. The charge pump is a good stopgap. OPA344 data sheet. Two factors influence an instrumentation amplifier's CMR. August 1993. This increased headroom allows for a single differential PMOS input stage to replace the composite differential input stage. with some distortion. In this equation. All rights reserved. 2012. Bonnie. you need a high-resolution converter to capture data during temperature extremes.66V. Bruce. If the ADC output is beyond a trip-point value. "Instrumentation amplifiers—avoiding a common pitfall. the amplifier's common-mode voltage remains constant. the problem arises when the INA inputs are connected without proper consideration to current paths or biasing. The quick and inexpensive solutions described here can help you avoid the INA circuit's input-stage pitfalls. June 2006. 2. Texas Instruments. 2. The amplifier output voltage swings from 140 mV to 4. References 1. 2. to keep the output range at V O between the powersupply rails. the differential amp rejects common-mode-voltage changes.or current-feedback amp with the circuit in Figure 1. Equation 1 might motivate an instrumentation-amplifier designer to ensure that there is plenty of gain available. In the region near ground. V B. microPower CMOS Operational Amplifiers. As the common-mode input voltage increases.Texas Instruments . If you use a current-feedback amp with the circuit in Figure 1. conference papers. These errors degrade the CMR of the instrumentation amplifier at higher gains. generally." Sensors magazine. I'd really like to hear from you! References 1. 24 Back to top Wringing out thermistor nonlinearities Thermistor-temperature-sensing devices present a design challenge if you intend to use such a device over its entire temperature range. At approximately 1. the picture becomes clear. Sept 15.7 MHz." Check out her latest blog. 50MHz. You can combine a series resistor. If necessary. sboa035. 3. The term "thermistor" originates from the descriptor "thermally sensitive resistor. 4Q 2005. Without the current-return path and the bias-voltage reference. and the amplifier output never reaches either rail. The two photodiodes let you find a light's position by monitoring the difference between their output signals. Using these equations. a unique zero-crossover input topology provides superior common-mode performance over the entire input range. For proper operation. where RIN is the input resistance. However. The more difficult interface challenge is to capture the nonlinear behavior of the thermistor in a digital representation with a linear ADC. the feedback resistor of a current-feedback-amp circuit must stay within a small range of values. "Getting themost out of your instrumentationamplifier design. If that scenario is a concern. if you use an amplifier that has a charge pump in its input stage to drive highprecision SAR or delta-sigma converters. In this 5V-supply system. Depending on the INA's silicon process. the NMOS offset error dominates. As Equation 1 states. If the circuit designer wants a distortion-free output from this amplifier circuit. if an operational amplifier with a complementary input stage has a THD specification of 0. In Figure 1. The goal is to select amps in which these parameters are as low as possible. Vendors of single-supply op amps claim their amplifiers have rail-to-rail input capability. This equivalent voltage has a nonlinear response. The simple idea of an op amp's CMR (common-mode rejection) has been around since the beginning of op-amp time. You can easily compensate for this type of error by replacing any of the four resistors with a potentiometer. The voltage at the inverting input of the amp tracks the voltage at the amp's noninverting input.and current-feedback amps are the same. and a voltage reference or the power supply (Figure 1). Alternatively. the amplifier's closed-loop bandwidth depends less on the closed-loop gain and the number of input channels. RT is the thermistor resistance at temperature T. December 2008. Figure 1: This composite input stage of the op amp uses PMOS and NMOS differential pairs so the input-voltage range can extend from above the positive rail to below the negative rail.com Single-supply-amplifier. R1 . Most product data sheets provide illustrations of the effects of input/gain-overload conditions. the PMOS transistors are in operation. As you work with these concepts. 2. First. The negative-temperature-coef ficient thermistor best suits precision temperature measurements. matching the R 1 to-R2 ratio to the R3 -to-R4 ratio–is critical. 26 Back to top About the author Bonnie Baker is a Senior Applications Engineer with the WEBENCH® team for Texas Instruments and has been involved with analog and digital designs and systems for over 25 years. More important. V OH is the absolute maximum voltage level with respect to V DD (drain-to-drain voltage) that the output can reach. an INA is easy to use. ΔV CM is the changing common-mode voltage that you apply equally to the system's input terminals with respect to ground. Photodiode Amplifiers: Op Amp Solutions. V OUT . you can use hardware-linearization techniques before digitization and a lower resolution ADC. Setting the PGA (programmable-gain amplifier) at a gain of 1V/V. 17 Back to top The non-negotiable single-supply operational amplifier Fundamental analog devices that serve applications such as high-resolution delta-sigma or SAR (successive-approximation-register) converter systems are feeling the crunch from amplifiers that have difficulty with achieving good rail-to-rail input performance." Electronics World. You may recall that this topology exhibited an offsetvoltage. making it difficult to detect the correctness of the INA input implementation. http://www. Figure 1: This single PMOS differential input stage in conjunction with a high-side chargepump eliminates the common-mode crossover distortion found in composite input stages. decades-old operational amplifier to gain a differential input signal. Since the V B voltage is static. R2 . the INA input stage saturates or floats to an undesirable voltage. The input voltages at V IN+ and V IN– and the gain of A 1 and A 2 cause the internal output voltages to increase or decrease. if you look at the common-mode range of the same device. As such. and a PGA to overcome the measurement difficulties of a nonlinear thermistor across a temperature range greater than ±25°C. as long the voltage at V O remains between the power-supply rails. A mismatch be-tween the resistor ratios creates an error at the output of A 3 . if the op amp's input stage has a charge pump with a THD specification that is 0. and the differential amp removes this effect. V B. even though the input signals continue to see a gain of –2V/V. This is true for the inverting-amplifier circuit found in Figure 3. requiring calibrated and impractical measurement conditions. The changes in output swing of A 1 and A 2 typically span the supply rails. you may notice that an input signal into an INA can produce an incorrect output signal that is nevertheless within the device's normal output range. the application-circuit configurations for voltage.com/ww/en/analog/Amplifiers-eBook/[1/14/14 1:13:55 PM] Back to top . and V OL is the absolute minimum voltage level that the output can reach. One technique is to place a resistor. A 3 and R2 form a difference amp. Figure 2: As the amplifier's common-mode voltage changes from ground to the positive supply. V L is the minimum voltage level at the output in the dc-open-loop-gain measurement. the incident light on the photodiode causes current to flow through the diode from cathode to anode. and ΔV OUT is the change in the system's output voltage with respect to the changing V CM values. an amplifier with a charge-pump input stage does not create this unwanted distortion across the entire input range. Circuit designers often misapply a thermocouple or even a two-wire microphone to the INA's input circuit. The calculated CMR (common-moderejection) error due to resistor mismatches is 100×(1+R 2 /R1 )/(% of mismatch error). the microcontroller sets the PGA gain to the next higher or lower gain setting. "OPA365. through a 10-kΩ resistor. Trump. Figure 3: The inverting-amplifier circuit keeps the amplifier's common-mode voltage at a constant voltage. low-power OPA344 input stage in Figure 1 has rail-to-rail input operation. March 2006. resistive device that eases one of the interface issues as you convert the thermistor resistance to voltage. RRI/O. Texas Instruments. Figure 1 shows a typical single-supply amplifier's output swing as you drive the output to the rails. with an input voltage of 4V p-p. In this circuit. Don't make this assumption! If you use a voltage. this type of mismatch can introduce nonlinearities in the system when the common-mode voltage of the two inputs changes. as V IN changes. 3. Kugelstadt." TI E2E Community. Eventually. The transfer function for the circuit is expressed as The input common-mode voltage (VIN) can vary between ground and V S. with a voltage-feedback amp. V IN. and V L is greater than V OL .3V/µsec. and the PMOS transistors are off. 2006. there are performance compromises that the circuit designer must address. if you have a voltage-feedback amp with a gain-bandwidth product of 180 MHz and there are three input channels (N=3) at a gain of –2V/V. bias current. Grame. if R1 equals R3 and R2 equals R4 . V S=5V). a noticeable gain error can occur between the two input signals. April 2. This specification encompasses the full amplifier input range. Nov 26. January 1995. it is easy to assume that the closed-loop bandwidth equals the gain-bandwidth product divided by each channel's gain. You must be aware of the limitations of V OA1 and V OA2 and then calculate whether your design is at risk using the equations in Figure 1. causes the INA's output to change to an invalid output voltage. but. the composite amplifier exhibits some limitations in linearity due to the input-stage topology. Although the precision. "OPA333. the NMOS transistors start to turn on. the thermistor is a high-impedance. or complementary. the microcontroller can again acquire a new ADC value. D). you would first pick the optimum feedback resistor. or |–2V/V|. For this amplifier. Let's consider the behavior of the composite and chargepump amplifier inputs in a single-supply inverting amplifier. if they have the proper input circuitry to support this function. and R4 are approximately the same value and the ratio of R3 to R4 is 1.and positive-temperature-coefficient devices. 21 Back to top . if you have a voltage-feedback amp with a gain-bandwidth product of 180 MHz and there are three input channels (N=3) at a gain of –2V/V. current-feedback amps can better solve high-speed problems than their voltage-feedback counterparts. Additional channels reduce the closed-loop bandwidth. the circuit's closed-loop bandwidth is 25. or |–2V/V|. Don't make this assumption! If you use a voltage.Voltage.and current-feedback amps are almost the same Current-feedback amplifiers have a higher slew rate than do voltage-feedback amplifiers. At first glance. For instance. If you design this circuit with such an amp. You would then select the appropriate value for RIN. equals the gain-bandwidth product divided by the noise gain. the closed-loop gain of each channel is –2V/V. the amplifier's closed-loop bandwidth depends less on the closed-loop gain and the number of input channels. even though the input signals continue to see a gain of –2V/V. generally. If that scenario is a concern. the application-circuit configurations for voltage. but. The voltage-feedback-amp's feedback-resistance value is more forgiving.or a voltage-feedback amp. per the manufacturer's specification and the circuit's noise gain. First. it is easy to assume that the closed-loop bandwidth equals the gain-bandwidth product divided by each channel's gain. The feedback resistor's higher values reduce the current-feedback amp's bandwidth. the noise gain is: (1) where N is the number of input channels. Lower value resistors reduce the current-feedback amp's stability. This amplifier's drive capability limits the resistor's minimum value. with a voltage-feedback amp.and current-feedback amps are the same. Figure 1 shows a circuit that is appropriate for either a current. but you can reduce the feedback-resistor value with the current-feedback-amp circuit and get an increase in circuit bandwidth. You can find the prescribed feedback-resistor value in the current-feedback amp's product data sheet. If the feedback resistance.and voltage-feedback amps. except for a few key points. go back and refine your feedback-resistor selection. where RIN is the input resistance. if you add channels to the circuit. This circuit's bandwidth. equals 2R IN. If you use a current-feedback amp with the circuit in Figure 1. For both current. you would first pick the optimum feedback resistor. RF . the feedback resistor of a current-feedback-amp circuit must stay within a small range of values.or current-feedback amp with the circuit in Figure 1. a small variation in the signal bandwidth and gain peaking in circuit may occur. and the overall circuit noise limits the maximum value. From this point. As such.7 MHz. The name "current-feedback amp" carries some mystique. the noise gain always equals the result of Equation 1. If you design this circuit with such an amp. the microcontroller sets the PGA gain to the next higher or lower gain setting. In this equation. requiring special considerations in some classes of circuits. Bonnie C. usually in the middle of the INA's output range. and A 3 are constants that the thermistor manufacturer provides. A 3 measures an erroneous difference voltage. Edgar Sánchez-Sinencio. At first glance. the instrumentation amplifier's CMR performance values tend to reach a maximum value at higher gains. In a classic. The microcontroller firmware's temperature-sensing algorithm reads the 10-bit-ADC digital value and passes it to a PGA hysteresis-software routine. Bonnie. the relationship of R1 . The voltage-feedback-amp's feedback-resistance value is more forgiving." BC Components data sheet. causes the INA's output to change to an invalid output voltage. you can use hardware-linearization techniques before digitization and a lower resolution ADC. 2005. As you work with these concepts. © Copyright 2014 EDN. At higher instrumentation-amplifier gains.and voltage-feedback amps. If the resistors around A 3 are not equal. you need a high-resolution converter to capture data during temperature extremes. generally. The quick and inexpensive solutions described here can help you avoid the INA circuit's input-stage pitfalls. the incident light on the photodiode causes current to flow through the diode from cathode to anode. These errors degrade the CMR of the instrumentation amplifier at higher gains. both inputs have a path to ground–in this case. The thermistor-resistance change over temperature is nonlinear. a 10-bit ADC. you may notice that an input signal into an INA can produce an incorrect output signal that is nevertheless within the device's normal output range." TI E2E Community. if you choose to use a rail-to-rail input amplifier. where she navigates the ins and outs of signal chain designs. Charles. Texas Instruments. The charge-pump input amplifier does not produce this same distortion. Bruce. however. a microcontroller. You can determine the surrounding thermistor temperature by using the Steinhart-Hart equation: T=1/(A 0 +A 1 (lnR T )+A 3 (lnR T 3 )). an INA is easy to use. An example of this type of input/gain-overload condition occurs when the in-range voltages on the INA's inputs drive A 1 . however. If that scenario is a concern. . ISBN: 0-07-024247. R3 . Consequently. and R4 to the instrumentation amplifier's CMR–specifically. April 1. AMSC.or current-feedback amp with the circuit in Figure 1. if you look at the common-mode range of the same device. it pays to understand the CMRR impact on your circuit. 3. If you use three op amps in a differential-photodiode-amp configuration. So what is the hang-up? Equation 1 yields the common CMR for a single op amp and instrumentation amplifier: Equation 1 where G is the system gain. or both to their positive or negative output-swing limit. Sept 15. the output swing never spans all the way to the rails. 2. RSER . the circuit's closed-loop bandwidth is 25. the dctransfer function of this circuit is 1V/V." EDN. A 1 and A 2 require low-inputcurrent CMOS or FET op amps. If the feedback resistance. A 3 rejects the common-mode signal and delivers the differential-voltage signal to the circuit output at V OUT . TAMU. Setting the PGA (programmable-gain amplifier) at a gain of 1V/V. The first and most dominant factor is the balance of the resistor ratios across A 3 . You can now use those graphs to your advantage. This story goes deeper. the differential amp rejects common-mode-voltage changes. March 27. Circuit designers often misapply a thermocouple or even a two-wire microphone to the INA's input circuit. Figure 1 shows a circuit that is appropriate for either a current. a 10-bit ADC in this circuit can sense a limited temperature range (approximately ±25°C). All rights reserved. At a real-world level. You can easily compensate for this type of error by replacing any of the four resistors with a potentiometer." The two basic types of thermistors are negative. August 1993. Bonnie has a drive to share her knowledge and experience. the amplifier's closed-loop bandwidth depends less on the closed-loop gain and the number of input channels. "Rail-to-Rail Op Amps. Baker. 4Q 2005. note that resolution is lost at high temperatures. The voltage at the inverting input of the amp tracks the voltage at the amp's noninverting input. As long as the resistors surrounding A 3 are equal. with a voltage-feedback amp.1% mismatch will cause a degradation of the instrumentation amplifier's CMR from ideal to a 66-dB level. Most product data sheets provide illustrations of the effects of input/gain-overload conditions. As you separate the parts.001 of R1 /R2 . 23 Back to top Instrumentation amplifier input-circuit strategies Many industrial and medical applications use instrumentation amplifiers to condition small signals in the presence of large common-mode voltages. the noise gain always equals the result of Equation 1. How would you solve this problem? Comment below. Nov 26. but A 1 and A 2 's open-loop gain error places a limit on this strategy. Note that the A 1 and A 2 stages do not gain the input common-mode voltage. The changes in output swing of A 1 and A 2 typically span the supply rails. the change of offset voltage is the only concern. you would first pick the optimum feedback resistor. From this point. the photodiode's currents flow through the R1 feedback resistors. Heed those considerations for the INA's input stage. December 2008. if you have a voltage-feedback amp with a gain-bandwidth product of 180 MHz and there are three input channels (N=3) at a gain of –2V/V. With CMR. and you can be confident that the INA's output voltage is representative of the thermocouple's voltage.ti. this configuration rejects external common-mode voltage and noise. the V OA1 and V OA2 output levels of A 1 and A 2 . But wait–there may be a problem with the output values of A 1 and A 2 (Figure 1).or a voltage-feedback amp. however. the amp's outputs change in voltage along with the IR drop across the R1 resistors. March 27. An input/gain-overload condition can occur if A 1 's output voltage.ti. In addition to her fascination with circuit design. but you can reduce the feedback-resistor value with the current-feedback-amp circuit and get an increase in circuit bandwidth. and Lew Counts. An equal illumination on the two photodiodes makes the output voltage 0V. the thermistor is a high-impedance.com. Bruce. and A 0 . the inner workings of the op amp are straightforward. you could use a single photodiode to determine the brightness of your bulbs. The common-mode behavior of the three-op-amp INA may surprise you if you ignore the nuances of the internal A 1 and A 2 output stages. References 1. The feedback resistor's higher values reduce the current-feedback amp's bandwidth. You can implement a differential amp discretely or with an off-the-shelf product. you will see greater accuracy in proximity and difference (Figure 1). this 0. Equation 1 might motivate an instrumentation-amplifier designer to ensure that there is plenty of gain available. Three basic kinds of nodes in the INA can cause input/gain-overload problems (Figure 1). Adding a matched photodiode and monitoring the difference between the two diode outputs removes the equal offsets the two diodes produce. the CMR of the three-op-amp instrumentation amplifier is ideally infinite. conference papers. However. unless you expanded your design task to using two photodiodes. the feedback resistor of a current-feedback-amp circuit must stay within a small range of values. it is easy to assume that the closed-loop bandwidth equals the gain-bandwidth product divided by each channel's gain. Additional channels reduce the closed-loop bandwidth. V OUT . A mismatch be-tween the resistor ratios creates an error at the output of A 3 . Jerald. or ground. 2009. instrumentation amplifiers are systems in which various parts contribute to the CMR error at different system gains. The negative-temperature-coef ficient thermistor best suits precision temperature measurements. The PGA hysteresis routine checks the PGA gain setting and compares the ADC digital value with the trip points that Figure 1 indicates. but. Kugelstadt. in series with the thermistor. Baker. R2 . 2. three-op-amp INA (Figure 1). R2 . and the V OUT output-swing capability of A 3 . One technique is to place a resistor. go back and refine your feedback-resistor selection. In Figure 1. 50 MHz. This amplifier's drive capability limits the resistor's minimum value. Even though you can run this type of algorithm in your microcontroller firmware. the open-loop gain error of the op amps dominates. 3. I'd really like to hear from you! References 1." Electronics World. through a 10-kΩ resistor. Under normal conditions. Thus. For both current. Consequently. In this condition. 2006. The applied input voltages to the INA are equivalent to the common-mode input voltage plus or minus the differential input signal. and a PGA to overcome the measurement difficulties of a nonlinear thermistor across a temperature range greater than ±25°C. Texas Instruments. You must be aware of the limitations of V OA1 and V OA2 and then calculate whether your design is at risk using the equations in Figure 1. Equation 2 gives the contribution to the CMR error with respect to the relationship of these resistors: Equation 2 For instance. January 1995. Photodiode Monitoring with Op Amps. Increasing the PGA's gain at these temperatures brings the output signal of the PGA back into a range at which the ADC can reliably provide conversions that identify the thermistor temperature. or |–2V/V|. A 1 . except for a few key points. It is true that circuit designers can use rail-to-rail input amplifiers in virtually any op-amp configuration. as long as you pay attention to the input stage and the output of the first stage. High CMRR." Sensors magazine.com/ww/en/analog/Amplifiers-eBook/[1/14/14 10:58:49 AM] Back to top . April 2. the input stage has two op amps in an adjustable-gain configuration and provides high input impedance on both the inverting (VIN−) and noninverting (VIN+) inputs. and authored a book: "A Baker's Dozen: Real Analog Solutions for Digital Designers. CMR A3 is equivalent to the CMR of the entire instrumentation amplifier. The input voltages at V IN+ and V IN– and the gain of A 1 and A 2 cause the internal output voltages to increase or decrease. The level shift is convenient in singlesupply applications. as the Steinhart-Hart equation describes. 25 Back to top A good holiday-season project Which of the bulbs on a Christmas tree is the brightest? If you had the time and desire to answer this question. The configurations of A 1 and A 2 act as traditional current-to-voltage converters or transimpedance amps. in a single-supply environment. References 1. The definition of an op amp's openloop gain has not changed. the microcontroller can again acquire a new ADC value. INAs have complete closed-loop operational amplifiers with feedback components included. The invalid voltage can appear to be legitimate. References 1. Two factors influence an instrumentation amplifier's CMR. where RIN is the input resistance. if R1 . drive a reference current through the thermistor to create an equivalent voltage.and current-feedback amps are the same." Check out her latest blog. "Introduction to NTCs: NTC Thermistors. Grame. A 2 's output voltage. and R4 are approximately the same value and the ratio of R3 to R4 is 1. However. per the manufacturer's specification and the circuit's noise gain.7 MHz. So how do designers calculate the INA's common-mode range? Consider the exposure to an input/gain-overload condition on the INA as a possibility. This circuit's bandwidth. http://www.com input stages Charge-pump input: performs without distortion in reaction to common-mode voltage changes Conclusion Manufacturers use several input topologies in their designs of rail-to-rail input amplifiers. 2. 3. subtracting the output voltages of A 1 and A 2 . sboa035. from the CMR perspective. ΔV CM is the changing common-mode voltage that you apply equally to the system's input terminals with respect to ground. RTHERM . You can reach Bonnie at ti_bonniebaker@list. The PGA gain and ADC value then pass to a microcontroller piecewise linear-interpolation routine. The composite input amplifier produces a crossover distortion. 24 Back to top Wringing out thermistor nonlinearities Thermistor-temperature-sensing devices present a design challenge if you intend to use such a device over its entire temperature range. requiring calibrated and impractical measurement conditions. the offset errors from the amplifier's open-loop gain also increase. So. the instrumentation amplifier's CMR increases as the system's gain increases–a nice feature. The two photodiodes let you find a light's position by monitoring the difference between their output signals. References 1. the input-current leakages are not dissipated. this type of mismatch can introduce nonlinearities in the system when the common-mode voltage of the two inputs changes. This equivalent voltage has a nonlinear response. The output voltages of A 1 and A 2 contain both difference and common-mode signals. As Equation 1 states. Without the current-return path and the bias-voltage reference. equals the gain-bandwidth product divided by the noise gain. offset. pg 69. 26 Back to top About the author Bonnie Baker is a Senior Applications Engineer with the WEBENCH® team for Texas Instruments and has been involved with analog and digital designs and systems for over 25 years. "The right way to use instrumentation amplifiers.TI. First. matching the R 1 to-R2 ratio to the R3 -to-R4 ratio–is critical. noise. FET gate. and temperature drift. When the circuit designer exercises proper precautions with the input pins. Trump. This condition is impossible to directly measure. D). The name "current-feedback amp" carries some mystique. At a gain of one. "Instrumentation amplifiers—avoiding a common pitfall. All of the internal amplifier's output voltages relate to the linear common-mode input range of the complete INA. The key performance parameters for A 1 and A 2 are input capacitance. Pay attention to the voltage levels of the INA's V IN+ and V IN– input pins." Analog Applications Journal. A 2 . Of all the performance characteristics of an INA. As the gain of A 1 and A 2 increases. Trump. where V OS is the offset voltage. Don't make this assumption! If you use a voltage. The term "thermistor" originates from the descriptor "thermally sensitive resistor. Obtaining data from a nonlinear thermistor sometimes can seem like an impossible task. V CM . or both violate the internal output-swing restrictions. Lower value resistors reduce the current-feedback amp's stability. even though the input signals continue to see a gain of –2V/V. 2012. A single photodiode provides some measure of a light's intensity through the magnitude of the diode's output signal. The goal is to select amps in which these parameters are as low as possible. resistive device that eases one of the interface issues as you convert the thermistor resistance to voltage. If you use a current-feedback amp with the circuit in Figure 1. the closed-loop gain of each channel is –2V/V. Precision Unity Gain Differential Amplifier. 20 Back to top Voltage.and current-feedback amps are almost the same Current-feedback amplifiers have a higher slew rate than do voltage-feedback amplifiers. Kitchin. the application-circuit configurations for voltage. Either condition creates an invalid output voltage. the noise gain is: (1) where N is the number of input channels. bias current. She has written hundreds of articles. of the INA. The final place to look for an input/gain-overload condition is at the output. Depending on the INA's silicon process. Single-Supply Operational Amplifier (Rev. The input stages of A 1 and A 2 limit the range of these two input voltages. if you add channels to the circuit. Finding that brightest bulb among the many and the background light would be a laborious task. the INA input stage saturates or floats to an undesirable voltage. or CMOS gate. The A 3 output restrictions are similar to those of any other amplifier. You can try to compensate for the thermistor's nonlinear response with a look-up table in your microcontroller. 2012. "Instrumentation amplifiers—they're not op amps butwhat are they?" TI E2E Community. Photodiode Amplifiers: Op Amp Solutions. and a voltage reference or the power supply (Figure 1). More important. You can find the prescribed feedback-resistor value in the current-feedback amp's product data sheet. You would then select the appropriate value for RIN. The output stage has four matched resistors around a single op amp. 2001. and the overall circuit noise limits the maximum value. Figure 2b illustrates a correct thermocouple connection to an INA. R3 . Because the inverting input of A 1 and A 2 has high impedance. Texas Instruments. The op amp's input offset-voltage error is easy to understand. You define the common-mode voltage of the input signals as (VA1OUT +V A2OUT )/2. This situation is not so mysterious when you think about the inside of this device. McGraw-Hill. The more difficult interface challenge is to capture the nonlinear behavior of the thermistor in a digital representation with a linear ADC. biased to a voltage within the INA's input range. Ideally. For instance. 2. This erroneous voltage plus the voltage reference to the INA is incorrect and may be inside the output range of A 3 . RF .Texas Instruments . the problem arises when the INA inputs are connected without proper consideration to current paths or biasing. pg 14). RG. if R1 equals R3 and R2 equals R4 . You can combine a series resistor. As with many INAs. Alternatively. 22 Back to top Common-mode range can bite hard My last column provided a glimpse inside the CMR (common-mode rejection) of a three-op-amp INA (instrumentation amplifier) and revealed the main contributors to total CMR error (see "Understanding CMR and instrumentation amplifiers. "Getting themost out of your instrumentationamplifier design. 27 Content provided courtesy of EDN. and ΔV OUT is the change in the system's output voltage with respect to the changing V CM values. "Advances in measuring with nonlinear sensors. Amplifiers and Linear. a small variation in the signal bandwidth and gain peaking in circuit may occur.and positive-temperature-coefficient devices. An amplifier's open-loop gain is 20log(ΔV OUT /ΔV OS ). the most misunderstood is the common-mode-range requirement. making it difficult to detect the correctness of the INA input implementation. On Board with Bonnie. That situation." Ax-09. For instance. which makes the magnitude of the output voltage a direct measurement of the difference between the direct light impinging on D1 and D2 . All inputs to the INA require a current-return path to ground and a bias-voltage reference. you can program the gain found in Figure 1 with a single resistor. In this circuit. SBOS145. The output stage has a reference voltage that level-shifts the output with respect to ground. A 3 and R2 form a difference amp. The simple idea of an op amp's CMR (common-mode rejection) has been around since the beginning of op-amp time. however. 2005." EDN. As such. The maximum and minimum input-voltage limits vary from device to device. backgroundlight conditions can influence this magnitude. V IN+ and V IN− connect to a bipolar transistor base. D1 and D2 respond linearly to illumination intensity. For instance. and the two inputs can float to any undefined voltage. If the ADC output is beyond a trip-point value. Low-Distortion. a noticeable gain error can occur between the two input signals. current-feedback amps can better solve high-speed problems than their voltage-feedback counterparts. T is the temperature in degrees Kelvin. in turn. "CMRR not created equal for all INAs. If necessary. Texas Instruments. design and application notes. RT is the thermistor resistance at temperature T. and the differential amp removes this effect. decades-old operational amplifier to gain a differential input signal. When measuring temperature. 2. 21 Back to top Chapter 2: INAs and PGAs Understanding CMR and instrumentation amplifiers The three-op-amp instrumentation amplifier in Figure 1 is seemingly a simple configuration in that it uses a basic. equals 2R IN. Thomas. Typically. The calculated CMR (common-moderejection) error due to resistor mismatches is 100×(1+R 2 /R1 )/(% of mismatch error). RRI/O. Background light adds only an offset to the photodiode's outputs. the picture becomes clear. The floating thermocouple circuit in Figure 2a does not provide a current path to ground or a bias-voltage reference for the INA's input pins. These four resistors combine with A 3 to subtract and gain the signals from the outputs of A 1 and A 2 . As long as the resistors surrounding A 3 are equal. The level shift is convenient in singlesupply applications. R3 . Equation 1 might motivate an instrumentation-amplifier designer to ensure that there is plenty of gain available. usually in the middle of the INA's output range. Increasing the PGA's gain at these temperatures brings the output signal of the PGA back into a range at which the ADC can reliably provide conversions that identify the thermistor temperature. If you use a current-feedback amp with the circuit in Figure 1. Either condition creates an invalid output voltage. it is easy to assume that the closed-loop bandwidth equals the gain-bandwidth product divided by each channel's gain. The op amp's input offset-voltage error is easy to understand. The voltage-feedback-amp's feedback-resistance value is more forgiving. if you choose to use a rail-to-rail input amplifier. RTHERM . a microcontroller. R2 . equals the gain-bandwidth product divided by the noise gain. INAs have complete closed-loop operational amplifiers with feedback components included. Thus. the application-circuit configurations for voltage. however. "CMRR not created equal for all INAs. the differential amp rejects common-mode-voltage changes. "The right way to use instrumentation amplifiers. CMR A3 is equivalent to the CMR of the entire instrumentation amplifier. Single-Supply Operational Amplifier (Rev. Without the current-return path and the bias-voltage reference. Lower value resistors reduce the current-feedback amp's stability. Figure 1 shows a circuit that is appropriate for either a current. Finding that brightest bulb among the many and the background light would be a laborious task. the offset errors from the amplifier's open-loop gain also increase. For instance. and Lew Counts. and you can be confident that the INA's output voltage is representative of the thermocouple's voltage. You would then select the appropriate value for RIN. a noticeable gain error can occur between the two input signals. Two factors influence an instrumentation amplifier's CMR. 2012. the input stage has two op amps in an adjustable-gain configuration and provides high input impedance on both the inverting (VIN−) and noninverting (VIN+) inputs. the CMR of the three-op-amp instrumentation amplifier is ideally infinite. the amp's outputs change in voltage along with the IR drop across the R1 resistors. go back and refine your feedback-resistor selection. you can program the gain found in Figure 1 with a single resistor. © Copyright 2014 EDN. this configuration rejects external common-mode voltage and noise. AMSC. 23 Back to top Instrumentation amplifier input-circuit strategies Many industrial and medical applications use instrumentation amplifiers to condition small signals in the presence of large common-mode voltages. You can combine a series resistor.com input stages Charge-pump input: performs without distortion in reaction to common-mode voltage changes Conclusion Manufacturers use several input topologies in their designs of rail-to-rail input amplifiers.or a voltage-feedback amp. the instrumentation amplifier's CMR performance values tend to reach a maximum value at higher gains. causes the INA's output to change to an invalid output voltage. the inner workings of the op amp are straightforward. or CMOS gate. Texas Instruments. The composite input amplifier produces a crossover distortion. you can use hardware-linearization techniques before digitization and a lower resolution ADC. if R1 equals R3 and R2 equals R4 . The microcontroller firmware's temperature-sensing algorithm reads the 10-bit-ADC digital value and passes it to a PGA hysteresis-software routine. The feedback resistor's higher values reduce the current-feedback amp's bandwidth. and a PGA to overcome the measurement difficulties of a nonlinear thermistor across a temperature range greater than ±25°C. Texas Instruments. however. With CMR. The A 3 output restrictions are similar to those of any other amplifier. and R4 to the instrumentation amplifier's CMR–specifically. Heed those considerations for the INA's input stage. How would you solve this problem? Comment below. Kugelstadt. Obtaining data from a nonlinear thermistor sometimes can seem like an impossible task. The changes in output swing of A 1 and A 2 typically span the supply rails. Depending on the INA's silicon process.com/ww/en/analog/Amplifiers-eBook/[1/14/14 10:58:49 AM] Back to top .ti. offset. A single photodiode provides some measure of a light's intensity through the magnitude of the diode's output signal. Consequently. If the resistors around A 3 are not equal. A 1 and A 2 require low-inputcurrent CMOS or FET op amps. and A 3 are constants that the thermistor manufacturer provides. generally. instrumentation amplifiers are systems in which various parts contribute to the CMR error at different system gains." Check out her latest blog. the input-current leakages are not dissipated. 2001.and current-feedback amps are the same. Thomas. 4Q 2005. The configurations of A 1 and A 2 act as traditional current-to-voltage converters or transimpedance amps. The two photodiodes let you find a light's position by monitoring the difference between their output signals. The term "thermistor" originates from the descriptor "thermally sensitive resistor. the instrumentation amplifier's CMR increases as the system's gain increases–a nice feature. D1 and D2 respond linearly to illumination intensity. A 3 and R2 form a difference amp. The first and most dominant factor is the balance of the resistor ratios across A 3 . As you work with these concepts. per the manufacturer's specification and the circuit's noise gain. As Equation 1 states. References 1. The negative-temperature-coef ficient thermistor best suits precision temperature measurements. This equivalent voltage has a nonlinear response. Low-Distortion. the dctransfer function of this circuit is 1V/V. 25 Back to top A good holiday-season project Which of the bulbs on a Christmas tree is the brightest? If you had the time and desire to answer this question. pg 69. or both to their positive or negative output-swing limit. or ground. Consequently. subtracting the output voltages of A 1 and A 2 . in turn. you will see greater accuracy in proximity and difference (Figure 1). References 1." Sensors magazine. 3. drive a reference current through the thermistor to create an equivalent voltage. the feedback resistor of a current-feedback-amp circuit must stay within a small range of values. In addition to her fascination with circuit design. A 3 measures an erroneous difference voltage. and R4 are approximately the same value and the ratio of R3 to R4 is 1. RT is the thermistor resistance at temperature T. 2. She has written hundreds of articles. both inputs have a path to ground–in this case. if R1 . This condition is impossible to directly measure. Baker. For instance. the output swing never spans all the way to the rails. An input/gain-overload condition can occur if A 1 's output voltage. but you can reduce the feedback-resistor value with the current-feedback-amp circuit and get an increase in circuit bandwidth. These four resistors combine with A 3 to subtract and gain the signals from the outputs of A 1 and A 2 . requiring special considerations in some classes of circuits. In a classic. RF . and the differential amp removes this effect.com. The PGA hysteresis routine checks the PGA gain setting and compares the ADC digital value with the trip points that Figure 1 indicates. As you separate the parts. The maximum and minimum input-voltage limits vary from device to device. if you look at the common-mode range of the same device. "Instrumentation amplifiers—they're not op amps butwhat are they?" TI E2E Community. however.and voltage-feedback amps. References 1. This circuit's bandwidth. Most product data sheets provide illustrations of the effects of input/gain-overload conditions. 21 Back to top Chapter 2: INAs and PGAs Understanding CMR and instrumentation amplifiers The three-op-amp instrumentation amplifier in Figure 1 is seemingly a simple configuration in that it uses a basic. 2012. Texas Instruments. April 2. The input stages of A 1 and A 2 limit the range of these two input voltages. Trump. All rights reserved. 20 Back to top Voltage. Bruce. The more difficult interface challenge is to capture the nonlinear behavior of the thermistor in a digital representation with a linear ADC.and current-feedback amps are almost the same Current-feedback amplifiers have a higher slew rate than do voltage-feedback amplifiers. You must be aware of the limitations of V OA1 and V OA2 and then calculate whether your design is at risk using the equations in Figure 1. the INA input stage saturates or floats to an undesirable voltage. Trump. The charge-pump input amplifier does not produce this same distortion. At first glance. The name "current-feedback amp" carries some mystique. High CMRR. If necessary. and temperature drift. this type of mismatch can introduce nonlinearities in the system when the common-mode voltage of the two inputs changes. A mismatch be-tween the resistor ratios creates an error at the output of A 3 . Bonnie has a drive to share her knowledge and experience. Equation 2 gives the contribution to the CMR error with respect to the relationship of these resistors: Equation 2 For instance. The calculated CMR (common-moderejection) error due to resistor mismatches is 100×(1+R 2 /R1 )/(% of mismatch error). as long as you pay attention to the input stage and the output of the first stage. as the Steinhart-Hart equation describes. The output voltages of A 1 and A 2 contain both difference and common-mode signals. 2." EDN. 27 Content provided courtesy of EDN. ISBN: 0-07-024247. The invalid voltage can appear to be legitimate. the picture becomes clear. All of the internal amplifier's output voltages relate to the linear common-mode input range of the complete INA. The thermistor-resistance change over temperature is nonlinear. At a real-world level. So what is the hang-up? Equation 1 yields the common CMR for a single op amp and instrumentation amplifier: Equation 1 where G is the system gain. For instance. resistive device that eases one of the interface issues as you convert the thermistor resistance to voltage. Photodiode Monitoring with Op Amps. That situation. V OUT . In this condition. Even though you can run this type of algorithm in your microcontroller firmware. you may notice that an input signal into an INA can produce an incorrect output signal that is nevertheless within the device's normal output range. Ideally. A 2 .or current-feedback amp with the circuit in Figure 1. V IN+ and V IN− connect to a bipolar transistor base. 2005. References 1. the most misunderstood is the common-mode-range requirement.and positive-temperature-coefficient devices. As with many INAs. decades-old operational amplifier to gain a differential input signal. "Instrumentation amplifiers—avoiding a common pitfall. 2006. with a voltage-feedback amp. Setting the PGA (programmable-gain amplifier) at a gain of 1V/V. The output stage has a reference voltage that level-shifts the output with respect to ground.7 MHz. where V OS is the offset voltage. The input voltages at V IN+ and V IN– and the gain of A 1 and A 2 cause the internal output voltages to increase or decrease. You define the common-mode voltage of the input signals as (VA1OUT +V A2OUT )/2.Texas Instruments . the closed-loop gain of each channel is –2V/V.TI. The simple idea of an op amp's CMR (common-mode rejection) has been around since the beginning of op-amp time. but. McGraw-Hill. Typically. V CM . If you design this circuit with such an amp. You can determine the surrounding thermistor temperature by using the Steinhart-Hart equation: T=1/(A 0 +A 1 (lnR T )+A 3 (lnR T 3 )). Grame. or |–2V/V|. Three basic kinds of nodes in the INA can cause input/gain-overload problems (Figure 1). The PGA gain and ADC value then pass to a microcontroller piecewise linear-interpolation routine. So." Electronics World. Bruce. Sept 15. This story goes deeper. The final place to look for an input/gain-overload condition is at the output. noise. RG. If that scenario is a concern. When the circuit designer exercises proper precautions with the input pins. You can now use those graphs to your advantage. the open-loop gain error of the op amps dominates. If you use three op amps in a differential-photodiode-amp configuration. Pay attention to the voltage levels of the INA's V IN+ and V IN– input pins. March 27. requiring calibrated and impractical measurement conditions. 2. March 27. An equal illumination on the two photodiodes makes the output voltage 0V. Texas Instruments. even though the input signals continue to see a gain of –2V/V. You can find the prescribed feedback-resistor value in the current-feedback amp's product data sheet. the noise gain is: (1) where N is the number of input channels. and the overall circuit noise limits the maximum value. The goal is to select amps in which these parameters are as low as possible. in series with the thermistor. 26 Back to top About the author Bonnie Baker is a Senior Applications Engineer with the WEBENCH® team for Texas Instruments and has been involved with analog and digital designs and systems for over 25 years. So how do designers calculate the INA's common-mode range? Consider the exposure to an input/gain-overload condition on the INA as a possibility. 24 Back to top Wringing out thermistor nonlinearities Thermistor-temperature-sensing devices present a design challenge if you intend to use such a device over its entire temperature range. matching the R 1 to-R2 ratio to the R3 -to-R4 ratio–is critical. conference papers. 50 MHz. The voltage at the inverting input of the amp tracks the voltage at the amp's noninverting input. 3. note that resolution is lost at high temperatures. A 2 's output voltage." The two basic types of thermistors are negative. The quick and inexpensive solutions described here can help you avoid the INA circuit's input-stage pitfalls. Bonnie. you need a high-resolution converter to capture data during temperature extremes. If the ADC output is beyond a trip-point value. and A 0 . D). the thermistor is a high-impedance. Adding a matched photodiode and monitoring the difference between the two diode outputs removes the equal offsets the two diodes produce. the relationship of R1 . from the CMR perspective. You can reach Bonnie at ti_bonniebaker@list. Kitchin. the microcontroller can again acquire a new ADC value. through a 10-kΩ resistor. Precision Unity Gain Differential Amplifier. an INA is easy to use. Under normal conditions. the amplifier's closed-loop bandwidth depends less on the closed-loop gain and the number of input channels. however. It is true that circuit designers can use rail-to-rail input amplifiers in virtually any op-amp configuration. The output stage has four matched resistors around a single op amp. 22 Back to top Common-mode range can bite hard My last column provided a glimpse inside the CMR (common-mode rejection) of a three-op-amp INA (instrumentation amplifier) and revealed the main contributors to total CMR error (see "Understanding CMR and instrumentation amplifiers. Charles. As the gain of A 1 and A 2 increases. the incident light on the photodiode causes current to flow through the diode from cathode to anode. three-op-amp INA (Figure 1). Edgar Sánchez-Sinencio. the change of offset voltage is the only concern. Of all the performance characteristics of an INA. From this point. current-feedback amps can better solve high-speed problems than their voltage-feedback counterparts. the problem arises when the INA inputs are connected without proper consideration to current paths or biasing." EDN. Don't make this assumption! If you use a voltage. In Figure 1. making it difficult to detect the correctness of the INA input implementation. which makes the magnitude of the output voltage a direct measurement of the difference between the direct light impinging on D1 and D2 . Baker. Alternatively." TI E2E Community. and authored a book: "A Baker's Dozen: Real Analog Solutions for Digital Designers. "Rail-to-Rail Op Amps. I'd really like to hear from you! References 1. RRI/O. backgroundlight conditions can influence this magnitude. The key performance parameters for A 1 and A 2 are input capacitance. On Board with Bonnie. you would first pick the optimum feedback resistor. One technique is to place a resistor. This situation is not so mysterious when you think about the inside of this device. bias current. it pays to understand the CMRR impact on your circuit. In this circuit. in a single-supply environment. R2 . this 0. a 10-bit ADC in this circuit can sense a limited temperature range (approximately ±25°C). Note that the A 1 and A 2 stages do not gain the input common-mode voltage. the microcontroller sets the PGA gain to the next higher or lower gain setting. August 1993. R3 . Additional channels reduce the closed-loop bandwidth. At higher instrumentation-amplifier gains. However. "Getting themost out of your instrumentationamplifier design. and a voltage reference or the power supply (Figure 1). if you add channels to the circuit. These errors degrade the CMR of the instrumentation amplifier at higher gains. . An amplifier's open-loop gain is 20log(ΔV OUT /ΔV OS ). April 1. and the V OUT output-swing capability of A 3 . Background light adds only an offset to the photodiode's outputs. The floating thermocouple circuit in Figure 2a does not provide a current path to ground or a bias-voltage reference for the INA's input pins. "Introduction to NTCs: NTC Thermistors." Analog Applications Journal. sboa035. a small variation in the signal bandwidth and gain peaking in circuit may occur. a 10-bit ADC. But wait–there may be a problem with the output values of A 1 and A 2 (Figure 1). January 1995. where RIN is the input resistance. the noise gain always equals the result of Equation 1. This amplifier's drive capability limits the resistor's minimum value. December 2008. Amplifiers and Linear. Because the inverting input of A 1 and A 2 has high impedance. If the feedback resistance." Ax-09. You can implement a differential amp discretely or with an off-the-shelf product.1% mismatch will cause a degradation of the instrumentation amplifier's CMR from ideal to a 66-dB level. the V OA1 and V OA2 output levels of A 1 and A 2 . Jerald. except for a few key points. the circuit's closed-loop bandwidth is 25. 2. The definition of an op amp's openloop gain has not changed. The applied input voltages to the INA are equivalent to the common-mode input voltage plus or minus the differential input signal. 2. In this equation. As such. ΔV CM is the changing common-mode voltage that you apply equally to the system's input terminals with respect to ground. of the INA. pg 14). "Advances in measuring with nonlinear sensors. Figure 2b illustrates a correct thermocouple connection to an INA. FET gate. First. The common-mode behavior of the three-op-amp INA may surprise you if you ignore the nuances of the internal A 1 and A 2 output stages. TAMU. All inputs to the INA require a current-return path to ground and a bias-voltage reference.ti. You can try to compensate for the thermistor's nonlinear response with a look-up table in your microcontroller. Photodiode Amplifiers: Op Amp Solutions. For both current. You can easily compensate for this type of error by replacing any of the four resistors with a potentiometer. the photodiode's currents flow through the R1 feedback resistors. This erroneous voltage plus the voltage reference to the INA is incorrect and may be inside the output range of A 3 . More important. you could use a single photodiode to determine the brightness of your bulbs. At a gain of one. RSER . biased to a voltage within the INA's input range. Nov 26. A 1 . and the two inputs can float to any undefined voltage. 3. design and application notes. SBOS145.001 of R1 /R2 . T is the temperature in degrees Kelvin. 2005. An example of this type of input/gain-overload condition occurs when the in-range voltages on the INA's inputs drive A 1 ." BC Components data sheet. 2009. where she navigates the ins and outs of signal chain designs. When measuring temperature. but A 1 and A 2 's open-loop gain error places a limit on this strategy. However. or both violate the internal output-swing restrictions. equals 2R IN. Bonnie C. A 3 rejects the common-mode signal and delivers the differential-voltage signal to the circuit output at V OUT . http://www. if you have a voltage-feedback amp with a gain-bandwidth product of 180 MHz and there are three input channels (N=3) at a gain of –2V/V. Circuit designers often misapply a thermocouple or even a two-wire microphone to the INA's input circuit. and ΔV OUT is the change in the system's output voltage with respect to the changing V CM values. unless you expanded your design task to using two photodiodes. A 3 and R2 form a difference amp. An example of this type of input/gain-overload condition occurs when the in-range voltages on the INA's inputs drive A 1 .and current-feedback amps are almost the same Current-feedback amplifiers have a higher slew rate than do voltage-feedback amplifiers. You can now use those graphs to your advantage. In this equation. If the resistors around A 3 are not equal. Adding a matched photodiode and monitoring the difference between the two diode outputs removes the equal offsets the two diodes produce. The output stage has a reference voltage that level-shifts the output with respect to ground. however. and a PGA to overcome the measurement difficulties of a nonlinear thermistor across a temperature range greater than ±25°C. 23 Back to top Instrumentation amplifier input-circuit strategies Many industrial and medical applications use instrumentation amplifiers to condition small signals in the presence of large common-mode voltages. ΔV CM is the changing common-mode voltage that you apply equally to the system's input terminals with respect to ground. the picture becomes clear. Texas Instruments. Typically. and temperature drift. a noticeable gain error can occur between the two input signals. and a voltage reference or the power supply (Figure 1). At first glance. The voltage at the inverting input of the amp tracks the voltage at the amp's noninverting input. 2012. The output voltages of A 1 and A 2 contain both difference and common-mode signals. Figure 2b illustrates a correct thermocouple connection to an INA. Either condition creates an invalid output voltage. where V OS is the offset voltage. An input/gain-overload condition can occur if A 1 's output voltage. generally. D1 and D2 respond linearly to illumination intensity. This condition is impossible to directly measure. T is the temperature in degrees Kelvin. Note that the A 1 and A 2 stages do not gain the input common-mode voltage." BC Components data sheet. in series with the thermistor. Trump. This circuit's bandwidth. the microcontroller sets the PGA gain to the next higher or lower gain setting. you can program the gain found in Figure 1 with a single resistor. "Advances in measuring with nonlinear sensors. Increasing the PGA's gain at these temperatures brings the output signal of the PGA back into a range at which the ADC can reliably provide conversions that identify the thermistor temperature. References 1. causes the INA's output to change to an invalid output voltage. Nov 26. The composite input amplifier produces a crossover distortion. this configuration rejects external common-mode voltage and noise. and authored a book: "A Baker's Dozen: Real Analog Solutions for Digital Designers. So how do designers calculate the INA's common-mode range? Consider the exposure to an input/gain-overload condition on the INA as a possibility. So what is the hang-up? Equation 1 yields the common CMR for a single op amp and instrumentation amplifier: Equation 1 where G is the system gain. If the ADC output is beyond a trip-point value." Sensors magazine. except for a few key points. R3 . RRI/O. 22 Back to top Common-mode range can bite hard My last column provided a glimpse inside the CMR (common-mode rejection) of a three-op-amp INA (instrumentation amplifier) and revealed the main contributors to total CMR error (see "Understanding CMR and instrumentation amplifiers. and you can be confident that the INA's output voltage is representative of the thermocouple's voltage. Edgar Sánchez-Sinencio. From this point. The applied input voltages to the INA are equivalent to the common-mode input voltage plus or minus the differential input signal. If you design this circuit with such an amp. The calculated CMR (common-moderejection) error due to resistor mismatches is 100×(1+R 2 /R1 )/(% of mismatch error). The level shift is convenient in singlesupply applications.or current-feedback amp with the circuit in Figure 1. One technique is to place a resistor. Even though you can run this type of algorithm in your microcontroller firmware. Setting the PGA (programmable-gain amplifier) at a gain of 1V/V. usually in the middle of the INA's output range. Trump. as long as you pay attention to the input stage and the output of the first stage. Charles. D). you would first pick the optimum feedback resistor. 24 Back to top Wringing out thermistor nonlinearities Thermistor-temperature-sensing devices present a design challenge if you intend to use such a device over its entire temperature range. however. More important. The PGA hysteresis routine checks the PGA gain setting and compares the ADC digital value with the trip points that Figure 1 indicates. the feedback resistor of a current-feedback-amp circuit must stay within a small range of values." EDN. the thermistor is a high-impedance. a 10-bit ADC in this circuit can sense a limited temperature range (approximately ±25°C). Bonnie has a drive to share her knowledge and experience. if you add channels to the circuit. In addition to her fascination with circuit design. Bruce.001 of R1 /R2 . biased to a voltage within the INA's input range. the offset errors from the amplifier's open-loop gain also increase. from the CMR perspective. Heed those considerations for the INA's input stage. However. At a gain of one. If necessary. go back and refine your feedback-resistor selection. You can implement a differential amp discretely or with an off-the-shelf product. 2012. of the INA. in turn. For instance. Obtaining data from a nonlinear thermistor sometimes can seem like an impossible task.1% mismatch will cause a degradation of the instrumentation amplifier's CMR from ideal to a 66-dB level. through a 10-kΩ resistor. "Instrumentation amplifiers—they're not op amps butwhat are they?" TI E2E Community. January 1995. RT is the thermistor resistance at temperature T. the inner workings of the op amp are straightforward. March 27. A 1 and A 2 require low-inputcurrent CMOS or FET op amps. and the V OUT output-swing capability of A 3 . 27 Content provided courtesy of EDN. First. You can combine a series resistor. Equation 1 might motivate an instrumentation-amplifier designer to ensure that there is plenty of gain available. A 2 's output voltage. On Board with Bonnie. the input stage has two op amps in an adjustable-gain configuration and provides high input impedance on both the inverting (VIN−) and noninverting (VIN+) inputs. As long as the resistors surrounding A 3 are equal. The key performance parameters for A 1 and A 2 are input capacitance. however. Because the inverting input of A 1 and A 2 has high impedance. The simple idea of an op amp's CMR (common-mode rejection) has been around since the beginning of op-amp time. The output stage has four matched resistors around a single op amp. If you use three op amps in a differential-photodiode-amp configuration. An amplifier's open-loop gain is 20log(ΔV OUT /ΔV OS ). pg 69. but." Ax-09. decades-old operational amplifier to gain a differential input signal. or both violate the internal output-swing restrictions. Sept 15. or both to their positive or negative output-swing limit. a microcontroller. Precision Unity Gain Differential Amplifier. which makes the magnitude of the output voltage a direct measurement of the difference between the direct light impinging on D1 and D2 . if you choose to use a rail-to-rail input amplifier. an INA is easy to use. References 1. A single photodiode provides some measure of a light's intensity through the magnitude of the diode's output signal. requiring special considerations in some classes of circuits. If you use a current-feedback amp with the circuit in Figure 1. resistive device that eases one of the interface issues as you convert the thermistor resistance to voltage.or a voltage-feedback amp. "Rail-to-Rail Op Amps. McGraw-Hill. SBOS145. if you look at the common-mode range of the same device. The invalid voltage can appear to be legitimate. or ground. The name "current-feedback amp" carries some mystique. subtracting the output voltages of A 1 and A 2 . 26 Back to top About the author Bonnie Baker is a Senior Applications Engineer with the WEBENCH® team for Texas Instruments and has been involved with analog and digital designs and systems for over 25 years. April 2. Thus. Depending on the INA's silicon process. a small variation in the signal bandwidth and gain peaking in circuit may occur. the closed-loop gain of each channel is –2V/V. this type of mismatch can introduce nonlinearities in the system when the common-mode voltage of the two inputs changes. For both current. All of the internal amplifier's output voltages relate to the linear common-mode input range of the complete INA. The quick and inexpensive solutions described here can help you avoid the INA circuit's input-stage pitfalls. The input voltages at V IN+ and V IN– and the gain of A 1 and A 2 cause the internal output voltages to increase or decrease. 3. requiring calibrated and impractical measurement conditions. At higher instrumentation-amplifier gains. You can reach Bonnie at ti_bonniebaker@list. Background light adds only an offset to the photodiode's outputs. if you have a voltage-feedback amp with a gain-bandwidth product of 180 MHz and there are three input channels (N=3) at a gain of –2V/V. 2005. Ideally. the noise gain always equals the result of Equation 1. you can use hardware-linearization techniques before digitization and a lower resolution ADC. If that scenario is a concern. The two photodiodes let you find a light's position by monitoring the difference between their output signals. It is true that circuit designers can use rail-to-rail input amplifiers in virtually any op-amp configuration. Low-Distortion. V IN+ and V IN− connect to a bipolar transistor base. 3. the amplifier's closed-loop bandwidth depends less on the closed-loop gain and the number of input channels. As you separate the parts. Bruce.and positive-temperature-coefficient devices. R2 .and voltage-feedback amps. The changes in output swing of A 1 and A 2 typically span the supply rails. Without the current-return path and the bias-voltage reference." TI E2E Community. All inputs to the INA require a current-return path to ground and a bias-voltage reference. the amp's outputs change in voltage along with the IR drop across the R1 resistors. or |–2V/V|. INAs have complete closed-loop operational amplifiers with feedback components included. the application-circuit configurations for voltage. where RIN is the input resistance. Photodiode Amplifiers: Op Amp Solutions. and R4 are approximately the same value and the ratio of R3 to R4 is 1. So. The configurations of A 1 and A 2 act as traditional current-to-voltage converters or transimpedance amps. "The right way to use instrumentation amplifiers. Figure 1 shows a circuit that is appropriate for either a current. References 1. Single-Supply Operational Amplifier (Rev. the input-current leakages are not dissipated. RF . You must be aware of the limitations of V OA1 and V OA2 and then calculate whether your design is at risk using the equations in Figure 1.com. © Copyright 2014 EDN. A mismatch be-tween the resistor ratios creates an error at the output of A 3 . April 1. the INA input stage saturates or floats to an undesirable voltage. "Introduction to NTCs: NTC Thermistors. References 1. In this condition. Finding that brightest bulb among the many and the background light would be a laborious task. 25 Back to top A good holiday-season project Which of the bulbs on a Christmas tree is the brightest? If you had the time and desire to answer this question. This equivalent voltage has a nonlinear response. High CMRR. Baker. as the Steinhart-Hart equation describes. The microcontroller firmware's temperature-sensing algorithm reads the 10-bit-ADC digital value and passes it to a PGA hysteresis-software routine. drive a reference current through the thermistor to create an equivalent voltage. in a single-supply environment. you could use a single photodiode to determine the brightness of your bulbs. The goal is to select amps in which these parameters are as low as possible. 21 Back to top Chapter 2: INAs and PGAs Understanding CMR and instrumentation amplifiers The three-op-amp instrumentation amplifier in Figure 1 is seemingly a simple configuration in that it uses a basic. backgroundlight conditions can influence this magnitude. You would then select the appropriate value for RIN. With CMR. "Getting themost out of your instrumentationamplifier design.com/ww/en/analog/Amplifiers-eBook/[1/14/14 10:58:49 AM] Back to top . Equation 2 gives the contribution to the CMR error with respect to the relationship of these resistors: Equation 2 For instance. RG.ti. This situation is not so mysterious when you think about the inside of this device. three-op-amp INA (Figure 1). Amplifiers and Linear.7 MHz. the CMR of the three-op-amp instrumentation amplifier is ideally infinite. The charge-pump input amplifier does not produce this same distortion. and the two inputs can float to any undefined voltage. note that resolution is lost at high temperatures. 2. and the differential amp removes this effect. As with many INAs. CMR A3 is equivalent to the CMR of the entire instrumentation amplifier. The definition of an op amp's openloop gain has not changed. the differential amp rejects common-mode-voltage changes. where she navigates the ins and outs of signal chain designs. In this circuit. Grame. 4Q 2005. you need a high-resolution converter to capture data during temperature extremes. These errors degrade the CMR of the instrumentation amplifier at higher gains. current-feedback amps can better solve high-speed problems than their voltage-feedback counterparts. You can easily compensate for this type of error by replacing any of the four resistors with a potentiometer. 2006. Thomas. All rights reserved. if R1 equals R3 and R2 equals R4 . The more difficult interface challenge is to capture the nonlinear behavior of the thermistor in a digital representation with a linear ADC. but you can reduce the feedback-resistor value with the current-feedback-amp circuit and get an increase in circuit bandwidth. Under normal conditions. She has written hundreds of articles. design and application notes. The A 3 output restrictions are similar to those of any other amplifier. R2 . both inputs have a path to ground–in this case. Most product data sheets provide illustrations of the effects of input/gain-overload conditions. You can find the prescribed feedback-resistor value in the current-feedback amp's product data sheet. This story goes deeper. the change of offset voltage is the only concern. The thermistor-resistance change over temperature is nonlinear. 20 Back to top Voltage. the instrumentation amplifier's CMR increases as the system's gain increases–a nice feature. 2. offset. Consequently.com input stages Charge-pump input: performs without distortion in reaction to common-mode voltage changes Conclusion Manufacturers use several input topologies in their designs of rail-to-rail input amplifiers. The op amp's input offset-voltage error is easy to understand. The maximum and minimum input-voltage limits vary from device to device. Additional channels reduce the closed-loop bandwidth. and Lew Counts. When measuring temperature. In Figure 1. it is easy to assume that the closed-loop bandwidth equals the gain-bandwidth product divided by each channel's gain. TAMU. R3 . the output swing never spans all the way to the rails. Consequently. Circuit designers often misapply a thermocouple or even a two-wire microphone to the INA's input circuit.TI. 50 MHz. . The negative-temperature-coef ficient thermistor best suits precision temperature measurements. the instrumentation amplifier's CMR performance values tend to reach a maximum value at higher gains. the incident light on the photodiode causes current to flow through the diode from cathode to anode. the relationship of R1 . 2. Of all the performance characteristics of an INA. sboa035. A 1 . However. This erroneous voltage plus the voltage reference to the INA is incorrect and may be inside the output range of A 3 . Kitchin. 3. however. As the gain of A 1 and A 2 increases. Texas Instruments. As such. even though the input signals continue to see a gain of –2V/V. RSER . These four resistors combine with A 3 to subtract and gain the signals from the outputs of A 1 and A 2 . Kugelstadt. You define the common-mode voltage of the input signals as (VA1OUT +V A2OUT )/2." Electronics World. and A 3 are constants that the thermistor manufacturer provides. AMSC. This amplifier's drive capability limits the resistor's minimum value. If the feedback resistance. March 27. Three basic kinds of nodes in the INA can cause input/gain-overload problems (Figure 1). if R1 . this 0. August 1993. V OUT . 2009. the microcontroller can again acquire a new ADC value. The input stages of A 1 and A 2 limit the range of these two input voltages. The feedback resistor's higher values reduce the current-feedback amp's bandwidth. Alternatively.Texas Instruments . The common-mode behavior of the three-op-amp INA may surprise you if you ignore the nuances of the internal A 1 and A 2 output stages. That situation. and A 0 . How would you solve this problem? Comment below. "CMRR not created equal for all INAs. Baker. Photodiode Monitoring with Op Amps. and ΔV OUT is the change in the system's output voltage with respect to the changing V CM values. RTHERM . As you work with these concepts. Jerald. the V OA1 and V OA2 output levels of A 1 and A 2 . the photodiode's currents flow through the R1 feedback resistors." Check out her latest blog. conference papers. pg 14). ISBN: 0-07-024247. But wait–there may be a problem with the output values of A 1 and A 2 (Figure 1).and current-feedback amps are the same. Bonnie. 2005. Don't make this assumption! If you use a voltage. A 3 measures an erroneous difference voltage. An equal illumination on the two photodiodes makes the output voltage 0V. you will see greater accuracy in proximity and difference (Figure 1). the most misunderstood is the common-mode-range requirement. and R4 to the instrumentation amplifier's CMR–specifically.ti. it pays to understand the CMRR impact on your circuit. At a real-world level. You can determine the surrounding thermistor temperature by using the Steinhart-Hart equation: T=1/(A 0 +A 1 (lnR T )+A 3 (lnR T 3 )). making it difficult to detect the correctness of the INA input implementation. or CMOS gate. As Equation 1 states. The PGA gain and ADC value then pass to a microcontroller piecewise linear-interpolation routine. Texas Instruments. 2. I'd really like to hear from you! References 1. The term "thermistor" originates from the descriptor "thermally sensitive resistor. noise. with a voltage-feedback amp. but A 1 and A 2 's open-loop gain error places a limit on this strategy. a 10-bit ADC. The first and most dominant factor is the balance of the resistor ratios across A 3 . "Instrumentation amplifiers—avoiding a common pitfall. A 2 . the circuit's closed-loop bandwidth is 25. The final place to look for an input/gain-overload condition is at the output. equals 2R IN. Two factors influence an instrumentation amplifier's CMR. You can try to compensate for the thermistor's nonlinear response with a look-up table in your microcontroller. The floating thermocouple circuit in Figure 2a does not provide a current path to ground or a bias-voltage reference for the INA's input pins. bias current. per the manufacturer's specification and the circuit's noise gain." Analog Applications Journal. Bonnie C. 2001. For instance. For instance. V CM . the open-loop gain error of the op amps dominates. matching the R 1 to-R2 ratio to the R3 -to-R4 ratio–is critical. The voltage-feedback-amp's feedback-resistance value is more forgiving. December 2008. the noise gain is: (1) where N is the number of input channels. you may notice that an input signal into an INA can produce an incorrect output signal that is nevertheless within the device's normal output range. Pay attention to the voltage levels of the INA's V IN+ and V IN– input pins. A 3 rejects the common-mode signal and delivers the differential-voltage signal to the circuit output at V OUT . 2. and the overall circuit noise limits the maximum value. Lower value resistors reduce the current-feedback amp's stability. unless you expanded your design task to using two photodiodes. equals the gain-bandwidth product divided by the noise gain. instrumentation amplifiers are systems in which various parts contribute to the CMR error at different system gains." The two basic types of thermistors are negative. Texas Instruments." EDN. In a classic. FET gate. the dctransfer function of this circuit is 1V/V. the problem arises when the INA inputs are connected without proper consideration to current paths or biasing. http://www. When the circuit designer exercises proper precautions with the input pins. as the Steinhart-Hart equation describes. "CMRR not created equal for all INAs. which makes the magnitude of the output voltage a direct measurement of the difference between the direct light impinging on D1 and D2 . An amplifier's open-loop gain is 20log(ΔV OUT /ΔV OS ).and voltage-feedback amps. So how do designers calculate the INA's common-mode range? Consider the exposure to an input/gain-overload condition on the INA as a possibility. but you can reduce the feedback-resistor value with the current-feedback-amp circuit and get an increase in circuit bandwidth. RTHERM . the noise gain is: (1) where N is the number of input channels. More important. Alternatively. Typically. 2012. March 27. You can reach Bonnie at ti_bonniebaker@list. If the resistors around A 3 are not equal. References 1. or CMOS gate. References 1. At a gain of one. Bonnie. "Instrumentation amplifiers—avoiding a common pitfall. The level shift is convenient in singlesupply applications. if you add channels to the circuit. if R1 . The first and most dominant factor is the balance of the resistor ratios across A 3 . Heed those considerations for the INA's input stage. D1 and D2 respond linearly to illumination intensity. noise. Background light adds only an offset to the photodiode's outputs. Jerald. Precision Unity Gain Differential Amplifier. Don't make this assumption! If you use a voltage. 2. and a voltage reference or the power supply (Figure 1)." Sensors magazine. if you have a voltage-feedback amp with a gain-bandwidth product of 180 MHz and there are three input channels (N=3) at a gain of –2V/V. In this equation. you would first pick the optimum feedback resistor. You can now use those graphs to your advantage. a 10-bit ADC. offset. you could use a single photodiode to determine the brightness of your bulbs. the INA input stage saturates or floats to an undesirable voltage. On Board with Bonnie. and A 3 are constants that the thermistor manufacturer provides. March 27. Note that the A 1 and A 2 stages do not gain the input common-mode voltage. the input-current leakages are not dissipated. you can program the gain found in Figure 1 with a single resistor. R3 . the feedback resistor of a current-feedback-amp circuit must stay within a small range of values. you can use hardware-linearization techniques before digitization and a lower resolution ADC. 3. You can implement a differential amp discretely or with an off-the-shelf product. A 3 and R2 form a difference amp. biased to a voltage within the INA's input range.com input stages Charge-pump input: performs without distortion in reaction to common-mode voltage changes Conclusion Manufacturers use several input topologies in their designs of rail-to-rail input amplifiers. The term "thermistor" originates from the descriptor "thermally sensitive resistor. usually in the middle of the INA's output range. the picture becomes clear. This amplifier's drive capability limits the resistor's minimum value. and ΔV OUT is the change in the system's output voltage with respect to the changing V CM values. April 1. For instance.and positive-temperature-coefficient devices.or current-feedback amp with the circuit in Figure 1. 50 MHz.and current-feedback amps are the same." EDN. You would then select the appropriate value for RIN. At first glance. "Introduction to NTCs: NTC Thermistors. and a PGA to overcome the measurement difficulties of a nonlinear thermistor across a temperature range greater than ±25°C. An example of this type of input/gain-overload condition occurs when the in-range voltages on the INA's inputs drive A 1 . The voltage at the inverting input of the amp tracks the voltage at the amp's noninverting input. equals 2R IN. 3. and temperature drift. All inputs to the INA require a current-return path to ground and a bias-voltage reference. 2. This situation is not so mysterious when you think about the inside of this device. Figure 2b illustrates a correct thermocouple connection to an INA. generally. three-op-amp INA (Figure 1). RRI/O. Consequently. the offset errors from the amplifier's open-loop gain also increase. A single photodiode provides some measure of a light's intensity through the magnitude of the diode's output signal.com/ww/en/analog/Amplifiers-eBook/[1/14/14 10:58:49 AM] Back to top . If that scenario is a concern. and the two inputs can float to any undefined voltage. . decades-old operational amplifier to gain a differential input signal. 25 Back to top A good holiday-season project Which of the bulbs on a Christmas tree is the brightest? If you had the time and desire to answer this question. August 1993. Ideally.and current-feedback amps are almost the same Current-feedback amplifiers have a higher slew rate than do voltage-feedback amplifiers." Check out her latest blog. the noise gain always equals the result of Equation 1. and A 0 . but A 1 and A 2 's open-loop gain error places a limit on this strategy. As you work with these concepts. the closed-loop gain of each channel is –2V/V. When measuring temperature. this 0. The negative-temperature-coef ficient thermistor best suits precision temperature measurements. both inputs have a path to ground–in this case. A 3 rejects the common-mode signal and delivers the differential-voltage signal to the circuit output at V OUT ." EDN. http://www. the change of offset voltage is the only concern. of the INA. note that resolution is lost at high temperatures. Texas Instruments. design and application notes. The PGA gain and ADC value then pass to a microcontroller piecewise linear-interpolation routine. At a real-world level. The invalid voltage can appear to be legitimate. the instrumentation amplifier's CMR performance values tend to reach a maximum value at higher gains. Additional channels reduce the closed-loop bandwidth." Ax-09. An input/gain-overload condition can occur if A 1 's output voltage. Photodiode Monitoring with Op Amps. As long as the resistors surrounding A 3 are equal. © Copyright 2014 EDN. The output voltages of A 1 and A 2 contain both difference and common-mode signals. and you can be confident that the INA's output voltage is representative of the thermocouple's voltage. For instance. Either condition creates an invalid output voltage. A 2 's output voltage. However. Two factors influence an instrumentation amplifier's CMR. "Instrumentation amplifiers—they're not op amps butwhat are they?" TI E2E Community. TAMU. and R4 are approximately the same value and the ratio of R3 to R4 is 1. RSER . Equation 2 gives the contribution to the CMR error with respect to the relationship of these resistors: Equation 2 For instance. instrumentation amplifiers are systems in which various parts contribute to the CMR error at different system gains. The changes in output swing of A 1 and A 2 typically span the supply rails. If you use three op amps in a differential-photodiode-amp configuration. you will see greater accuracy in proximity and difference (Figure 1). The op amp's input offset-voltage error is easy to understand. The floating thermocouple circuit in Figure 2a does not provide a current path to ground or a bias-voltage reference for the INA's input pins. Bruce. and R4 to the instrumentation amplifier's CMR–specifically. ISBN: 0-07-024247. Amplifiers and Linear. the V OA1 and V OA2 output levels of A 1 and A 2 . sboa035. Of all the performance characteristics of an INA. The final place to look for an input/gain-overload condition is at the output. You can try to compensate for the thermistor's nonlinear response with a look-up table in your microcontroller. The charge-pump input amplifier does not produce this same distortion. For instance. a small variation in the signal bandwidth and gain peaking in circuit may occur. where RIN is the input resistance. These four resistors combine with A 3 to subtract and gain the signals from the outputs of A 1 and A 2 . The two photodiodes let you find a light's position by monitoring the difference between their output signals. the thermistor is a high-impedance. This story goes deeper. Kitchin. Three basic kinds of nodes in the INA can cause input/gain-overload problems (Figure 1). McGraw-Hill.Texas Instruments . D). it is easy to assume that the closed-loop bandwidth equals the gain-bandwidth product divided by each channel's gain. High CMRR. Charles. per the manufacturer's specification and the circuit's noise gain. AMSC. The input stages of A 1 and A 2 limit the range of these two input voltages. and Lew Counts. causes the INA's output to change to an invalid output voltage. drive a reference current through the thermistor to create an equivalent voltage. 21 Back to top Chapter 2: INAs and PGAs Understanding CMR and instrumentation amplifiers The three-op-amp instrumentation amplifier in Figure 1 is seemingly a simple configuration in that it uses a basic. With CMR. the circuit's closed-loop bandwidth is 25. So what is the hang-up? Equation 1 yields the common CMR for a single op amp and instrumentation amplifier: Equation 1 where G is the system gain. You can find the prescribed feedback-resistor value in the current-feedback amp's product data sheet. and the V OUT output-swing capability of A 3 . however. The thermistor-resistance change over temperature is nonlinear. Sept 15. Adding a matched photodiode and monitoring the difference between the two diode outputs removes the equal offsets the two diodes produce. Trump. A 1 . or both violate the internal output-swing restrictions. References 1. the amp's outputs change in voltage along with the IR drop across the R1 resistors. RT is the thermistor resistance at temperature T. or both to their positive or negative output-swing limit. As Equation 1 states. Thus. 2001. V IN+ and V IN− connect to a bipolar transistor base. A 1 and A 2 require low-inputcurrent CMOS or FET op amps. How would you solve this problem? Comment below. if R1 equals R3 and R2 equals R4 .TI. Nov 26. requiring special considerations in some classes of circuits. From this point. Baker. A 3 measures an erroneous difference voltage. Finding that brightest bulb among the many and the background light would be a laborious task. Without the current-return path and the bias-voltage reference. The feedback resistor's higher values reduce the current-feedback amp's bandwidth. The applied input voltages to the INA are equivalent to the common-mode input voltage plus or minus the differential input signal. If the ADC output is beyond a trip-point value. 23 Back to top Instrumentation amplifier input-circuit strategies Many industrial and medical applications use instrumentation amplifiers to condition small signals in the presence of large common-mode voltages. This condition is impossible to directly measure. But wait–there may be a problem with the output values of A 1 and A 2 (Figure 1). This equivalent voltage has a nonlinear response. the dctransfer function of this circuit is 1V/V. except for a few key points. "Advances in measuring with nonlinear sensors. INAs have complete closed-loop operational amplifiers with feedback components included." Analog Applications Journal. Because the inverting input of A 1 and A 2 has high impedance. through a 10-kΩ resistor. this type of mismatch can introduce nonlinearities in the system when the common-mode voltage of the two inputs changes. the most misunderstood is the common-mode-range requirement. These errors degrade the CMR of the instrumentation amplifier at higher gains. 2009. The maximum and minimum input-voltage limits vary from device to device. the application-circuit configurations for voltage. CMR A3 is equivalent to the CMR of the entire instrumentation amplifier. The common-mode behavior of the three-op-amp INA may surprise you if you ignore the nuances of the internal A 1 and A 2 output stages. current-feedback amps can better solve high-speed problems than their voltage-feedback counterparts. December 2008. Bonnie C. in turn. The goal is to select amps in which these parameters are as low as possible. 2012. "The right way to use instrumentation amplifiers. the photodiode's currents flow through the R1 feedback resistors. the incident light on the photodiode causes current to flow through the diode from cathode to anode. or |–2V/V|. V OUT . requiring calibrated and impractical measurement conditions. 20 Back to top Voltage. where V OS is the offset voltage. you need a high-resolution converter to capture data during temperature extremes. in series with the thermistor. even though the input signals continue to see a gain of –2V/V. Bonnie has a drive to share her knowledge and experience. a microcontroller. 22 Back to top Common-mode range can bite hard My last column provided a glimpse inside the CMR (common-mode rejection) of a three-op-amp INA (instrumentation amplifier) and revealed the main contributors to total CMR error (see "Understanding CMR and instrumentation amplifiers. So. The definition of an op amp's openloop gain has not changed. RF . resistive device that eases one of the interface issues as you convert the thermistor resistance to voltage. A mismatch be-tween the resistor ratios creates an error at the output of A 3 .com. In a classic. the differential amp rejects common-mode-voltage changes. the inner workings of the op amp are straightforward. Texas Instruments. An equal illumination on the two photodiodes makes the output voltage 0V. pg 69. The composite input amplifier produces a crossover distortion. The input voltages at V IN+ and V IN– and the gain of A 1 and A 2 cause the internal output voltages to increase or decrease. You can easily compensate for this type of error by replacing any of the four resistors with a potentiometer. the CMR of the three-op-amp instrumentation amplifier is ideally infinite. as long as you pay attention to the input stage and the output of the first stage. 27 Content provided courtesy of EDN. Circuit designers often misapply a thermocouple or even a two-wire microphone to the INA's input circuit." BC Components data sheet. the relationship of R1 . 2005. 3. The A 3 output restrictions are similar to those of any other amplifier.1% mismatch will cause a degradation of the instrumentation amplifier's CMR from ideal to a 66-dB level. R3 . The simple idea of an op amp's CMR (common-mode rejection) has been around since the beginning of op-amp time. For both current. FET gate. That situation. R2 . A 2 . pg 14). First." Electronics World. with a voltage-feedback amp. but. At higher instrumentation-amplifier gains. 2. Baker. a 10-bit ADC in this circuit can sense a limited temperature range (approximately ±25°C). SBOS145. however. Texas Instruments. where she navigates the ins and outs of signal chain designs. All of the internal amplifier's output voltages relate to the linear common-mode input range of the complete INA.or a voltage-feedback amp. As the gain of A 1 and A 2 increases. unless you expanded your design task to using two photodiodes.001 of R1 /R2 . R2 . This circuit's bandwidth. the microcontroller can again acquire a new ADC value. Consequently. subtracting the output voltages of A 1 and A 2 . however. the input stage has two op amps in an adjustable-gain configuration and provides high input impedance on both the inverting (VIN−) and noninverting (VIN+) inputs. Texas Instruments. Equation 1 might motivate an instrumentation-amplifier designer to ensure that there is plenty of gain available. The microcontroller firmware's temperature-sensing algorithm reads the 10-bit-ADC digital value and passes it to a PGA hysteresis-software routine. the open-loop gain error of the op amps dominates. In addition to her fascination with circuit design. however. January 1995. As such. the amplifier's closed-loop bandwidth depends less on the closed-loop gain and the number of input channels. you may notice that an input signal into an INA can produce an incorrect output signal that is nevertheless within the device's normal output range. Photodiode Amplifiers: Op Amp Solutions. If you use a current-feedback amp with the circuit in Figure 1. I'd really like to hear from you! References 1. a noticeable gain error can occur between the two input signals. 2. the output swing never spans all the way to the rails. In Figure 1. if you look at the common-mode range of the same device. The PGA hysteresis routine checks the PGA gain setting and compares the ADC digital value with the trip points that Figure 1 indicates. In this circuit. and the overall circuit noise limits the maximum value. Edgar Sánchez-Sinencio.7 MHz. April 2. 2006. One technique is to place a resistor. If necessary. the microcontroller sets the PGA gain to the next higher or lower gain setting. Under normal conditions.ti. Setting the PGA (programmable-gain amplifier) at a gain of 1V/V. You can determine the surrounding thermistor temperature by using the Steinhart-Hart equation: T=1/(A 0 +A 1 (lnR T )+A 3 (lnR T 3 )). ΔV CM is the changing common-mode voltage that you apply equally to the system's input terminals with respect to ground. The name "current-feedback amp" carries some mystique. equals the gain-bandwidth product divided by the noise gain. matching the R 1 to-R2 ratio to the R3 -to-R4 ratio–is critical. Lower value resistors reduce the current-feedback amp's stability. However. Single-Supply Operational Amplifier (Rev. making it difficult to detect the correctness of the INA input implementation. or ground. The output stage has a reference voltage that level-shifts the output with respect to ground. The more difficult interface challenge is to capture the nonlinear behavior of the thermistor in a digital representation with a linear ADC. and the differential amp removes this effect. She has written hundreds of articles. bias current. The quick and inexpensive solutions described here can help you avoid the INA circuit's input-stage pitfalls. You must be aware of the limitations of V OA1 and V OA2 and then calculate whether your design is at risk using the equations in Figure 1. References 1. Even though you can run this type of algorithm in your microcontroller firmware. Figure 1 shows a circuit that is appropriate for either a current. go back and refine your feedback-resistor selection." The two basic types of thermistors are negative. from the CMR perspective. the instrumentation amplifier's CMR increases as the system's gain increases–a nice feature. All rights reserved. the problem arises when the INA inputs are connected without proper consideration to current paths or biasing. and authored a book: "A Baker's Dozen: Real Analog Solutions for Digital Designers. Trump. As with many INAs. T is the temperature in degrees Kelvin. this configuration rejects external common-mode voltage and noise. "Getting themost out of your instrumentationamplifier design. The configurations of A 1 and A 2 act as traditional current-to-voltage converters or transimpedance amps. Low-Distortion. RG." TI E2E Community. conference papers. if you choose to use a rail-to-rail input amplifier. The calculated CMR (common-moderejection) error due to resistor mismatches is 100×(1+R 2 /R1 )/(% of mismatch error). "Rail-to-Rail Op Amps. You define the common-mode voltage of the input signals as (VA1OUT +V A2OUT )/2. Depending on the INA's silicon process. it pays to understand the CMRR impact on your circuit. This erroneous voltage plus the voltage reference to the INA is incorrect and may be inside the output range of A 3 . 2. Thomas. In this condition. V CM . Pay attention to the voltage levels of the INA's V IN+ and V IN– input pins. an INA is easy to use. backgroundlight conditions can influence this magnitude. As you separate the parts. Most product data sheets provide illustrations of the effects of input/gain-overload conditions. When the circuit designer exercises proper precautions with the input pins. You can combine a series resistor. 26 Back to top About the author Bonnie Baker is a Senior Applications Engineer with the WEBENCH® team for Texas Instruments and has been involved with analog and digital designs and systems for over 25 years. If you design this circuit with such an amp. Increasing the PGA's gain at these temperatures brings the output signal of the PGA back into a range at which the ADC can reliably provide conversions that identify the thermistor temperature. 2005. Kugelstadt.ti. Obtaining data from a nonlinear thermistor sometimes can seem like an impossible task. 4Q 2005. If the feedback resistance. 24 Back to top Wringing out thermistor nonlinearities Thermistor-temperature-sensing devices present a design challenge if you intend to use such a device over its entire temperature range. The voltage-feedback-amp's feedback-resistance value is more forgiving. The output stage has four matched resistors around a single op amp. in a single-supply environment. Grame. The key performance parameters for A 1 and A 2 are input capacitance. It is true that circuit designers can use rail-to-rail input amplifiers in virtually any op-amp configuration. Bruce. Baker. All rights reserved. 2005. Adding a matched photodiode and monitoring the difference between the two diode outputs removes the equal offsets the two diodes produce. August 1993. © Copyright 2014 EDN.ti. however. D1 and D2 respond linearly to illumination intensity. Finding that brightest bulb among the many and the background light would be a laborious task." Sensors magazine. You can easily compensate for this type of error by replacing any of the four resistors with a potentiometer. References 1. where she navigates the ins and outs of signal chain designs. Bonnie C. You can reach Bonnie at ti_bonniebaker@list. A 3 and R2 form a difference amp. Texas Instruments. the differential amp rejects common-mode-voltage changes. the dctransfer function of this circuit is 1V/V. Texas Instruments. The key performance parameters for A 1 and A 2 are input capacitance. Bonnie has a drive to share her knowledge and experience. you could use a single photodiode to determine the brightness of your bulbs. On Board with Bonnie. You can combine a series resistor. ISBN: 0-07-024247. a microcontroller. A 3 rejects the common-mode signal and delivers the differential-voltage signal to the circuit output at V OUT . Jerald. Photodiode Monitoring with Op Amps. The voltage at the inverting input of the amp tracks the voltage at the amp's noninverting input. backgroundlight conditions can influence this magnitude. The configurations of A 1 and A 2 act as traditional current-to-voltage converters or transimpedance amps. 2. the amp's outputs change in voltage along with the IR drop across the R1 resistors. In addition to her fascination with circuit design. You can implement a differential amp discretely or with an off-the-shelf product. If you use three op amps in a differential-photodiode-amp configuration. however." Check out her latest blog. January 1995. 26 Back to top About the author Bonnie Baker is a Senior Applications Engineer with the WEBENCH® team for Texas Instruments and has been involved with analog and digital designs and systems for over 25 years. Because the inverting input of A 1 and A 2 has high impedance. A single photodiode provides some measure of a light's intensity through the magnitude of the diode's output signal. Precision Unity Gain Differential Amplifier. As long as the resistors surrounding A 3 are equal.com indicates. and authored a book: "A Baker's Dozen: Real Analog Solutions for Digital Designers. design and application notes. and temperature drift. March 27. which makes the magnitude of the output voltage a direct measurement of the difference between the direct light impinging on D1 and D2 . 2006. the microcontroller can again acquire a new ADC value. April 1. the incident light on the photodiode causes current to flow through the diode from cathode to anode. 2. If the resistors around A 3 are not equal.com/ww/en/analog/Amplifiers-eBook/[1/14/14 10:48:43 AM] Back to top . The two photodiodes let you find a light's position by monitoring the difference between their output signals. The PGA gain and ADC value then pass to a microcontroller piecewise linear-interpolation routine. the microcontroller sets the PGA gain to the next higher or lower gain setting. McGraw-Hill. SBOS145. An equal illumination on the two photodiodes makes the output voltage 0V. You define the common-mode voltage of the input signals as (VA1OUT +V A2OUT )/2.com. "Introduction to NTCs: NTC Thermistors. More important. Ideally. conference papers. Grame. this type of mismatch can introduce nonlinearities in the system when the common-mode voltage of the two inputs changes.TI. a noticeable gain error can occur between the two input signals. http://www. requiring calibrated and impractical measurement conditions. subtracting the output voltages of A 1 and A 2 . She has written hundreds of articles. "Advances in measuring with nonlinear sensors. sboa035. Consequently. Obtaining data from a nonlinear thermistor sometimes can seem like an impossible task. 3. 27 Content provided courtesy of EDN. If the ADC output is beyond a trip-point value. A 1 and A 2 require low-inputcurrent CMOS or FET op amps. the photodiode's currents flow through the R1 feedback resistors. 2001. The goal is to select amps in which these parameters are as low as possible. The calculated CMR (common-moderejection) error due to resistor mismatches is 100×(1+R 2 /R1 )/(% of mismatch error). a 10-bit ADC. If necessary. you will see greater accuracy in proximity and difference (Figure 1). Photodiode Amplifiers: Op Amp Solutions. Background light adds only an offset to the photodiode's outputs.Texas Instruments . offset. However. The output voltages of A 1 and A 2 contain both difference and common-mode signals.ti. and the differential amp removes this effect. References 1. In this circuit. noise. unless you expanded your design task to using two photodiodes." BC Components data sheet. bias current. and a PGA to overcome the measurement difficulties of a nonlinear thermistor across a temperature range greater than ±25°C. 25 Back to top A good holiday-season project Which of the bulbs on a Christmas tree is the brightest? If you had the time and desire to answer this question. and 4 kHz and so on. However. subtracting the output voltages of A 1 and A 2 . In this circuit. D1 and D2 respond linearly to illumination intensity. The key performance parameters for A 1 and A 2 are input capacitance. Consequently. backgroundlight conditions can influence this magnitude. "Instrumentation amplifiers—they're not op amps butwhat are they?" TI E2E Community. When you drive the output high.Amplifiers eBook . Baker. 4Q 2005. if R1 . the charge-pump design can provide an output ripple voltage that is low enough so as to not produce undesirable noise or distortion at the output of the op amp. the transfer function for this circuit is expressed as The V B voltage can be anywhere between ground and V S. Kitchin. Figure 4 The noninverting gained amplifier configuration allows the amplifier’s commonmode voltage to change with input signals. June 2006." Texas Instruments. Figure 2: As the amplifier's common-mode voltage changes from ground to the positive supply. In the region near the positive power supply. You can now use those graphs to your advantage.ti. with the PMOS transistors completely off. For instance. Two factors influence an instrumentation amplifier's CMR. Bonnie has a drive to share her knowledge and experience. articles. the noninverting amplifier circuit may require a bias voltage. Most product data sheets provide illustrations of the effects of input/gain-overload conditions. This article is an expansion of the blog post "What does 'rail to rail' input operation really mean?" By examining an additional inputstage topology and implementing these op amps into some common op-amp circuits. 2009. as long as you pay attention to the input stage and the output of the first stage." EDN. The simple rail-to-rail operational amplifier must have a transistor design that spans the power supply with minimal distortion. August 1993. Line 1 verifies that the amplifier has true rail-to-rail input capability. The common-mode behavior of the three-op-amp INA may surprise you if you ignore the nuances of the internal A 1 and A 2 output stages. Trump.input pins. where V OS is the offset voltage. the output cannot go all the way to the rails. and V L is greater than V OL . At a real-world level. V IN. Although the precision. the noise gain always equals the result of Equation 1. instrumentation amplifiers are systems in which various parts contribute to the CMR error at different system gains. but A 1 and A 2 's open-loop gain error places a limit on this strategy.4V. The difference between these two topologies in this figure is obvious. 2012. The output stage has four matched resistors around a single op amp. In this circuit. the output swing never spans all the way to the rails. From approximately 1. For proper operation. Figure 2a illustrates the nonlinearity-output-stage effects with a single-supply CMOS amplifier by showing distortion at 2. The two stages shared.7 MHz. RRI/O. however. there are performance compromises that the circuit designer must address. Nov 26. an op amp with a composite input topology may limit the input range of the buffer. Low-Distortion.8V below the positive power-supply rail as 92 dB (typ). V OH is the absolute maximum voltage level with respect to V DD (drain-to-drain voltage) that the output can reach. The conditions of the dc-open-loop-gain specification define the amplifier's linear operating output range. if you have a voltage-feedback amp with a gain-bandwidth product of 180 MHz and there are three input channels (N=3) at a gain of -2V/V.ti.com. the circuit's closed-loop bandwidth is 25. You can reach Bonnie at ti_bonniebaker@list. The PGA gain and ADC value then pass to a microcontroller piecewise linear-interpolation routine. the feedback resistor of a current-feedback-amp circuit must stay within a small range of values. and the 16-bit SAR ADC has a THD specification of -99 dB. The amplifier output voltage swings from 140 mV to 4. This story goes deeper. the incident light on the photodiode causes current to flow through the diode from cathode to anode. 20 Back to top Voltage. however. You can determine the surrounding thermistor temperature by using the Steinhart-Hart equation: T=1/(A 0 +A 1 (lnR T )+A 3 (lnR T 3 )). with a voltage-feedback amp. or |-2V/V|. A typical common-mode rejection specification for an amplifier with a charge-pump input stage has one CMRR specification. the system THD is -88 dB. Application solutions Circuit designers can use rail-to-rail input op amps in virtually any op-amp configuration. As with many INAs. you can see an op amp's output limitations to swinging rail to rail when the op amp is driving an ADC. Equation 2 gives the contribution to the CMR error with respect to the relationship of these resistors: Equation 2 For instance. © Copyright 2014 EDN. The specification conditions can subtly describe the amplifier's rail-to-rail input topology. Under normal conditions. The amplifier's typical closed-loop bandwidth is about 3 MHz with a typical slew rate of 2. The microcontroller firmware's temperature-sensing algorithm reads the 10-bit-ADC digital value and passes it to a PGA hysteresis-software routine. So. you can use hardware-linearization techniques before digitization and a lower resolution ADC. Finding that brightest bulb among the many and the background light would be a laborious task. the input range extends at least 100 mV beyond both power-supply rails. an INA is easy to use. The definition of an op amp's openloop gain has not changed." Check out her latest blog. Trump. and R4 are approximately the same value and the ratio of R3 to R4 is 1. Switching from one differential input stage to the other causes this distortion. 21 Back to top Chapter 2: INAs and PGAs Understanding CMR and instrumentation amplifiers The three-op-amp instrumentation amplifier in Figure 1 is seemingly a simple configuration in that it uses a basic. The maximum and minimum input-voltage limits vary from device to device. These errors degrade the CMR of the instrumentation amplifier at higher gains. through a 10-kΩ resistor. You can also expect almost a tenfold decrease in the amplifier's THD (total-harmonic-distortion) performance. In this 5V-supply system. the system THD is: where THDOPA =20log(THD OPA-%×100) and THDOPA-% is the THD specification in the operational amplifier's data sheet in units of percentage. The article discusses a composite input-stage topology for single-supply op amps. So. and the V OUT output-swing capability of A 3 . McGraw-Hill. With CMR. current-feedback amps can better solve high-speed problems than their voltage-feedback counterparts. You would then select the appropriate value for RIN. V B. and you can be confident that the INA's output voltage is representative of the thermocouple's voltage. and R4 to the instrumentation amplifier's CMR–specifically. Zero-Drift Series. the INA input stage saturates or floats to an undesirable voltage. 2. V OUT . 3. the thermistor is a high-impedance. conference papers. combining the advantages of those stages to achieve actual rail-to-rail input operation (Figure 1). if you choose to use a rail-to-rail input amplifier. You can easily compensate for this type of error by replacing any of the four resistors with a potentiometer. the V OL minimum specification is 15 mV above ground. This erroneous voltage plus the voltage reference to the INA is incorrect and may be inside the output range of A 3 . the THD of an ADC driven by an op amp in a buffer configuration is the root-sum square of distortion contributions of the ADC and op amp. 50 MHz. as long as the combination of the elements in this circuit (VB. Consequently. When using composite input amplifiers. Sept 15. An equal illumination on the two photodiodes makes the output voltage 0V. RTHERM . In a classic. the closed-loop gain of each channel is -2V/V. allowing the amplifier's output swing to go to and well beyond the power-supply rails. Obtaining data from a nonlinear thermistor sometimes can seem like an impossible task. from the CMR perspective. RF . offset. or both to their positive or negative output-swing limit. the composite input amplifier produces distortion at the circuit output. If you design this circuit with such an amp. Ideally. design and application notes. April 2. Grame. Bonnie. As the gain of A 1 and A 2 increases. and the differential amp removes this effect. note that resolution is lost at high temperatures. V IN+ and V IN− connect to a bipolar transistor base. Single-supply amplifiers continue to keep pace with high-resolution converters because engineers implement innovative amplifier-circuit topologies. A common single-supply amplifier input topology has parallel PMOS and NMOS differential input stages. Alternatively. The feedback resistor's higher values reduce the current-feedback amp's bandwidth. 2. the NMOS section of the circuit finally takes over. Here V B establishes the common-mode voltage of the amplifier. However. 25 Back to top A good holiday-season project Which of the bulbs on a Christmas tree is the brightest? If you had the time and desire to answer this question. you would first pick the optimum feedback resistor. Amplifiers and Linear. the differential amp rejects common-mode-voltage changes." Analog Applications Journal. Texas Instruments. A typical common-mode rejection specification for an amplifier with a composite input stage has two or more CMRR specifications (Table 1). Figure 2: The op-amp offset voltage of an op amp with a charge-pump input remains constant across changes in the common-mode voltage. you will see what I mean. the circuit will not create distortion from the composite’s input stage. This equivalent voltage has a nonlinear response. A single photodiode provides some measure of a light's intensity through the magnitude of the diode's output signal. In this equation. the input signal (VIN) can traverse from one rail to the other rail. V L is the minimum voltage level at the output in the dc-open-loop-gain measurement. circuit designers need to consider the behavior of the single-supply op-amp input stage. crossover phenomena. So what is the hang-up? Equation 1 yields the common CMR for a single op amp and instrumentation amplifier: Equation 1 where G is the system gain. the PMOS transistors are in operation. "The right way to use instrumentation amplifiers. read the fine print! Some single-supply amps have output-stage charge pumps. including a monthly column in EDN magazine and on edn. If the input signal always remains less than the composite input stage’s transition voltage. "CMRR not created equal for all INAs. 2. if you look at the common-mode range of the same device." The two basic types of thermistors are negative. but you can reduce the feedback-resistor value with the current-feedback-amp circuit and get an increase in circuit bandwidth. The charge-pump input amplifier does not produce this same distortion. How would you solve this problem? Comment below. and ΔV OUT is the change in the system's output voltage with respect to the changing V CM values. 2005. both inputs have a path to ground–in this case. 2005. microPower CMOS Operational Amplifiers. in turn. The composite input stage is one example. The applied input voltages to the INA are equivalent to the common-mode input voltage plus or minus the differential input signal. The V OH maximum specification is V DD -20 mV. The single differential-input stage with a charge pump buys you a 20. In addition to her fascination with circuit design. and V OL is the absolute minimum voltage level that the output can reach. where she navigates the ins and outs of signal chain designs. A 3 rejects the common-mode signal and delivers the differential-voltage signal to the circuit output at V OUT . March 27. Additional channels reduce the closed-loop bandwidth. If you use three op amps in a differential-photodiode-amp configuration. Without the current-return path and the bias-voltage reference. All rights reserved. the input stage of the CMOS amplifier completely changes from its PMOS input pair to its NMOS input pair at -2V below the 3V positive supply rail. 18 Back to top What does "rail to rail" input operation really mean? A hot discussion topic with single-supply operational amplifiers is whether they are capable of rail-to-rail input or output operation. This specification encompasses the full amplifier input range. biased to a voltage within the INA's input range. and temperature drift. where she navigates the ins and outs of signal chain designs. Bruce.001 of R1 /R2 . if you add channels to the circuit. The floating thermocouple circuit in Figure 2a does not provide a current path to ground or a bias-voltage reference for the INA's input pins. If the ADC output is beyond a trip-point value. Bonnie has authored a book: "A Baker's Dozen: Real Analog Solutions for Digital Designers.and the gain of A 1 and A 2 cause the internal output voltages to increase or decrease. V CM . Either condition creates an invalid output voltage. Texas Instruments. drive a reference current through the thermistor to create an equivalent voltage. By reducing the amplifier's output signal to 272 mV from each rail. V B. the picture becomes clear. the rail-to-rail input operation across the complete amplifier's rail-to-rail common-mode range (Reference 1). A 2 's output voltage. In this configuration. the CMR of the three-op-amp instrumentation amplifier is ideally infinite. Background light adds only an offset to the photodiode's outputs. Typically. generally. 22 Back to top Common-mode range can bite hard My last column provided a glimpse inside the CMR (common-mode rejection) of a three-op-amp INA (instrumentation amplifier) and revealed the main contributors to total CMR error (see "Understanding CMR and instrumentation amplifiers. read your data sheet and refer to the conditions on the open-loop-gain test. You can find the prescribed feedback-resistor value in the current-feedback amp's product data sheet.to 30-dB increase in the amplifier's CMMR (common-mode-rejection ratio). Depending on the INA's silicon process. RG. You can implement a differential amp discretely or with an off-the-shelf product.and positive-temperature-coefficient devices. pg 69. At approximately 1. of the INA. if an operational amplifier with a complementary input stage has a THD specification of 0. More important. Bonnie.and current-feedback amps are the same. The level shift is convenient in singlesupply applications. The amplifier's linearity starts to degrade long before reaching the output-swing maximums. R2 . crossover phenomena. The quick and inexpensive solutions described here can help you avoid the INA circuit's input-stage pitfalls. Bruce. the headroom between the signal and rails is 140 mV. This disparity in specification only points out that the amplifier's input stage has a composite input topology. the data looks perfect with only the ADC distortion (Figure 2b). Note that this specification does not describe the range specified in Line 1. If the resistors around A 3 are not equal. The circuit requires a bias voltage. requiring calibrated and impractical measurement conditions. December 2008." Ax-09. TAMU.8V. if they have the proper input circuitry to support this function. Texas Instruments. All inputs to the INA require a current-return path to ground and a bias-voltage reference. the NMOS offset error dominates. Heed those considerations for the INA's input stage. and the two inputs can float to any undefined voltage. The transfer function of the circuit in Figure 5 is shown here: The input common-mode voltage (VIN) can vary between power-supply rails. A 3 and R2 form a difference amp. a noticeable gain error can occur between the two input signals. Pay attention to the voltage levels of the INA's V IN+ and V IN. which in this case is an amplifier with a composite input stage. the circuit does not create distortion from the composite’s input stage. I'll talk about the rail-to-rail amp's output stage and its ability to achieve rail-to-rail performance. you will see greater accuracy in proximity and difference (Figure 1). 2006. Bottom line: inverting-amplifier circuit Composite input: performs without distortion with V B outside the PMOS/NMOS transition region Charge-pump input: performs without distortion The noninverting amplifier’s common-mode voltage (Figure 4) is equal to the input voltage. Figure 1 shows a circuit that is appropriate for either a current. The trend toward designing single-supply op amps started in the 1970s with a single differential-input stage that spanned a portion of the common-mode input range. The noninverting amplifier circuit gains the input signal with a voltage level-shift implemented with V B. 50MHz. and the overall circuit noise limits the maximum value. the amplifier's closed-loop bandwidth depends less on the closed-loop gain and the number of input channels. For example. When using a single-supply amplifier.Best of Baker's Best . It is true that circuit designers can use rail-to-rail input amplifiers in virtually any op-amp configuration. V IN establishes the amplifier’s common-mode voltage. low-power OPA344 input stage in Figure 1 has rail-to-rail input operation. Lower value resistors reduce the current-feedback amp's stability. "OPA365. the photodiode's currents flow through the R1 feedback resistors. First. the composite amplifier exhibits some limitations in linearity due to the input-stage topology. or complementary. however. however. Of all the performance characteristics of an INA. The goal is to select amps in which these parameters are as low as possible.or a voltage-feedback amp. Equation 1 might motivate an instrumentation-amplifier designer to ensure that there is plenty of gain available. equals the gain-bandwidth product divided by the noise gain. 17 Back to top The non-negotiable single-supply operational amplifier Fundamental analog devices that serve applications such as high-resolution delta-sigma or SAR (successive-approximation-register) converter systems are feeling the crunch from amplifiers that have difficulty with achieving good rail-to-rail input performance. Line 3 specifies the CMRR across the entire input range as 70 dB (typ. I'd really like to hear from you! References 1. References 1. Near the rail. both the PMOS and NMOS transistors are operating. Figure 1: This single PMOS differential input stage in conjunction with a high-side chargepump eliminates the common-mode crossover distortion found in composite input stages. Texas Instruments. Figure 2b illustrates a correct thermocouple connection to an INA. In fact. An amplifier's open-loop gain is 20log(ΔV OUT /ΔV OS ). The design topology in Figure 1 can have wide variations in offset voltage across the amplifier's common-mode input range. If the circuit designer wants a distortion-free output from this amplifier circuit. This increase has a positive effect on amplifiers in buffer configurations. Figure 2 compares the response of the charge pump to composite amplifier inputs. OPA2333 1. You may recall that this topology exhibited an offsetvoltage. designers added a second. requiring special considerations in some classes of circuits. where V OUT is the output voltage and V OS is the input offset voltage. Alternatively. The invalid voltage can appear to be legitimate. pg 14). rail-to-rail input/output LMP7701 CMOS amplifier in Figure 2 exhibits the offset-voltage-error crossover behavior around 1. the relationship of R1 .and voltage-feedback amps. Don't make this assumption! If you use a voltage. Single-Supply Operational Amplifier (Rev. As long as the resistors surrounding A 3 are equal. Single-Supply Rail-to-Rail Operational Amplifiers. and buffer circuits. Adding a matched photodiode and monitoring the difference between the two diode outputs removes the equal offsets the two diodes produce. 3. "Introduction to NTCs: NTC Thermistors. V IN. the open-loop gain error of the op amps dominates. Bottom line: noninverting-amplifier circuit Composite input: the input signal (VIN) must not travel into the transition region of the PMOS and CMOS differential input stages Charge-pump input: performs without distortion in reaction to common-mode voltage or V IN changes With a unity-gain buffer (Figure 5). Composite input op amps can create distortion at the output. The voltage at the inverting input of the amp tracks the voltage at the amp's noninverting input. A mismatch be-tween the resistor ratios creates an error at the output of A 3 . The input voltages at V IN+ and V IN. For both current. and a PGA to overcome the measurement difficulties of a nonlinear thermistor across a temperature range greater than ±25°C. V O. You define the common-mode voltage of the input signals as (VA1OUT +V A2OUT )/2. with an input voltage of 4V p-p." TI E2E Community. The output voltages of A 1 and A 2 contain both difference and common-mode signals. you can read about them in "Rail-to-rail input amplifier application solutions. If that scenario is a concern. http://www. Thomas. the PMOS offset-error portion of the input stage is dominant. Edgar Sánchez-Sinencio. Line 2 specifies the CMRR from ground to 1. Photodiode Monitoring with Op Amps. Texas Instruments. For linear operation. the NMOS transistors are in use. In many applications." Single-supply amplifier manufacturers also claim they have devices that will swing rail-to-rail on the output. Alternatively. "OPA333. References 1.004%. "Advances in measuring with nonlinear sensors. and V IN) keeps the output (VO) between V S and ground. with the NMOS transistors turned off. Charles. V S=5V). this type of mismatch can introduce nonlinearities in the system when the common-mode voltage of the two inputs changes. as V IN changes. References 1. a microcontroller. If necessary. ΔV CM is the changing common-mode voltage that you apply equally to the system's input terminals with respect to ground. "Getting themost out of your instrumentationamplifier design. it pays to understand the CMRR impact on your circuit. The op amp's input offset-voltage error is easy to understand. March 2006. the output of single-supply amplifiers can come within only 50 to 300 mV of each rail (Figure 1). the V OA1 and V OA2 output levels of A 1 and A 2 . Because the inverting input of A 1 and A 2 has high impedance. T is the temperature in degrees Kelvin. sboa035. to keep the output range at V O between the powersupply rails. RT is the thermistor resistance at temperature T. as the Steinhart-Hart equation describes. Single-supply-amplifier. OPA2365 2. as long the voltage at V O remains between the power-supply rails. On Board with Bonnie. the offset errors from the amplifier's open-loop gain also increase. or ground. however. Figure 1: This composite input stage of the op amp uses PMOS and NMOS differential pairs so the input-voltage range can extend from above the positive rail to below the negative rail. Eventually. With those types of amplifiers. The A 3 output restrictions are similar to those of any other amplifier. Figure 1 shows a typical single-supply amplifier's output swing as you drive the output to the rails. References 1. INAs have complete closed-loop operational amplifiers with feedback components included. decades-old operational amplifier to gain a differential input signal. the amplifier is nonlinear. the input stage has two op amps in an adjustable-gain configuration and provides high input impedance on both the inverting (VIN−) and noninverting (VIN+) inputs. Three basic kinds of nodes in the INA can cause input/gain-overload problems (Figure 1). The changes in output swing of A 1 and A 2 typically span the supply rails. you need a high-resolution converter to capture data during temperature extremes. From a signal-chain perspective.3V/µsec. The charge pump is a good stopgap. the system THD becomes -98 dB. 2. the amp's outputs change in voltage along with the IR drop across the R1 resistors. the microcontroller can again acquire a new ADC value. April 1. When measuring temperature.com called "Baker’s Best. This increased headroom allows for a single differential PMOS input stage to replace the composite differential input stage. When the circuit designer exercises proper precautions with the input pins. CMR A3 is equivalent to the CMR of the entire instrumentation amplifier. So how do designers calculate the INA's common-mode range? Consider the exposure to an input/gain-overload condition on the INA as a possibility. but it can get close. If the circuit designer uses the amplifier in a voltage follower or buffer configuration. The FFT plot in Figure 2a shows the amplifier/ADC-combination response to a 1-kHz signal in a 5V system. The voltage-feedback-amp's feedback-resistance value is more forgiving. As you work with these concepts. Let's consider the behavior of the composite and chargepump amplifier inputs in a single-supply inverting amplifier. In every case. R2 . V H is the maximum voltage level at the output in the dc-open-loop-gain measurement. to keep the output range at V O between the power-supply rails. Figure 5 The input signal. OPA344 data sheet. In Figure 1. There are more ways to tackle this input topology problem with the IC design. except for a few key points. To achieve optimum performance. and the NMOS transistors are completely off. If you use a current-feedback amp with the circuit in Figure 1. A 1 and A 2 require low-inputcurrent CMOS or FET op amps. An example of this type of input/gain-overload condition occurs when the in-range voltages on the INA's inputs drive A 1 . This condition is impossible to directly measure. per the manufacturer's specification and the circuit's noise gain. Setting the PGA (programmable-gain amplifier) at a gain of 1V/V. LMP7701 data sheet. A 2 . the input-current leakages are not dissipated. If the input signal always remains less or more than the composite input-stage’s transition voltage. RSER . IC designers borrowed a technology from other devices to solve this problem. matching the R 1 to-R2 ratio to the R3 -to-R4 ratio–is critical. and A 0 .or current-feedback amp with the circuit in Figure 1. When you bring V IN+ toward the negative rail.2V. At lower common-mode input voltages. For instance. April 23. The calculated CMR (common-moderejection) error due to resistor mismatches is 100×(1+R 2 /R1 )/(% of mismatch error). which makes the magnitude of the output voltage a direct measurement of the difference between the direct light impinging on D1 and D2 . if R1 equals R3 and R2 equals R4 . V B. . however. From this point. R3 . Neither of these approaches produced an amplifier adequate for the high-precision systems to span the amplifier's full common-mode input range.and current-feedback amps are almost the same Current-feedback amplifiers have a higher slew rate than do voltage-feedback amplifiers. You can combine a series resistor. a 10-bit ADC in this circuit can sense a limited temperature range (approximately ±25°C). bias current. 27 Content provided courtesy of EDN. equals 2R IN. Thus. Texas Instruments. As the common-mode input voltage increases. causes the INA's output to change to an invalid output voltage. usually in the middle of the INA's output range. The offset voltage of an op amp with a composite input varies across changes in the common-mode voltage. V H is less than V OH. These specification tables provide evidence of the characteristics of the input-stage topology. The first and most dominant factor is the balance of the resistor ratios across A 3 ." BC Components data sheet. March 27. Revisiting single-supply input topologies The CMRR (common-mode rejection ratio) specification describes changes in the amplifier's offset-voltage versus common-mode input changes. Low-Noise. Baker. R1 . But wait–there may be a problem with the output values of A 1 and A 2 (Figure 1). This is true for the inverting-amplifier circuit found in Figure 3. These four resistors combine with A 3 to subtract and gain the signals from the outputs of A 1 and A 2 . At first glance.com/ww/en/analog/Amplifiers-eBook/[1/17/14 2:03:58 PM] Back to top . the microcontroller sets the PGA gain to the next higher or lower gain setting. Increasing the PGA's gain at these temperatures brings the output signal of the PGA back into a range at which the ADC can reliably provide conversions that identify the thermistor temperature. When V B establishes the commonmode voltage of the amplifier. choose the V B voltage to be below or above the PMOS/NMOS transition region. "Rail-to-Rail Op Amps. even though the input signals continue to see a gain of -2V/V. The output stage has a reference voltage that level-shifts the output with respect to ground. As a second ICdesign strategy. engineers continue to demand lower system power supplies and insist on better signal integrity. R2 . the instrumentation amplifier's CMR performance values tend to reach a maximum value at higher gains. ISBN: 0-07-024247. resistive device that eases one of the interface issues as you convert the thermistor resistance to voltage. noise. you may notice that an input signal into an INA can produce an incorrect output signal that is nevertheless within the device's normal output range. 3. it is easy to assume that the closed-loop bandwidth equals the gain-bandwidth product divided by each channel's gain. For this amplifier. The more difficult interface challenge is to capture the nonlinear behavior of the thermistor in a digital representation with a linear ADC. 2001. if you use an amplifier that has a charge pump in its input stage to drive highprecision SAR or delta-sigma converters. a small variation in the signal bandwidth and gain peaking in circuit may occur. or CMOS gate.TI. Table 1: The single-supply op-amp data sheet describes the CMRR test conditions as well as the actual specified values. This amplifier's drive capability limits the resistor's minimum value.8V above the powersupply voltage.66V. The name "current-feedback amp" carries some mystique.1V." Additionally. Bonnie C. The two photodiodes let you find a light's position by monitoring the difference between their output signals. making it difficult to detect the correctness of the INA input implementation. The simple idea of an op amp's CMR (common-mode rejection) has been around since the beginning of op-amp time. As Equation 1 states. They began to use the all-too-common charge pump to push a single differential-input stage of the amplifier above the positive-power supply (Figure 1). "Where did all the racket come from?" EDN. In Table 1. the amplifier's common-mode input voltage can remain at a static voltage. In the region near ground. "Instrumentation amplifiers—avoiding a common pitfall." EDN. All of the internal amplifier's output voltages relate to the linear common-mode input range of the complete INA. such as an input stage with a charge pump. the noise gain is: where N is the number of input channels. you could use a single photodiode to determine the brightness of your bulbs. Amplifier designers place the switching mechanism's frequency above the amplifier's bandwidth and keep the switching noise lower than the amplifier's thermal noise floor. the amplifier's common-mode voltage remains constant.0004%. The configurations of A 1 and A 2 act as traditional current-to-voltage converters or transimpedance amps. in series with the thermistor. Bottom line: buffer circuit Composite input: the op amp’s input range (VIN) must not travel into the transition region between the PMOS and CMOS differential input stages Charge-pump input: performs without distortion in reaction to common-mode voltage changes Conclusion Manufacturers use several input topologies in their designs of rail-to-rail input amplifiers. unless you expanded your design task to using two photodiodes.1% mismatch will cause a degradation of the instrumentation amplifier's CMR from ideal to a 66-dB level. References 1. In Figure 1. The PGA hysteresis routine checks the PGA gain setting and compares the ADC digital value with the trip points that Figure 1 indicates. SBOS145. The thermistor-resistance change over temperature is nonlinear. This circuit's bandwidth. but the chip designer has to make some compromises to achieve this type of performance. the change of offset voltage is the only concern.1 to 2V. the instrumentation amplifier's CMR increases as the system's gain increases–a nice feature. You may think that the charge pump's output ripple voltage will become a noise problem. 2012. Since the V B voltage is static. Baker. FET gate. Jerald. the NMOS transistors start to turn on. The term "thermistor" originates from the descriptor "thermally sensitive resistor." Sensors magazine. differential-input stage. You can try to compensate for the thermistor's nonlinear response with a look-up table in your microcontroller. In this condition. However. 26 Back to top About the author Bonnie Baker is a Senior Applications Engineer with the WEBENCH® team for Texas Instruments and has been involved with analog and digital designs and systems for over 25 years. Next time. The input stages of A 1 and A 2 limit the range of these two input voltages. There are circuit-design tricks at your disposal for minimizing this input-stage crossover effect. you can program the gain found in Figure 1 with a single resistor. A 1 . The negative-temperature-coef ficient thermistor best suits precision temperature measurements. with some distortion. As you separate the parts. Circuit designers often misapply a thermocouple or even a two-wire microphone to the INA's input circuit. the PMOS transistors are completely on. rail-to-rail-output ads can give a false sense of security. Even though you can run this type of algorithm in your microcontroller firmware. the most misunderstood is the common-mode-range requirement. three-op-amp INA (Figure 1). An input/gain-overload condition can occur if A 1 's output voltage. an amplifier with a charge-pump input stage does not create this unwanted distortion across the entire input range. High CMRR. The op-amp topology in Figure 1 provides superior common-mode performance over the entire input range. The dc open loop gain in decibels is 20 log(ΔV OUT /ΔV OS ). For instance. of the buffer circuit changes the amplifier’s common-mode voltage. You must be aware of the limitations of V OA1 and V OA2 and then calculate whether your design is at risk using the equations in Figure 1.com Single-supply amplifiers do not truly swing rail to rail at the output. and the amplifier output never reaches either rail. At a gain of one. Using these equations. The transfer function for the circuit is expressed as The input common-mode voltage (VIN) can vary between ground and V S. the inner workings of the op amp are straightforward. References 1. a 10-bit ADC. Figure 3: The inverting-amplifier circuit keeps the amplifier's common-mode voltage at a constant voltage. and A 3 are constants that the thermistor manufacturer provides. noninverting amplifier. if the op amp's input stage has a charge pump with a THD specification that is 0. One technique is to place a resistor. She has written hundreds of articles. The composite input amplifier produces a crossover distortion. 2. 2009. V O. this 0. The final place to look for an input/gain-overload condition is at the output. Vendors of single-supply op amps claim their amplifiers have rail-to-rail input capability. 23 Back to top Instrumentation amplifier input-circuit strategies Many industrial and medical applications use instrumentation amplifiers to condition small signals in the presence of large common-mode voltages. Precision Unity Gain Differential Amplifier. Later. This situation is not so mysterious when you think about the inside of this device. 19 Back to top Rail-to-rail input amplifier application solutions Many single-supply operational amplifiers are capable of rail-to-rail input operation. 2. Note that the A 1 and A 2 stages do not gain the input common-mode voltage. If V IN travels across this entire range. 3. VB can be assigned so that the input signal is not required to go across the composite input stage’s crossover region. and Lew Counts. a regulated charge pump lifts the top of the differential input as well as its biasing current source to 1. 3. At higher instrumentation-amplifier gains." Texas Instruments. As such. but. 2. a unique zero-crossover input topology provides superior common-mode performance over the entire input range. That situation. in a single-supply environment. D). the application-circuit configurations for voltage. crossover distortion as the input common-mode voltage traveled across the full rail-to-rail input voltage range. When you bring the input terminals to the positive rail. and the PMOS transistors are off. V S. R3 . The inverting-amplifier circuit gains the input signal as well as changes the signal polarity. or both violate the internal output-swing restrictions. and a voltage reference or the power supply (Figure 1). this configuration rejects external common-mode voltage and noise. The best way to view the input stage's behavior is to look at the offset voltage versus the common-mode input voltage (Figure 2). the problem arises when the INA inputs are connected without proper consideration to current paths or biasing. the dctransfer function of this circuit is 1V/V. If the feedback resistance. the CMRR is equal to 20*log (ΔV CM /V OS ). Photodiode Amplifiers: Op Amp Solutions. The 4. AMSC. 24 Back to top Wringing out thermistor nonlinearities Thermistor-temperature-sensing devices present a design challenge if you intend to use such a device over its entire temperature range. your system's performance will improve. Kugelstadt. where RIN is the input resistance. January 1995. A 3 measures an erroneous difference voltage.6MHz." Electronics World. go back and refine your feedback-resistor selection. enhancements.ti. such components are subject to these terms. copyright.com/audio Automotive and Transportation www. or a license from TI under the patents or other intellectual property of TI.com/omap TI E2E Community e2e. Products Applications Audio www.ti.com/wirelessconnectivity Mailing Address: Texas Instruments. Information published by TI regarding third-party products or services does not constitute a license to use such products or services or a warranty or endorsement thereof. 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