Biopotential AmplifiersECG Amplifier Basic Requirements • Essential function of a biopotential amplifier is to take a weak electric signals of biological origin and increase its amplifier • They must have high input impedance so that they provide minimal loading to avoid distortion of the signal. Typical input impedances are 1MΩ. • Input circuit must provide protection. No currents must appear at the input terminals. • Output circuit is primarily used to drive the amplifier load – output p impedance p should be low. • Biopotential amplifiers must be designed to be optimal in a particular frequency range as needed by the signal to obtain optimal p signal g to noise ratios. l The h voltage l is i in i the h range of 1 mV ~ 5 mV • The second stage is an instrumentation amplifier. .ECG Recording System • The first stage is a transducer (AgCl electrode).04 Hz to 150 Hz filter. Normally implemented by cascading a low‐pass filter and a high pass filter. which has a very high CMRR (90dB) and high gain (1000) • Opto‐coupler to isolate the input and output of amplifier by converting the electrical signal to light and then back • Bandpass filter of 0. which convert ECG into i electrical l i l voltage. Cardiac Vector • • Heart generates an electrical signal Electrical activity of the heart can be modeled as an electric dipole located in a conducting medium where a dipole consists of points of equal positive and negative charge separated from one another and is denoted by the dipole moment The dipole moment is a vector from negative charge to positive charge having the magnitude proportional to the separation of these charges. This dipole moment is called the cardiac vector. • • • • . The cardiac vector indicates the direction of the depolarization in time. represented by M Its magnitude and direction vary during the cardiac cycle as the dipole field varies itself. so no voltage component is seen in this lead. The component of M in the direction of a1 is given by the dot product of these two vectors and denoted on the figure by val.a1=|M| cosθ M a2 θ υa1 + Figure 6. Lead vector a2 is perpendicular to the cardiac vector. We can do that by connecting leads on the surface of the body to d detect biopotentials.We want to capture the cardiac vector M by looking at t vector t components t . . b l then h a1 the voltage difference introduced in the lead is the projection of the cardiac vector A lead is defined as a connection between 2 electrodes placed on the body Va1=M.2 6 2 Relationships between the two lead vectors a1 and a2 and the cardiac vector M. LL and RA are fed to the inputs of the instrumentation ‐ diff differential ti l amplifier lifi I+III=II .Example of Leads – Eindhoven’s triangle • Connection between 2 electrodes • The Th primary i leads l d are – – – – Lead I: LA to RA Lead II: LL to RA Lead III: LL to LA RL for ground • For a lead II system which is very y common. . Concept of Wilson’s Central Terminal • Wilson et al. • Wilson’s terminal is not ground – but the average of the limb potentials with the total current at this point to be zero • There are other lead configurations called Augmented Leads . suggested the use of the central terminal as a reference for measuring the electrode potentials • This reference was formed by connecting a 5 kW resistor from the limb electrode to the common point point. Other Leads – Augmented For signal augmentation – Disconnect the unipolar electrode y you are measuring g from the wilson’s terminal and then measure . Chest Leads V1‐V6 Chest leads V3‐V4 best for septal defects The most commonly used clinical ECG‐system. V5. V2. the 12‐ lead ECG system. III aVR. V4. V6 . aVF V1. consists of the following 12 leads leads. V3. II. which are: I. aVL. 12 0.16 0 02 t 0.13 13 0.000 to 0.06 to 0.0 to 0.ECG Wave ECG Nominal Data wave P Q R S T Lead I 0.42 Lead II 0.18 0 18 t 0.49 0.0 to 0.015 to 0.02 to 1 1.55 Lead III ‐0.06 to 0.073 to 0.03 to 1 1.06 to 0.30 .68 68 0.18 to 1 1.19 0.0 to 0.0 to 0.28 0 03 t 0.31 31 0.55 0.13 0.36 0.0 to 0.0 to 0. 7 Block diagram of an electrocardiograph .Design of an ECG circuit Right leg electrode Sensing electrodes Lead‐fail detect Driven right leg circuit ADC Memory Amplifier protection circuit Lead selector Preamplifier Isolation circuit Driver amplifier Recorder Ð printer Auto Baseline calibration restoration Isolated power supply Parallel circuits for simultaneous recordings from different leads Microcomputer Control program ECG analysis program Operator display Ke board Keyboard Figure 6. Main Components of the ECG Circuit Preamplifier ‐Initial Amplification ‐Needs very high I/P impedance ‐High CMRR ‐Typically. it is a 3 opamp differential amplifier with a gain control switch Driver Amplifier ‐Amplification of the ECG signal for appropriate recording Isolation circuitry ‐Blocks the ECG from power line frequencies eque c es Driven right leg circuit ‐Provides a reference point on the body instead of ground . Preamplifier Design Design g Specifications p Amplification Range: 20‐2000 Frequency Range (0. Gd=4 4. We can replace R4 in y a potentiometer p to this circuit by adjust to increase common mode rejection.7 7 and Gc=0.0047 which is good Common Mode rejection. .5MΩ Hi h CMRR (Ex High (E 60dB) Step 1: Single Opamp Differential Amplifier For this differential amplifier VOUT = (V1 – V2)R4/R3 For a CMRR>60dB or CMRR>1000 Gd/Gc>1000 Gd is governed by R4/R3 if we choose R4=47K R4 47K and R3 R3=10K 10K.05‐150Hz) High Input Impedance 2. • Step 2: Consider the 2 opamp stage and design it for high gain VOUT Gain= = (V1 – V2)(1+2R2/R1) VOUT 1+2R2/R1 If we choose R2=22K R2 22K and R1=10K R1 10K.4 . then gain=(1+(2*22)/10))=5.Preamplifier Design Cont. 4~25 VOUT = – (V1 – V2)(1 + 2R2/R1)(R4/R3) .Preamplifier Design Cont.7*5. • Step 3: Cascade the 2 opamp stage with the differential amplifier Total Gain of the instrumentation amplifier =4. STEP5 Preamplifier with Filtering Low Pass f=1/(2*pi*RC)~106Hz Truncates frequencies>106Hz STEP6 Non‐inverting amplifier Gain=(1+150K/4.7K)~32 ( / ) Total Gain=25*32=800 STEP4 High Pass τ=RC=3.3s f=1/(2*pi*RC)~0.05Hz Passes frequencies>0.05Hz . the High Pass Filter stage should be placed immediately after the d ff differential l amplifier l f to chop h off ff the h DC component of its output.Some additional design considerations High gain stages early in the signal path. Otherwise. this DC component will be amplified by the gain stage g g and may y saturate the following op‐amps . However. 2nd order filter Salley‐Key high pass filter .Its gain is determined by the resistor Rg. This coupling is modeled as a capacitor. This is quite high. VA ‐ VB = Id1*(Z1‐Z2) ~120µV if Id1 is in nA and difference of Z1‐Z2 is in KΩ. Interference from Electric Devices – Power line interference Power line C2 Z1 Id1 Id2 C1 120 V C3 There is electric field coupling between the power line and the lead wires and/or ECG amplifier. A B Electrocardiograph G ZG Id1+ Id2 Figure 6. Also lowering skin‐ electrode impedances may help.Problems with ECG. Hence the voltage VA ‐ VB = Id1*Z1‐Id2*Z2. If the electrodes are placed close together the currents are approximately the same. Coupling capacitance between the hot side of the power line and lead wires causes current to flow through skin‐electrode impedances on its way to ground. . This can be minimized by shielding the leads and grounding each shield at the ECG unit.10 A mechanism of electric‐ field pickup of an electrocardiograph resulting from the power line. Body impedance is low ~ 500Ω. It causes a current to flow from the Z2 power line through the skin‐electrode impedance through the body to ground. Hence the h skin k electrode l d impedances d become b critical l in the design of the biopotential amplifiers .2µA and ZG=50KΩ. Any imbalance in the input contribute to the common mode signal. υcm Z1 υcm Electrocardiograph A Zin B Zin Z2 υcm ZG idb G Figure 6. But for real amplifiers with finite input impedance. mode voltages such as powerline interference . VA‐VB=Vcm ((Z2‐Z1)/Zin) if Z1 and Z2 are <<Zin. there is some Vcm that appears in the output output.11 Current flows from the power line through the body and ground d impedance. Typical values are 10mV for idb=0. The magnitude of this signal is Vcm=idb*Z ZG. Power line 120 V Cb idb There is also a possibility of current from the power line to flow through the body as shown causing a common model voltage to appear in the signal.Problems with ECG. i d th thus creating ti a common‐mode voltage everywhere on Hence we need to keep input impedance high And skin‐electrode impedance equal to remove common the body. For a perfect amplifier this is no problem as the differential amplifier with reject the common mode signal. Electrom ographic interference on the ECG. • Other sources of interference – Magnetic field pickup EMG i interference t f Figure 6.Problems with ECG Cont. Figure 6. . (b) This effect can be minimized by twisting the lead wires together and keeping them close to the body in order to subtend a much smaller area.9 (a) 60 Hz power‐line interference (b) Electromyographic interference. The change in magnetic field passing through this area induces a current in the loop.12 Magnetic‐field pickup by the elctrocardiograph (a) Lead wires for lead I make a closed loop (shaded area) when patient and electrocardiograph are considered in the circuit. • Voltage limiting devices such as diodes are used for protecting the ECG circuitry and are connected between the lead and RL ground.14 6 14 Voltage‐limiting devices (a) Current‐ voltage characteristics of a voltage‐limiting device. (b) Parallel silicon‐diode voltage‐limiting circuit. • These occur for example in the operating room when the ECG is combined with the use of an electrosurgical unit that will induce high transient voltages into the patient.13 A voltage‐protection scheme at the input of an electrocardiograph Figure 6. Figure 6. (c) Back‐to‐back silicon Zener‐diode .Problems with Transients • To protect the ECG circuit against high voltages we need voltage limiting circuitry. Other Problems frequently encountered with the ECG • Frequency Distortion: High frequency distortion ‐ Rounding off the QRS waveform and diminishing its amplitude. take long time for recovery due ECG to the large charge built up in the capacitors. g the p patient current will flow through presenting a safety problem as well as •Artifacts from Large Transients – elevating the patients body potential Cause a large abrupt deflection in the projecting erroneous voltages in the ECG. Low frequency distortion – baseline is no longer horizontal after an event. and a finite period of time is Peaks of the QRS are cutoff required for the charge to bleed off enough to bring the ECG back into the amplifier’s Ground Loops – If 1 ground of 1 device active region of operation. • • . saturate. Saturation or cutoff distortion – High Figure 6. a a first‐order recovery of the system. This is followed by is higher than the ECG ground.8 Effect of a voltage transient on an ECG recorded on an electrocardiograph in offset voltages and improperly adjusted which the transient causes the amplifier to amplifiers can produce saturated ECGs. we can try to eliminate the common model signal at the source. For instance Electric l and d Magnetic f field ld pickup k can be minimized by electrostatic shielding υ RL and twisting of lead wires. potential This negative feedback causes the output common mode signal to be low. cm id − + υ3 Ra − + υ4 Ra Rf − Auxiliary op p amp p + Ro • RL . Another h solution l i i is the h Driven i ‐Right i h Leg R System where the RL electrode is connected to the O/P of an auxiliary opamp The common mode signal opamp. Even though the amplifier will help in eliminating these because of the high CMRR. sensed by the voltage followers is amplified and fed‐back to the body – raising the RL potential.Common mode reduction circuits • Common mode signal from the body or power line is a problem. EMG is the electromyogram.16 Voltage and frequency ranges of some common biopotential signals.Design considerations with other p amplifiers Figure 6. EOG is the electrooculogram. EEG is the elctroencephalogram. dc potentials include intracellular voltages as well as voltages measured from several points on the body. ECG is the electrocardiogram. and AAP is the axon action potential. . Typical surface EMG signals for large muscles. To observe an EMG signal. Muscles generate voltages around 100 mV when they contract. These voltages lt are greatly tl attenuated tt t d b by i internal t l tissue ti and d the th skin.EMG Amplifier – Basics and Design • • • • • • • • • EMG stands for electromyogram It is measurement of electrical potentials created by the contraction of muscles. We will also want to use a circuit the draws nearly zero current from the p leads. ki and d they th are weak but measurable at the surface of the skin. EMG signals contain frequencies ranging from 10 Hz or lower up to 1 kHz or higher. We reject this signal by looking at the difference in voltage between two nearby points on the skin over the muscle of interest. are around 1‐2 mV in amplitude. we need to build a differential amplifier with high common‐mode rejection The dominant common mode voltage signals on our bodies is usually a 60‐ Hz sine wave that is capacitively coupled to us from the 120‐VAC wiring in the walls. such as the bicep. since dc current p passed through g EMG electrodes can lead to input large dc offsets and degrade the long‐term usefulness of the electrodes. . . 2 nA or devices with MOSFETS (lower input currents. To observe an EMG we need EMG electrodes. Amplitudes visualized should be 100‐300mV. but they generally exhibit higher levels of noise). You can essentially plot gain over frequency for varying I/P frequency. For example you can set a gain of 201. . noise) TL084 is identical to that of the LM324 in the pin diagram For safety the best method is to connect two 9V batteries for power supply You can design the instrumentation amplifier (3 opamp one) we discussed in class. 201 Gain is (1 + 2R2/R1)(R4/R3) We can use values like 10KΩ for all resistors except R2 and 1MΩ for R2 to get an overall gain of 201. Connect the other two electrodes to the input of the opamp and observe the response on the oscilloscope by flexing the bicep. You can measure the overall gain of your circuit by applying a small amplitude 1KHz sine wave from a function generator. this is a good g grounding g g p point. The 3rd electrode can be b stuck t k to t th the b bone i in your elbow lb of f the th same arm and d that th t is i connected t d to t ground d i in the EMG circuit. We can stick two of these electrodes on the muscle of interest (ex. This will keep your body potential near your circuit’s ground potential. Connect your elbow to circuit ground. With cutoff around 10 Hz . To avoid any DC offsets from the electrodes – we can add high pass filter to the instrumentation amplifier. Since there are no muscles at your elbow to generate electric p potentials.• • • • • We can build an EMG circuit using an instrumentation amplifier with opamps such as LM741 and LM324(BJT devices input currents of 100‐500 nA) or TL084 device with JFETs – input currents <0. close to each other but not overlapping). bicep.