Welcome to “A Precision Low-Level DAS/ECG Cardio tachometer Demo board” presentation.The presentation will focus on an interesting application of analog circuits where they are utilized to amplify and condition the very low level electrical signals associated with the human cardiac system. Often these applications involve detecting very small electrical signals and amplifying them in the presence of very large, potentially interfering signals. A cardiotachometer demonstration board has been developed for this purpose and our session today will underscore its capabilities and the difficulties that it overcomes in the harsh monitoring environment. The cardiotachometer is an instrument for measuring the rapidity of the heartbeat and can provide the details of the heart rhythm as it progresses from one beat to the next. In case you are not familiar with the acronyms DAS/ECG it is appropriate to explain them. DAS represents Data Acquisition System, which is an electronics system used to collect information, and condition the information such that it can be analyzed. For example, collecting and analyzing the heartbeat or other biophysical characteristics over a period of time. Electrocardiography, is a non-invasive procedure for recording the electrical changes in the heart. The record, which is called an electrocardiogram (ECG or EKG), shows the series of waves that relate to the electrical impulses which occur during each beat of the heart1. 1 www.healthatoz.com 1 This is an outline of the subjects that will be touched upon during this presentation. 2 Most often the stimulus behind biophysical activity taking place in a living organism is the result of small electrical changes that occur within muscle and nerve cells. These electrical changes are the result of biopotential differences. As the name implies biopotenials are biologically based electrical potentials acting as minute batteries. The diagram illustrates the resting potential which remains steady at about -70mV. But when commanded by the brain, a shift in the biopotential takes place and moves from -70mV to +20mV when the muscle reaction is undertaken. The shift amounts to a change of nearly 100mV as the muscle transitions from a resting state to an action state. These minute electrical changes within the muscle cells can be electrically observed through external instrumentation. The heart (myocardium) is a multichambered muscle and its health is central to life itself. Therefore the heart is often monitored using electrocardiography. The electrocardiograph is the instrument that detects, signal conditions, records and displays the heart’s activity. An important point to keep in mind is that even though the biopotential is strongest at the source, by time it is detected at the body surface it has been greatly attenuated making biophysical occurrences more difficult to detect and separate from interfering electrical sources. 3 Biopotentials are developed from electrochemical gradients established across cell membranes. These are voltage differences that exist between separated points in living cells, tissues, and organelles. The potential difference measured with electrodes between a living cell’s interior cytoplasm and the exterior aqueous medium is generally called the membrane potential or resting potential (ERP). This potential is relatively constant in striated muscle cells with a potential of about -50 to -100mV. Nerve cells show a similar range2. Related to these biopotentials are the ionic charge transfers, or currents that give rise to much of the electrical changes occurring in nerve, muscles and other electrically active cells3. This current is the direct result of the electrochemistry associated with ions internal and external to the cell. The biopotential plot has a rising section depicting depolarization and a falling section indicating repolarization. Depolarization can simply be though of as the electrical stimulation of the heart muscle cells. During depolarization the muscle fibers shorten causing contraction. While during repolarization the muscle cells relax, lengthen, and return to the resting state4. 2,3 4 Biopotentials and Ionic currents, “Answers.com” Welch Allyn Protocol Clinical Support 4 the Systole and Diastole. right ventricle. left atrium and left ventricle. The individual waves associated with each portion of the heart’s function sequence combine to produce the ECG waveform monitored on the body surface. Although these phases will not be further explored here. The function of the right side of the heart is to deliver deoxygenated blood from the body to the lungs. The resulting ECG waveform is shown at the bottom of the waveform diagram.The human heart cutaway shown in the diagram exposes the four chambers – the right atrium. The function of the left side of the heart is to deliver oxygenated blood from the lungs to the body. the waveform diagram accompanying the cutaway shows the relative timing and amplitude of the biophysical signals as the heart components go through a complete cycle. The cardiac cycle consists of two phases . 5 . 5 Welch Allyn Protocol Clinical Support 6 . The Bundle of HIS is a thick bundle of nerves that transmits the electrical impulses from the AV node to the Purkinje fibers. These fibers distribute the electrical impulses to the individual heart muscle cells5. Each wave and interval appear on the ECG display as the result of a particular electrical function of the heart6. These individual functions are observed on the ECG display and labeled as P. It controls the contraction of the heart.S.R. corresponding to the particular heart interval. It generates the electrical impulse and sets the pace of the heart.Q.T and U. The SA node is often referred to as the heart’s pacemaker. Cardiologist assess the functionality and gross condition of the heart muscle from these different segments of the ECG waveform.The Cardiac Conduction System is the name given to the heart’s electrical conduction system. They can be thought of as an ion to electron converter. When placed against the skin chloride is exchanged from the skin to the electrode. The DAS/ECG board that will be described is designed to perform these external functions. The electrode is composed of silver (Ag) with a silver chloride (AgCl) surface. In doing so there is a free two-way exchange of ions. This conversion allows the electrical currents to be amplified and conditioned by external circuitry. so no double layer is formed at the surfaces.The electrodes are transducers that detect the minute ionic currents associated with the biopotenials. and silver is exchanged from the electrode to the skin. 7 . 5V shown in the diagram which provides DC biasing. provides a common mode drive voltage. it provides common-mode signal feedback to aid in common-mode noise cancellation. the tiny differential signals are coupled to an instrumentation amplifier (INA) for the first level of amplification. From the arm electrodes. One electrode is placed on each arm. it may be used to impose a common DC level on the patient. while a third is placed on the right leg. first. The latter is very important because common-mode noise may be hundreds to thousands of times greater than the detected ECG biopotentials. connected to the right leg. And second. An example would the +2. The arm electrodes are intended to detect the minute differential biopotentials associated with the heart’s activity. This third electrode serves two purposes.For ECG applications three or more electrodes are placed on the body. 8 . The third electrode. to the two differential sensors. The diagram shows one of the most commonly used connections between the body and ECG equipment. With time this was perfected into the more commonly used connections today. The equilateral triangle is formed by raising the arms and positioning the points on the limbs equidistant. The lead vectors associated with Einthoven’s lead system are conventionally found based on the assumption that the heart is located at the center of a infinite.cc. the voltages measured by the three limb leads are proportional to the projections of the electric heart vector on the sides of the lead vector triangle7. which may include as many as 12 electrodes. 7 buttler. homogenous volume conductor (at the center of a homogeneous sphere representing the torso).tut.fi 9 . This allows the heart biopotential activity to be monitored through many different planes.The ECG Einthoven triangle dates back to the earliest days of electrocardiography and provides the basis for electrode placement. With these assumptions. Either leg may be used for a lead connection and the other leg then becomes the reference to which the other limbs are referenced. Einthoven’s Law provides the voltage relationships between the leads. The right-hand diagram shows how each of these subcircuits interconnect to create an overall equivalent circuit. a greater harmonic bandwidth is needed. The electrode itself can be modeled as a 1μF capacitor in parallel with a 10kΩ resistor. Although this may appear to be a low cutoff frequency it is sufficient to pass the frequency components associated with the ECG. For example.When the ECG electrode is physically contacted with the body a complex electrical model is created. with a heartbeat rate of 60bpm. Even the fast R-wave potion with a duration of about 0. 10 . The 1μF capacitor in conjunction with the 1kΩ skin resistor inserts a simple RC. skin contact resistances and a parallel resistance and capacitance associated with the probe. low-pass filter function in the ECG path to the amplifier. But because this is a quickly ramping up and down pulse. the fundamental frequency is 1Hz. The 159Hz satisfies the requirement for even shorter R-waves. has a fundamental frequency of about 33Hz. The model includes the body biopotential and resistance. Its cutoff frequency is: fC = 1/(2πRC) For the values shown fC is 159Hz.03 seconds at 60bpm. The bandwidth limited electrode/skin interface helps reduce the circuit’s response to unwanted higher frequency electrical interference. but note the smoothness of the waveform as compared to the ECG waveform. Therefore. the bandwidth requirements are much less for a blood pressure monitoring application. 11 .This is comparison of the fundamental frequency and bandwidth requirements for monitoring blood pressure in the head and an ECG. The blood pressure waveform has a period that coincides with the R pulses of the ECG. while the others are much smaller. The P. 12 . Any electrical interference can easily mask these important portions of the waveform.Here is an example of a normal ECG chart recoding for a heartbeat of 62bpm.Q. Note that the R wave pulse has an amplitude about equal to 1mV. The rate can be determined from the rate of R wave occurrences. A 1mV calibration pulse is posted for comparison and has an amplitude of 500uV per vertical division.R.and U portions of the ECG are labeled for convenience.S.T. The drift in the baseline is normal and can be due to the long charging time constant of AC coupled circuits and/or the subtle changes in the electrode halfcell potentials associated with the ionic charge transfers (current). referred to as a somatic tremor. coupling circuit and/or changes in the ionic current levels. 13 . Muscle shaking is an example of an irregularity caused by internal muscle tremors. Sixty hertz AC pick-up is the result of induced electric field energy present in the vicinity of the ECG equipment. but any induced frequency such as RF can disturb the ECG adding noise to the baseline. Short-term DC instability may be an indication of an issue with the ECG equipment. So this characteristic is connected with the equipment rather an internal bodily function. Not only 60Hz. The gradual baseline drift discussed in the previous slide is due to charging of the high-pass. often received by the electrodes or electrode leads.These displays provide examples of irregular ECG tracings caused by both internal and external factors. A variety of different sensors may be directly interfaced to the board making possible other types of medical-related and non-medical measurements. 14 . The demo board contacts provide the input for the differential ECG signals via the right and left thumbs. and digital indication of heart rate.The DAS/ECG demo board functions as a self-contained heart-rate monitor providing a visual. If necessary. The three ECG electrodes are built in and conveniently accessed off one end of the board. audible. external leads and contacts may be connected to the board as well. Common-mode drive is accessed via a finger electrode under the board. Since the board is only being used to detect heart rate and not a detailed ECG pattern. precise Einthoven electrode connection are not required. If the amplitude of the waveform is sufficient.The biopotentials detected at the body surface by the ECG are highly attenuated relative to their point of origination. Often. Other body signals such as brain waves may have amplitudes a fraction of this level. Additionally. the amplitude is on the order of a few hundred microvolts (μV). to key a 1kHz burst oscillator. Once the low-level signals are amplified the output is applied to the cardiotach circuit. it will trigger a one-shot multivibrator. on-board circuitry is provided so that the amplifiers may be configured for sensor interfacing and filtering functions. This is accomplished through the use of high performance instrumentation and operational amplifiers on the demo board. The amplified waveform is passed through a 150 μV peak-to-peak threshold detector. 15 . used to pulse an LED. These will be discussed in more detail a little later. The one-shot output may be counted. Very high voltage gain (V/V) is required to bring these minute signals to a level where signal processing may be reliably applied. The DAS/ECG board also provides a probe point where the amplified ECG waveform may be observed with an oscilloscope. U10 – Provides a stable +2. U1. U6 – Peak-to-peak detector and monostable multivibrator circuit. U8 – Auto power down circuit which is especially useful when using battery power.Moving to the next level of circuit complexity reveals the IC building blocks used in the demo board: 1. U9 – An uncommitted op-amp useful for providing sensor interface. 4.5V reference voltage for mid-scale commonmode biasing. 7. U11 – An optional socket for the OPT101 Monolithic Photodiode/SingleSupply Transimpedance Amplifier. 2. U2. 5. 16 . 3. 6. U3 – Input instrumentation amplifier and gain stages. U7 – Low dropout regulator supplies +5V to power the circuitry. U5. U4. a minimum CMRR of 100dB (114dB typ. The INA326 gain is set to -5V/V in this example. it is necessary to establish a mid-scale voltage. centered about +2. The INA326 is followed by an OPA335 auto-zeroing operational amplifier that features a maximum voltage offset of 5μV. low-pass filter may be configured within the stage by the addition and selection of a feedback capacitor. That is accomplished by connecting the +2. or 2400V/V.) and an adjustable gain from less than 0. A 1mVP-P input is amplified to 4. The overall gain is the product of the individual gains of the two stages.8VP-P. Likewise. Here the OPA335 is set to an inverting gain of -480V/V. 17 . (-5V/V)(-480V/V). a voltage offset drift of 0.5V. rail-to-rail INA326 instrumentation amplifier is at the front end providing low offset (<100uV).Here the analog front-end has been separated from the remaining circuits.000V/V. common-mode DC voltage is rejected by the amplifier.05μV/ºC and maximum operating current of 285μA. A precision. A first-order.5V reference voltage as a common-mode voltage to both the INA326 and OPA335. Since the board is powered by a single supply.1V/V to >10. The high common-mode rejection of the INA326 rejects the 60Hz and other common-mode interference picked up by the electrodes. The inversion is important because it will be used to counter a DC common-mode. A2 will amplify the difference in voltage applied to its two inputs and in turn drive the common-mode potential applied to the right leg until it is equal to the +2. 18 .5V reference voltage. This voltage is buffered by A1. This auto-zero feature keeps the DC level constant which is necessary for a stable ECG display baseline.5V common mode voltage is applied to A2’s non-inverting input via a resistive divider. R G is split into two equal resistors.5V/V. and then applied to A2 which has an inverting gain of minus 19. Any DC common mode voltage present at the two inputs will shift the DC level at the resistor junction.Just the front-end portion of the INA326 is shown illustrating how the right-leg DC drive voltage is developed and controlled. The INA326 gain set resistor. electrode potential on the electrodes.5V voltage is the mid-scale voltage level for all the analog circuitry. A +2. The +2. AC coupling removes the DC electrode offset. The gain is set by selecting an input resistor via a jumper. The AC high-pass frequency response is selected at 0. The INA326 is followed by ½ of a OPA2335.05Hz. Additionally. or 2.5Hz. a low pass filter function is provided by this stage.This very busy circuit portion of the DAS/ECG circuit diagram provides the remainder of the analog front-end circuit.0Hz using a resistor-jumper provision. 19 . Its cutoff frequency is set by connecting the appropriate capacitor into the feedback path with another jumper. 0. gain stage. The INA326 circuit includes a provision for DC or AC coupling. This offset is taken care of using a DC restorer circuit that will be discussed in the next slide. As the frequency is increased the gain of the integrator rapidly falls off. This reference voltage is sometimes referred to as a pedestal voltage. It also provides a high-pass transfer characteristic with a cutoff frequency that is a function of the integrator RC constant.5V within the output bounds. The net result is a DC restorer circuit that compensates for a DC commonmode voltage. If 0V is applied to the non-inverting input. but without any capacitors directly in the signal path.5V on the noninverting input. If the reference pin is set to +2. Thus. At DC the integrator’s gain is very large and any deviation from +2.will result in a large DC voltage at the output. 20 . then the output can swing above and below +2. pin 5. This DC voltage is then applied to the INA326 reference input in such a manner as to drive the INA’s output back to +2. AC signals having a frequency above the integrator’s -3dB cutoff frequency have virtually no affect on the reference voltage applied to the INA. High quality.The INA326 output voltage may be referenced to a voltage applied to the reference pin.5V. This results in a circuit equivalent to a capacitively coupled amplifier. The integrator shown in the schematic is referenced to +2. high capacitance capacitors are often large and costly and are avoided using this technique.5V seen at the inverting input – as the result of a common-mode DC voltage on the INA’s inputs . then the output will be referenced to zero volts and the swing can move up from 0V. such as may be present with the electrodes.5V. because it raises the output up from ground (0V). or the onboard facilities provided on the DAS/ECG demo board may be utilized. These pulses trigger a one-shot multivibrator which stretches the pulses to a uniform time duration. or be counted by a BPM meter.The output from the amplifier section may be sampled and processed by external circuitry. The burst oscillator has audible tone that is available through the speaker. These pulses may also be used to flash an LED as a visual indictor of BPM. 21 . The stretched pulses from the one-shot are then used to key a 1kHz burst oscillator for a time period that corresponds to the one-shot pulse duration. The amplified ECG waveform is passed to a differentiator and peak-to-peak detector that produces pulses at the heartbeat rate. The input signal is amplified to a level such that the second stage output runs rail-to-rail. This is ideal for triggering the first TLC556 section. U4’s second section is a peak detector where the peak value of the ECG waveform is stored on C11 (0. The TLC556’s second section is arranged as a 1kHz. producing a positive going replication of the positive or negative going ECG wave. which is configured as a 100ms. astable multivibrator. one-shot. 22 . U5’s first section buffers the peak detector output. keyed by the preceding one-shot stage.The first section of U4 is connected as an absolute value amplifier.1μF). The circuit has a lower threshold of about ±150μV. nearly 0 to 5V. while the second section amplifies and “squares up” the waveform. 8V. Its +5V regulator output is supplied to the TLV3491 comparator supply pin. Then supply current is delivered to the DAS/ECG circuits through the comparator’s output stage.5V the comparator output is high. Current varies from <1mA to about 15mA depending on the board’s operating mode.The DAS/ECG board may be powered by either a 9V alkaline battery. An RC timing circuit is connected from the +5V supply line to the inverting input. in about 40 minutes.5V. A TPS71550. 23 .5V voltage is developed at the non-inverting input. at about +4. When that voltage is below +4. A small amount of hysteresis is added to the comparator to improve noise immunity at the threshold. the output voltage drops to nearly zero volts removing power from the DAS/ECG circuits. while a +4. The voltage at the RC node is initially zero and eventually charges to a voltage exceeding +4. Once the comparator RC input voltage crosses +4. 5V low-drop out (LDO) regulator provides the supply voltage for the board.5V. or an external supply. The DAS/ECG demo board has a number of features that make it easy to use for testing circuit ideas and experimentation. it may be used for other portable applications where high voltage gain and high common-mode rejection are required. 24 . In addition to the EGC cardiotachometer application. The left arm (LA) and right arm (RA) “electrodes” are located on the end of the board.This image displays the top side of the DAS/ECG cardiotachometer board. while the right finger drive electrode is placed underneath the board. 25 . power ON timer function. There is an ON/OFF switch and start switch for the 40 minute. LED and supply connections are also shown. The speaker. The gain and low-pass and high-pass cut-off frequencies care established using jumpers and can be changed as needed.This shows some of the user selectable functions on the board. 26 . Here’s a more detailed layout showing the location of the analog circuits and the tachometer circuits that follow them. 27 . 5V when powered by a +5V supply. These add to the AC common-mode signals on the body and help in the cancellation of these unwanted signals.The back side of the ECG/DAS board contains the important common-mode drive pad. This is typically biased at +2. 28 . It is important from the standpoint that it sources complementary phase. The image also shows the back side of the pin sockets that are used for wires connections to the board and the +9V battery holder. A brief set of instructions for the cardiotachometer use are provided on the board. in the upper right-hand corner. AC common-mode signals back to the body. The board is easier to steady and maintain an even contact while sitting. John Brown the DAS/ECG demo board developer.Here. 29 . demonstrates how the board is held while in the standing position. The key to obtaining a good cardiotachometer result is to gently grasp the electrode pads as shown while holding the board steady. so do so if possible. but also analytical and scientific instrumentation. Therefore. and some automotive and industrial sensor applications as well. 30 . it is equally suitable for other applications where very small signals may be buried amongst large common-mode signals. Certainly other biomedical monitoring applications fall into this category. industrial monitoring. in the presence common-mode AC interference with an amplitude of about 2Vp.The cardiotachometer amplifier circuit is capable of detecting a biopotential of about 200uVp. A puffing tube in conjunction with a bridge transducer will be used to detect a change in gas pressure. 31 . This application will demonstrate how a bridge transducer can be directly interfaced with the DAS/ECG board. The magnitude of the breath pressure can then be used to control a medical assist apparatus such as a wheelchair.Some other applications will be explored now to show the versatility of the DAS/ECG design. Medical uses include applications where the tube serves to direct the breath pressure of a user to the bridge transducer. Puffing tubes find application in industrial gas lines and valves where the gas pressure and flow characteristics require monitoring. 5VP for a range of 1.16 to 2. Current for the bridge transducer may be supplied by the DAS/ECG.4mVp-p.2V to 4. such as the puffing tube.8V.5V center voltage is from the pedestal voltage applied to the INA326 reference pin. Any input common-mode DC voltage and bridge offset voltage will be auto-zeroed by U3 as previously discussed. The 2. then the output range would span from 0. If the differential voltage measured 2.Silicon Microsystems manufactures a thin film pressure bridge transducer that interfaces with a air lines.0 to 4. A nominal value of 1. The transducer’s sensitivity in this application results in a differential voltage of about 0. The bridge transducer may have an offset.4mVp-p.5mVp-p is used for illustrative purposes. or imbalance between the two sides as great as 50mV.5VDC ±1. 32 . on-board +5V reference. The gain is set to 2000V/V and this produces an output voltage of 2. The bridge connects directly to each INA326 differential input.0V. while the bridge produces a 0. The output phase between the mechanical gauge and the DAS/ECG board are set the same so that both result in an upscale reading. This is adequate for the bridge sensor output range. The DAS/ECG board has been set with a bandwidth of 2Hz to 17Hz in this application.The ECG/DAS board bridge sensor input is shown coupled to a mechanical pressure gauge in the puffing pressure bridge application.75mVpk change for the same input. the DAS/ECG board gain has been set to 2000V/V. The sensitivity of the mechanical gauge is established at Δ5mm Hg for a 0. 33 .0375psi pressure change. As mentioned. but illustrates the ability of the board to detect and amplifier the bridge sensor output – even at this higher rate.Here’s the actual oscilloscope display for the DAS/ECG board output with a simulated puffing input (upper trace). 34 . The output swings approximately 1. and is centered about the +2.5VP-P.5V pedestal voltage. The lower trace indicates that the burst oscillator is being activated and it provides pulses. The pulses can be counted and used to arrive at the puffing rate.2s than a human can deliver. The input puffing rate is a much faster 0. Now the bridge offset must be taken into account to assure the DAS/ECG board output does not saturate. The DAS/ECG will now be configured to provide DC coupling .versus the AC coupling used in the previous applications. 35 . In this example the pressure change associated within a squeezing a tube will be observed and measured.This is pressure bridge application where DC or very low frequency signals require monitoring. one must be cognizant of a sensor’s DC offset and the direction it will drive the output. Therefore. 36 . That could result in an voltage level that would exceed the amplifier’s minimum or maximum output level.If the DAS/ECG board is configured for DC coupling any offset associated with the bridge will be amplified by the very high circuit gain. The bridge offset is so large that the gain has to be limited to this much lower value. This is to prevent the offset from driving the output into the positive output rail. the bridge “phase” has been selected such that when the squeezing pressure is applied the bridge resistances shift in the direction that moves the output downward and away from the positive rail. For this example the bridge offset is 43mV and when multiplied 100x the output is about +4. down to 100V/V.3V. placing the output close to the positive rail. However. 37 . Notice that the overall gain has been reduced substantially from its previous AC setting of 2000V/V.The auto-zero feature has been disabled and the INA326 reference pin is connected to zero volts. If the offset was as high as 50mV. but note that doing so does reduce the bridge output by 50%. Bridge bias is provided by the on-board TPS71550 LDO regulator. An alternative to lowering the gain would be to reduce the voltage applied to the bridge. The output is then used to bias the bridge. 38 . then the output would be up against the rail. it can easily be configured as a unity-gain buffer. The device is specified with a maximum offset of ±50mV.This image depicts how the squeezing tube is connected to the bridge and also some of the DAS/ECG board settings for DC operation. the dual OPA2335 (or OPA2336) is not used in this application. The same pressure bridge transducer is used here as earlier. A resistive divider in located on the board and divides the +5V down to +2.5V. Since U2. Gain resistors have been changed and the low-pass bandwidth jumper set as needed. but the bridge circuit connections have been changed to assure the amplifiers operate within their linear range. It has been observed that the particular bridge used for this example resulted in a differential offset of 43mV. This is still within the board’s AC passband when the high-pass filter is set to a cut-off frequency such as 0. Notice that the amplitude change during the squeezing event is about half with +2.5V excitation as compared to that with +5V excitation.The output response of the DC coupled DAS/ECG board during a squeeze is displayed in this oscilloscope image.05Hz. This is as expected. Some examples of low frequency uses are geophysical. while directly below it is the response with a +2.5V bridge excitation. This translates to a frequency of about 0. mechanical and industrial process control applications.2Hz. Also observe that the event had a duration of about 5 seconds. Setting the board for DC coupling may be the best option for use at even lower frequencies. The upper trace is the response with a +5V bridge excitation. 39 . One 499k resistor is connected directly across the INA326 differential inputs. This same common-mode voltage appears at both of the INA326 inputs through the resistors.5V. body members or the whole body. 499k resistors across the photodiode. photo generated current flows through the diode and through the 3 resistors. A sensitive photodiode or a combined photodiode/transimpedance amplifier such as the OPT101 is located on the other side of the finger. Fluctuations in the blood volume within the finger changes the transmission path between the LED light source and that reaching the photodiode. The blood volume coincides with the pressure and the DAS/ECG board provides a relative indication of the pressure. The cathode end of the diode is referenced to +2.Here’s an interesting AC coupled application for the DAS/ECG board where relative blood pressure may be optically detected and monitored. The circuit configuration is that of a plethysmograph. 40 . An LED is positioned so that its light output is directed through the finger. When light shines on the photodiode. an instrument used for measuring changes in volume within an organ. As the photo generated current changes in response to the blood volume fluctuations a differential voltage is created across the resistor and is amplified by the DAS/ECG board amplifiers. Notice the connection of the photo diode and the 3 series-connected. An overall gain of 6kV/V is used with the application and the bandwidth has been set from 2Hz to 17Hz. 499k bias resistors have been added to the board. The three. 41 .The DAS/ECG board is shown outfitted with the monolithic OPT101 photodiode/transimpedance amplifier. This oscilloscope display provides the output traces when the DAS/ECG is connected in the plethysmograph application. This pulse is used to key the output burst oscillator. The middle two traces are the 1st timer’s input and output pulses. 42 . The upper trace tracks the changing blood volume within the finger indicating the blood pressure level. The output pulse corresponds with the peak blood pressure. A 700mVP-P output amplitude results when the overall gain is set to 6kV/V. evaluate and optimize circuit performance in medical and non-medical sensor applications. front-end applications. the DAS/ECG board is useful for demonstrating the ability of highperformance analog circuits in low signal level.In summary. 43 . The board’s versatility allows one to experiment.