UNIVERSITY OF N.S.W. SCHOOL OF ELECTRICAL ENGINEERING AND TELECOMMUNICATIONS ELEC4623 / ELEC9734 BIOMEDICAL ENGINEERING LABORATORY 3: DESIGN, TESTING AND ANALYSIS OF A HIGH QUALITY ISOLATED BIOPOTENTIAL AMPLIFIERS INTRODUCTION: In this laboratory you will be analysing an isolated ecg preamplifier design and testing its performance under a variety of conditions. Some signal processing tasks will have direct applicability to this laboratory. You will be expected to have carried out a preliminary analysis of the circuit prior to coming to the laboratory. The second part of this laboratory will involve some basic signal processing of the ecg signal and will take place during the following tutorial session. LABORATORY EQUIPMENT AVAILABLE FOR THIS LABORATORY We will use MyDAQ for data acquisition and display at 500 samples/sec. The isolated ecg preamplifier. This is a newly developed card which is powered from approximately 7.5 9 volts from a laboratory power supply and sits within a small plastic enclosre. The input to the device is a patient cable with three leads terminated with 4mm banana plugs. The output is a BNC connector that may be connected to a supplementary antialising filter and / or to the A/D connector box. A hand held multimeter. As this is battery powered, it may also be used to safely probe around the isolated part of the preamplifier circuit. You should use this instrument to record all ac rms measurements as it is quite accurate within the frequency range relevant for this experiment. A medical grade isolation transformer. You can use this to isolate any instrument you may wish to attach to the front end patient circuit. In our experiment you MUST isolate the Signal Generator when you sweep the frequency response of the circuit from less than 0.05Hz to 200Hz A signal generator. This will be used primarily to generate a Bode plot of the circuit frequency response over the range 0.05Hz to 200Hz. A small transformer providing approximately 3.5Vac. This will be used as the test- signal for most of your measurements including the tuning of the CMRR. As the output is at mains frequency, the signal will be stable. The ecg unit has a built in resistive divider which gives 1000: 1 division, with a centre tap. A capacitive divider is also provided for testing the performance of the Driven Right Leg. Note that these banana sockets are NOT connected to any signal internally. - 1 -
RUNNING THE MyDAQ PROGRAM To run the program, simply double click on the MyDAQ icon on the desktop of Windows 2000. You will see the main window of the program MyDAQ.vi. This loads a simple program to allow up to four channels of Analogue Input (CH0 to CH3) to be collected, displayed on the screen as shown below. The sampling frequency and number of samples can be set from the Samples/second and Samples/channel textboxes respectively. The Total Duration textbox cannot be modified directly (it is locked), however, it can be modified by changing the sampling frequency and/or the number of samples. To RUN the measurement, click on the Run continuously button ( ) and click the Acquire Data button. At the end of the measurement, a window will appear and ask you to enter a filename. Save the file (without any extension) in the C:\ELEC4623 folder. This file can be loaded into MatLab workplace by using the LOAD command. The file name becomes the name of a matrix variable and each column is for one channel. You should run the program for 20 seconds and with sampling frequency of 500 Hz unless otherwise stated. VIEWING CRO OUTPUT ON COMPUTER SCREEN: To view the CRO output on the computer screen, connect the CRO to the computer via USB and run the program Freeview. Make sure you switch to color off if you want to print. DESCRIPTION OF THE ECG UNIT: The ecg unit supplied has been designed to be patient safe, with the inputs isolated from the output circuits via a medical grade isolated power supply, and the signal isolated via - 2 -
a linear opto-isolator (IL300). The input ecg signal can be amplified either by a high precision instrumentation amplifier OR through a classical instrumentation amplifier formed from three discrete operational amplifiers. The unit is powered by 7.5v from a laboratory power supply and Isolated Ground (GISO), VOFF=5v, VISO+ +13v, VISO- -13v and VG (Virtual Ground)=2.5v, are produced internally. LOCATION OF JUMPERS (Larger view from top of board with BNC connector on right) LK1, LK2, LK3 and LK8 Connect isolated power supplies to circuit LK4 Selects connection to RL input lead LK5 Selects High Pass frequency as 0.05Hz or 0.5Hz LK6 Selects output from A1 or A2 LK7 Selects CM signal from A1 or A2 LK9 Connects input lead to A2 + LK10 Connects input lead to A2 - Figure 3: Jumper positions. Jumpers as shown connect RL to GISO, Inputs to A2 (three op amp instrumentation amplifier), connects A2 to the next stage and selects the CM signal from A2 AIMS: 1. To analyse and test in detail the design of an isolated ecg preamplifier supplied. Specifically, to determine, The differential and common mode gain of the amplifier The frequency response of the preamplifier. 2. To maximise the CMRR by trimming the gain of the differential amplifier 3. To measure the Common Mode Rejection Ratio (CMRR) at 50Hz. 4. To evaluate the performance of the Driven Right Leg in reducing the common mode voltage. 5. To calculate the overall noise figure for the amplifier, referred to the input. - 3 -
6. To investigate the influence of electrode placement on the size and shape of the ecg recorded and to investigate factors which influence the quality of the recording. 7. To record signal (ii) above without the driven right leg circuit in operation and the RL lead connected (i) through a 1MΩ resistor to GISO and (ii) directly to GISO METHOD: NOTE: the front end of the amplifier circuit is isolated from earth for reasons of patient safety and noise reduction. Any measurements taken on the isolated side of the circuit must be done relative to the isolated common (also called isolated ground GISO), either with an oscilloscope isolated from earth or with the battery powered multimeter. Note that if you isloate the CRO you MUST disconnect the USB cable which connects the CRO to the computer. CONNECTING POWER Both the ecg and the filter units are powered from the dual voltage laboratory supply. Use your multimeter to set the output voltages to +8v (positive side) and -8v (negative side). REMEMBER TO CHECK THE VOLTAGES TO MAKE SURE THAT THEIR VALUES ARE CORRECT, as a high voltage may cause damage to the circuitry. TURN THE POWER SUPPLY OFF after checking that voltages are correct. Connect the dual conductor power lead to the ecg unit and the black lead to common terminal and the red lead to the +8v terminal. If you are using the filter unit, connect the three conductor power cable to the filter unit and connect the black lead to common, the red lead to the +8v supply and the blue lead to the -8v supply. ASK THE TUTOR TO CHECK POWER CONNECTIONS BEFORE SWITCHING ON POWER An overall schematic diagram is given in Fig. 1. The component layout is given in Fig. 2. Note in particular the location of the large banana sockets and the the jumper blocks. Familiarise yourself with the components used, the circuit design and the physical layout of the board. Input signals are connected via the input 15 pin D Type connector. The input cables are connected to 4mm banana plugs: BLACK RL Right Leg RED (-) RA Right Arm YELLOW (+) LL Left leg - 4 -
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FIGURE 2: COMPONENT LAYOUT OF ECG UNIT SHOWING LOCATION OF JUMPERS AND CONNECTORS FIGURE 2: COMPONENT LAYOUT OF ECG UNIT SHOWING LOCATION OF JUMPERS AND CONNECTOR - 6 -
CONNECTORS CONNECTOR (NC/NO) LABEL FUNCTION LK1 1-2 (NC) Connect VOFF to circuit LK2 1-2 (NC) Connect VISO+ to circuit LK3 1-2 (NC) Connect GISO to circuit LK4 1-2 3-4 5-6 (NC) Connect RL to Driven Right Leg (DRL) out Connect RL to GISO through 1MΩ Connect RL directly to GISO LK5 1-2 (NC) Increase HP frequency to 0.5Hz LK6 1-2 3-4 (NC) Signal source is high precision instrumentation amplifier (A1) Signal source is three op amp instrumentation amplifier (A2) LK7 1-2 3-4 (NC) Vcm is sourced from A1 Vcm is sourced from A2 LK8 1-2 (NC) Connect VISO- to circuit LK9 1-2 (NC) Connects + input to A2 LK10 1-2 (NC) Connects input to A2 4mm BANANA SOCKETS SK1 Signal Source + polarity SK2 Signal Source + / 1000 SK3 Midpoint of voltage divider SK4 Signal Source - /1000 Sk5 Signal Source polarity SK6 Capacitive divider, +ve polarity SK7 Capacitive divider midpoint reference SK8 Capacitive divider, - ve polarity SK9 Connected to GISO The jumper selections shown as NC are correct for the first part of the experiment. Jumpers may be need to be changed for different parts of the experiment. Test points have been placed in the circuit to facilitate viewing of signals and debugging Nam Function Name Function e TP1 VOFF (5v) TP8 V Output (NOT isolated) TP2 VISO+ (15v nominal) TP9 Vcm of A2 TP3 GISO TP10 Vout of A1 TP4 VISO- (-15v nominal) TP11 Vcm of A2 TP5 RL TP12 Vout of A2 TP6 Driven right leg output TP13 Output of ac coupled amplifier TP7 Shield drive output TP14 Output of opto isolator (NOT isolated) You will be able to change a number of characteristics of the circuit operation, namely; Outputs can be selected either from a precision instrumentation amplifier (INA118) or from a three discrete op-amp instrumentation amplifier CMRR, by adjusting the potentiometer P1 (R40) Overall gain by adjusting P2 (R47) High pass filter characteristic by inserting or removing jumper LK5 so that the high pass characteristic of the circuit is set either by the 3.3MΩ resistor (R17) alone (0.05Hz 3dB cutoff) or the 3.3MΩ and the 330KΩ (R19) resistors in parallel (0.5Hz 3dB cutoff). - 7 -
EXPERIMENTAL PROCEDURES Jumper settings: LK1, LK2, LK3, LK4(5-6), LK5, LK6(3-4), LK7(3-4), LK8, LK9, LK10 ARE ALL CONNECTED SET OPTIMUM CMRR 1. Connect the output of the small transformer across the capacitive divider terminals SK6 and SK8. Connect the black input lead (RL) to SK8. Connect the Red and Yellow input leads to the midpoint SK7. This will supply a common mode input signal of approximately 1.7v rms to the input of A2. Check this with your CRO and your multimeter and make sure they agree (Remember to use AC coupling in the CRO and the multimeter for a sinuosoidal signal generated by the transformer or signal generator). Connect your CRO or multimeter to the BNC connector and obtain the optimum CMRR by adjusting P1 (R40) until the signal is at a minimum. Record the rms common mode signals at the input (V CMin measured between SK7 and SK8) and at the output (V CMout measured at BNC output). Calculate the common mode gain as G cm = V CMout /V CMin. CHECK MAXIMUM GAIN AND SIGNAL QUALITY 2. Connect the output of the small transformer across the 1000:1 divider to SK1 and SK5. Connect the input leads Red to SK2, Black to SK3 and Yellow to SK4. Connect SK3 to the GISO banana socket SK9. This will provide a differential input signal to the biopotential amplifier. 3. Observe the signal quality at the output BNC connector. If the signal appears distorted (clipping or saturation occurs), reduce the gain pot P2 (R47) until the distortion disappears. If the distorted signal perserveres at minimum gain, reduce the input signal from 10v pp to 5v pp. Then increase gain to point just before any distortion becomes evident. 4. Record all voltages and calculate the differential gain as G D = V Dout /V D. The differential input voltage V D is roughly equal to V TRANS /1000 where V TRANS is the transformer voltage. Note that this is at 50Hz, the mains frequency. 5. With the gain setting now fixed, repeat (1) above to recalculate the common mode gain as G cm = V CMout /V Cmin. 6. Power the oscilloscope through the isolation transformer and make sure that the USB cable connecting the CRO to the computer is DISCONNECTED as this would break the isolation. Similarly make sure that no other input is connected. Whilst you are inputting a large common mode mode signal as in (5) above, use the oscilloscope (powered through the isolation transformer) to look at the SHIELD voltage (TP7) and the DRL voltage (TP6). Are the signals as you would expect? 7. Calculate the Common mode rejection ratio (CMRR) in db as 20 log (G D /G CM ). 8. Replace the small transformer with your signal generator (which is connected to the mains through the isolation transformer), and make the same connection as in (2). Using 10Hz as the midband reference (0db) briefly scan the frequency response from 0.1-250Hz (but no need to plot the full frequency response), when LK5 is inserted or not inserted. Record the frequency of the high pass 3dB point in both cases (are they as expected?). 9. Connect the red input lead and the yellow input lead to ecg clamps and place on the left and right wrists. Connect the black input lead to another clamp and position somewhere convenient further up the right arm. Observe the output signal on the oscilloscope, then connect it to the A/D converter and make a recording of the signal for 30 seconds at a sampling rate of 500Hz (If this exceeds the recording capacity of MyDAQ, just record the maximum number of data points permitted). Save the file - 8 -
with.txt extension. If the signals appear noisy, use some electrode gel and observe whether there is a significant improvement in the signal quality. WITH SUBJECT CONNECTED TO GISO THROUGH A 1MΩ RESISTOR 11. When jumper LK4(5-6) is connected, the subject is directly connected to GISO (refer to the full circuit schematic). This will not satisfy regulatory standards, although it won t have any safefy issue since the circuit is isolated. Now remove jumper LK4(5-6) and connect LK4(3-4), so that the subject is connected to GISO through the 1 MΩ resistor. Repeat 9 above, making sure to record on the computer at least one 30 second record at a sampling rate of 500Hz and save the file as before. WITH THE DRIVEN RIGHT LEG (DRL) CIRCUIT CONNECTED TO SUBJECT 12. Remove jumper LK4(3-4) and connect LK4(1-2), so that the subject is connected to GISO through the DRL circuit. Repeat 9 above, making sure to record on the computer at least one 30 second record at a sampling rate of 500Hz and save the file as before. 13. With all three input leads shorted together, record a sample of the internal noise. Does twisting the leads or spreading them apart make any difference? Calculate the rms noise referred to the input. Remember that you previously calculated G D. EFFECT OF DRIVEN RIGHT LEG CIRCUIT 14. Connect SK8 to GISO (SK9). Connect the output of the small transformer across the capacitive divider terminals SK6 and SK8. Connect the Red and Yellow input leads to the midpoint SK7. This will supply a common mode input signal of approximately 1.7v rms to the input of A2. Measure this V CMin voltage with your multimeter (or CRO) relative to GISO. Whilst still measuring the midpoint voltage, also connect the Black input lead to the same midpoint SK7. Record the new value of V CMin. Calculate the db reduction in V Cmin arising from the action of the DRL circuit (you should see a substantial decrease in common mode voltage here after connecting the DRL circuit). OPTIONAL ACTIVITY TO BE ATTEMPTED IF TIME PERMITS COMPARISON OF CIRCUIT PERFORMANCE WHEN PRECISION INSTRUMENTATION AMPLIFIER (INA118) IS USED 15. Remove LK6(3-4) and connect LK6(1-2). Remove LK7(3-4) and connect LK7(1-2). This connects the precision instrumentation amplifier into the circuit. Connect LK4(5-6) to connect the RL to GISO. 16. Using the same methods as above, recalculate the G D and G CM and then the new CMRR. Record a number of ecg samples (30 second duration, 500Hz sampling rate), with RL connected to GISO through a 1MΩ resistor, and then to the DRL circuit. Can you see any improvement? 17. With all three input leads shorted together, record a sample of the internal noise. 18. Using the BNC connector cable supplied, connect the output of the ecg unit to the input of the low pass antialiasing filter. Calculate the addirional gain contributed by the antialising filter at around 10Hz. 19. Briefly plot a Bode diagram of the frequency response above 1 Hz and confirm that the 3 db point is now at approximately 40 Hz. Estimate the rate of attenuation over the octave of 50Hz to 100Hz. 20. Record a number of ecg samples (30 second duration, 500Hz sampling rate). 21. With all three input leads shorted together, record a sample of the internal noise. Does the low pass filter improve the overall noise performance? Don t forget to - 9 -
compensate for any gain that may be contributed by the filter. Calculate the rms noise referred to the input. 22. Using the suction electrodes and gel provided, make a number of recordings with the RL permanently connected at the medial extremity of the rib gage on the right hand side, and the active leads placed at different locations on the chest. SWITCHED CAPACITOR ANTI-ALIASING FILTER U11 (MAX292) is being used as an anti-aliasing filter. Read the attached report on switched capacitor filters. Answer these questions in your lab report. 1) If switched capacitor filters operate in the z - domain and themselves are subject to aliasing, how can they function as anti-aliasing filters? 2) What role do R11 and C19 play? 3) What subcircircuit function do U10A, R22, R24, C21 and C22 form and what does it do? 4) What is the approximate corner frequency of the filter formed by U11? (HINT: The CLK input plays a role in deciding the corner frequency. You need to refer to a graph in the MAX292 datasheet and know how the clock frequency relates to the corner frequency.) WRITING THE REPORT Report all data and comment on the results obtained. Use MATLAB to plot all ecg signals recorded. In your report, try to address the following questions: Explain what you are trying to achieve in part 1-5 and 7. What is the relevance of this procedure in the design of a biopotential amplifier, which is used for recording a bipolar ECG limb lead? By studying the full circuit schematic (next page), explain what shield voltage and DRL voltage you are expecting and compare them with your measurements. Explain how the 3 ways of patient right leg connection via adjusting LK4 (a. directly to GISO, b. via 1MΩ resistor and c. DRL) can affect the 50 Hz common mode noise, and compare them with your observations (i.e. your ECG plots). For part 14, has the DRL circuit achieved its purpose? Answer the switched capacitor filter questions. Please refer to the syllabus for how the lab report should be written. - 10 -