This is an outline of the subjects that will be touched upon during this presentation.

Transcription

1 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 heart

2 This is an outline of the subjects that will be touched upon during this presentation. 2

3 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

4 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 (E RP ). This potential is relatively constant in striated muscle cells with a potential of about -50 to -100mV. Nerve cells show a similar range 2. 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 cells 3. 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 state 4. 2,3 Biopotentials and Ionic currents, Answers.com 4 Welch Allyn Protocol Clinical Support 4

5 The human heart cutaway shown in the diagram exposes the four chambers the right atrium, right ventricle, left atrium and left ventricle. The function of the right side of the heart is to deliver deoxygenated blood from the body to the lungs. The function of the left side of the heart is to deliver oxygenated blood from the lungs to the body. The cardiac cycle consists of two phases - the Systole and Diastole. Although these phases will not be further explored here, 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 individual waves associated with each portion of the heart s function sequence combine to produce the ECG waveform monitored on the body surface. The resulting ECG waveform is shown at the bottom of the waveform diagram. 5

6 The Cardiac Conduction System is the name given to the heart s electrical conduction system. It controls the contraction of the heart. The SA node is often referred to as the heart s pacemaker. It generates the electrical impulse and sets the pace of the heart. 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 cells 5. Each wave and interval appear on the ECG display as the result of a particular electrical function of the heart 6. These individual functions are observed on the ECG display and labeled as P,Q,R,S,T and U, corresponding to the particular heart interval. Cardiologist assess the functionality and gross condition of the heart muscle from these different segments of the ECG waveform. 5 Welch Allyn Protocol Clinical Support 6

7 The electrodes are transducers that detect the minute ionic currents associated with the biopotenials. They can be thought of as an ion to electron converter. This conversion allows the electrical currents to be amplified and conditioned by external circuitry. 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. When placed against the skin chloride is exchanged from the skin to the electrode, and silver is exchanged from the electrode to the skin. In doing so there is a free two-way exchange of ions, so no double layer is formed at the surfaces. 7

8 For ECG applications three or more electrodes are placed on the body. The diagram shows one of the most commonly used connections between the body and ECG equipment. One electrode is placed on each arm, while a third is placed on the right leg. The arm electrodes are intended to detect the minute differential biopotentials associated with the heart s activity. The third electrode, connected to the right leg, provides a common mode drive voltage. This third electrode serves two purposes; first, it may be used to impose a common DC level on the patient. An example would the +2.5V shown in the diagram which provides DC biasing, to the two differential sensors. And second, it provides common-mode signal feedback to aid in common-mode noise cancellation. The latter is very important because common-mode noise may be hundreds to thousands of times greater than the detected ECG biopotentials. From the arm electrodes, the tiny differential signals are coupled to an instrumentation amplifier (INA) for the first level of amplification. 8

9 The ECG Einthoven triangle dates back to the earliest days of electrocardiography and provides the basis for electrode placement. The equilateral triangle is formed by raising the arms and positioning the points on the limbs equidistant. Either leg may be used for a lead connection and the other leg then becomes the reference to which the other limbs are referenced. 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, homogenous volume conductor (at the center of a homogeneous sphere representing the torso). With these assumptions, 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 triangle 7. Einthoven s Law provides the voltage relationships between the leads. With time this was perfected into the more commonly used connections today, which may include as many as 12 electrodes. This allows the heart biopotential activity to be monitored through many different planes. 7 buttler.cc.tut.fi 9

10 When the ECG electrode is physically contacted with the body a complex electrical model is created. The model includes the body biopotential and resistance, skin contact resistances and a parallel resistance and capacitance associated with the probe. The right-hand diagram shows how each of these subcircuits interconnect to create an overall equivalent circuit. The electrode itself can be modeled as a 1μF capacitor in parallel with a 10kΩ resistor. The 1μF capacitor in conjunction with the 1kΩ skin resistor inserts a simple RC, low-pass filter function in the ECG path to the amplifier. Its cutoff frequency is: f C = 1/(2πRC) For the values shown f C is 159Hz. Although this may appear to be a low cutoff frequency it is sufficient to pass the frequency components associated with the ECG. For example, with a heartbeat rate of 60bpm, the fundamental frequency is 1Hz. Even the fast R-wave potion with a duration of about 0.03 seconds at 60bpm, has a fundamental frequency of about 33Hz. But because this is a quickly ramping up and down pulse, a greater harmonic bandwidth is needed. The 159Hz satisfies the requirement for even shorter R-waves. The bandwidth limited electrode/skin interface helps reduce the circuit s response to unwanted higher frequency electrical interference. 10

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, 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

12 Here is an example of a normal ECG chart recoding for a heartbeat of 62bpm. The rate can be determined from the rate of R wave occurrences. The P,Q,R,S,T,and U portions of the ECG are labeled for convenience. A 1mV calibration pulse is posted for comparison and has an amplitude of 500uV per vertical division. Note that the R wave pulse has an amplitude about equal to 1mV, while the others are much smaller. Any electrical interference can easily mask these important portions of the waveform. 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). 12

13 These displays provide examples of irregular ECG tracings caused by both internal and external factors. Muscle shaking is an example of an irregularity caused by internal muscle tremors, referred to as a somatic tremor. The gradual baseline drift discussed in the previous slide is due to charging of the high-pass, coupling circuit and/or changes in the ionic current levels. So this characteristic is connected with the equipment rather an internal bodily function. Sixty hertz AC pick-up is the result of induced electric field energy present in the vicinity of the ECG equipment; often received by the electrodes or electrode leads. Not only 60Hz, 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. 13

14 The DAS/ECG demo board functions as a self-contained heart-rate monitor providing a visual, audible, and digital indication of heart rate. The three ECG electrodes are built in and conveniently accessed off one end of the board. If necessary, external leads and contacts may be connected to the board as well. The demo board contacts provide the input for the differential ECG signals via the right and left thumbs. 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. A variety of different sensors may be directly interfaced to the board making possible other types of medical-related and non-medical measurements. 14

15 The biopotentials detected at the body surface by the ECG are highly attenuated relative to their point of origination. Often, the amplitude is on the order of a few hundred microvolts (μv). Other body signals such as brain waves may have amplitudes a fraction of this level. Very high voltage gain (V/V) is required to bring these minute signals to a level where signal processing may be reliably applied. This is accomplished through the use of high performance instrumentation and operational amplifiers on the demo board. Additionally, on-board circuitry is provided so that the amplifiers may be configured for sensor interfacing and filtering functions. These will be discussed in more detail a little later. Once the low-level signals are amplified the output is applied to the cardiotach circuit. The amplified waveform is passed through a 150μV peak-to-peak threshold detector. If the amplitude of the waveform is sufficient, it will trigger a one-shot multivibrator. The one-shot output may be counted, used to pulse an LED, to key a 1kHz burst oscillator. The DAS/ECG board also provides a probe point where the amplified ECG waveform may be observed with an oscilloscope. 15

16 Moving to the next level of circuit complexity reveals the IC building blocks used in the demo board: 1. U1, U2, U3 Input instrumentation amplifier and gain stages. 2. U4, U5, U6 Peak-to-peak detector and monostable multivibrator circuit. 3. U7 Low dropout regulator supplies +5V to power the circuitry. 4. U8 Auto power down circuit which is especially useful when using battery power. 5. U9 An uncommitted op-amp useful for providing sensor interface. 6. U10 Provides a stable +2.5V reference voltage for mid-scale commonmode biasing. 7. U11 An optional socket for the OPT101 Monolithic Photodiode/Single- Supply Transimpedance Amplifier. 16

17 Here the analog front-end has been separated from the remaining circuits. A precision, rail-to-rail INA326 instrumentation amplifier is at the front end providing low offset (<100uV), a minimum CMRR of 100dB (114dB typ.) and an adjustable gain from less than 0.1V/V to >10,000V/V. The INA326 gain is set to -5V/V in this example. The INA326 is followed by an OPA335 auto-zeroing operational amplifier that features a maximum voltage offset of 5μV, a voltage offset drift of 0.05μV/ºC and maximum operating current of 285μA. Here the OPA335 is set to an inverting gain of -480V/V. A first-order, low-pass filter may be configured within the stage by the addition and selection of a feedback capacitor. Since the board is powered by a single supply, it is necessary to establish a mid-scale voltage. That is accomplished by connecting the +2.5V reference voltage as a common-mode voltage to both the INA326 and OPA335. The overall gain is the product of the individual gains of the two stages; (-5V/V)(-480V/V), or 2400V/V. A 1mV P-P input is amplified to 4.8V P-P, centered about +2.5V. The high common-mode rejection of the INA326 rejects the 60Hz and other common-mode interference picked up by the electrodes. Likewise, common-mode DC voltage is rejected by the amplifier. 17

18 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, R G is split into two equal resistors. Any DC common mode voltage present at the two inputs will shift the DC level at the resistor junction. This voltage is buffered by A1, and then applied to A2 which has an inverting gain of minus 19.5V/V. The inversion is important because it will be used to counter a DC common-mode, electrode potential on the electrodes. A +2.5V common mode voltage is applied to A2 s non-inverting input via a resistive divider. The +2.5V voltage is the mid-scale voltage level for all the analog circuitry. 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 +2.5V reference voltage. This auto-zero feature keeps the DC level constant which is necessary for a stable ECG display baseline. 18

19 This very busy circuit portion of the DAS/ECG circuit diagram provides the remainder of the analog front-end circuit. The INA326 circuit includes a provision for DC or AC coupling. AC coupling removes the DC electrode offset. This offset is taken care of using a DC restorer circuit that will be discussed in the next slide. The AC high-pass frequency response is selected at 0.05Hz, 0.5Hz, or 2.0Hz using a resistor-jumper provision. The INA326 is followed by ½ of a OPA2335, gain stage. The gain is set by selecting an input resistor via a jumper. Additionally, a low pass filter function is provided by this stage. Its cutoff frequency is set by connecting the appropriate capacitor into the feedback path with another jumper. 19

20 The INA326 output voltage may be referenced to a voltage applied to the reference pin, pin 5. If 0V is applied to the non-inverting input, then the output will be referenced to zero volts and the swing can move up from 0V. If the reference pin is set to +2.5V, then the output can swing above and below +2.5V within the output bounds. This reference voltage is sometimes referred to as a pedestal voltage, because it raises the output up from ground (0V). The integrator shown in the schematic is referenced to +2.5V on the noninverting input. At DC the integrator s gain is very large and any deviation from +2.5V seen at the inverting input as the result of a common-mode DC voltage on the INA s inputs - will result in a large DC voltage at the output. This DC voltage is then applied to the INA326 reference input in such a manner as to drive the INA s output back to +2.5V. As the frequency is increased the gain of the integrator rapidly falls off. Thus, AC signals having a frequency above the integrator s -3dB cutoff frequency have virtually no affect on the reference voltage applied to the INA. The net result is a DC restorer circuit that compensates for a DC commonmode voltage, such as may be present with the electrodes. It also provides a high-pass transfer characteristic with a cutoff frequency that is a function of the integrator RC constant. This results in a circuit equivalent to a capacitively coupled amplifier, but without any capacitors directly in the signal path. High quality, high capacitance capacitors are often large and costly and are avoided using this technique. 20

21 The output from the amplifier section may be sampled and processed by external circuitry, or the onboard facilities provided on the DAS/ECG demo board may be utilized. The amplified ECG waveform is passed to a differentiator and peak-to-peak detector that produces pulses at the heartbeat rate. These pulses trigger a one-shot multivibrator which stretches the pulses to a uniform time duration. 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 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, or be counted by a BPM meter. 21

22 The first section of U4 is connected as an absolute value amplifier, producing a positive going replication of the positive or negative going ECG wave. U4 s second section is a peak detector where the peak value of the ECG waveform is stored on C11 (0.1μF). The circuit has a lower threshold of about ±150μV. U5 s first section buffers the peak detector output, while the second section amplifies and squares up the waveform. The input signal is amplified to a level such that the second stage output runs rail-to-rail, nearly 0 to 5V. This is ideal for triggering the first TLC556 section, which is configured as a 100ms, one-shot. The TLC556 s second section is arranged as a 1kHz, astable multivibrator, keyed by the preceding one-shot stage. 22

23 The DAS/ECG board may be powered by either a 9V alkaline battery, or an external supply. Current varies from <1mA to about 15mA depending on the board s operating mode. A TPS71550, 5V low-drop out (LDO) regulator provides the supply voltage for the board. Its +5V regulator output is supplied to the TLV3491 comparator supply pin. An RC timing circuit is connected from the +5V supply line to the inverting input, while a +4.5V voltage is developed at the non-inverting input. The voltage at the RC node is initially zero and eventually charges to a voltage exceeding +4.5V, in about 40 minutes. When that voltage is below +4.5V the comparator output is high, at about +4.8V. Then supply current is delivered to the DAS/ECG circuits through the comparator s output stage. Once the comparator RC input voltage crosses +4.5V, the output voltage drops to nearly zero volts removing power from the DAS/ECG circuits. A small amount of hysteresis is added to the comparator to improve noise immunity at the threshold. 23

24 The DAS/ECG demo board has a number of features that make it easy to use for testing circuit ideas and experimentation. In addition to the EGC cardiotachometer application, it may be used for other portable applications where high voltage gain and high common-mode rejection are required. 24

25 This image displays the top side of the DAS/ECG cardiotachometer board. The left arm (LA) and right arm (RA) electrodes are located on the end of the board, while the right finger drive electrode is placed underneath the board. 25

26 This shows some of the user selectable functions on the board. The gain and low-pass and high-pass cut-off frequencies care established using jumpers and can be changed as needed. There is an ON/OFF switch and start switch for the 40 minute, power ON timer function. The speaker, LED and supply connections are also shown. 26

27 Here s a more detailed layout showing the location of the analog circuits and the tachometer circuits that follow them. 27

28 The back side of the ECG/DAS board contains the important common-mode drive pad. This is typically biased at +2.5V when powered by a +5V supply. It is important from the standpoint that it sources complementary phase, AC common-mode signals back to the body. These add to the AC common-mode signals on the body and help in the cancellation of these unwanted signals. 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. 28

29 Here, John Brown the DAS/ECG demo board developer, 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. The board is easier to steady and maintain an even contact while sitting, so do so if possible. 29

30 The cardiotachometer amplifier circuit is capable of detecting a biopotential of about 200uVp, in the presence common-mode AC interference with an amplitude of about 2Vp. Therefore, 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, but also analytical and scientific instrumentation, industrial monitoring, and some automotive and industrial sensor applications as well. 30

31 Some other applications will be explored now to show the versatility of the DAS/ECG design. This application will demonstrate how a bridge transducer can be directly interfaced with the DAS/ECG board. A puffing tube in conjunction with a bridge transducer will be used to detect a change in gas pressure. Puffing tubes find application in industrial gas lines and valves where the gas pressure and flow characteristics require monitoring. Medical uses include applications where the tube serves to direct the breath pressure of a user to the bridge transducer. The magnitude of the breath pressure can then be used to control a medical assist apparatus such as a wheelchair. 31

32 Silicon Microsystems manufactures a thin film pressure bridge transducer that interfaces with a air lines, such as the puffing tube. The bridge connects directly to each INA326 differential input. Current for the bridge transducer may be supplied by the DAS/ECG, on-board +5V reference. The transducer s sensitivity in this application results in a differential voltage of about 0.16 to 2.4mVp-p. A nominal value of 1.5mV p-p is used for illustrative purposes. The gain is set to 2000V/V and this produces an output voltage of 2.5VDC ±1.5V P for a range of 1.0 to 4.0V. If the differential voltage measured 2.4mV p-p, then the output range would span from 0.2V to 4.8V. The 2.5V center voltage is from the pedestal voltage applied to the INA326 reference pin. The bridge transducer may have an offset, or imbalance between the two sides as great as 50mV. Any input common-mode DC voltage and bridge offset voltage will be auto-zeroed by U3 as previously discussed. 32

33 The ECG/DAS board bridge sensor input is shown coupled to a mechanical pressure gauge in the puffing pressure bridge application. The output phase between the mechanical gauge and the DAS/ECG board are set the same so that both result in an upscale reading. The sensitivity of the mechanical gauge is established at Δ5mm Hg for a psi pressure change, while the bridge produces a 0.75mV pk change for the same input. As mentioned, the DAS/ECG board gain has been set to 2000V/V. 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. 33

34 Here s the actual oscilloscope display for the DAS/ECG board output with a simulated puffing input (upper trace). The input puffing rate is a much faster 0.2s than a human can deliver, but illustrates the ability of the board to detect and amplifier the bridge sensor output even at this higher rate. The output swings approximately 1.5V P-P, and is centered about the +2.5V 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. 34

35 This is pressure bridge application where DC or very low frequency signals require monitoring. In this example the pressure change associated within a squeezing a tube will be observed and measured. The DAS/ECG will now be configured to provide DC coupling - versus the AC coupling used in the previous applications. Now the bridge offset must be taken into account to assure the DAS/ECG board output does not saturate. 35

36 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. That could result in an voltage level that would exceed the amplifier s minimum or maximum output level. Therefore, one must be cognizant of a sensor s DC offset and the direction it will drive the output. 36

37 The auto-zero feature has been disabled and the INA326 reference pin is connected to zero volts. Notice that the overall gain has been reduced substantially from its previous AC setting of 2000V/V, down to 100V/V. 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. For this example the bridge offset is 43mV and when multiplied 100x the output is about +4.3V, placing the output close to the positive rail. However, 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. 37

38 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 same pressure bridge transducer is used here as earlier, but the bridge circuit connections have been changed to assure the amplifiers operate within their linear range. Gain resistors have been changed and the low-pass bandwidth jumper set as needed. Bridge bias is provided by the on-board TPS71550 LDO regulator. It has been observed that the particular bridge used for this example resulted in a differential offset of 43mV. The device is specified with a maximum offset of ±50mV. If the offset was as high as 50mV, then the output would be up against the rail. An alternative to lowering the gain would be to reduce the voltage applied to the bridge. A resistive divider in located on the board and divides the +5V down to +2.5V. Since U2, the dual OPA2335 (or OPA2336) is not used in this application, it can easily be configured as a unity-gain buffer. The output is then used to bias the bridge, but note that doing so does reduce the bridge output by 50%. 38

39 The output response of the DC coupled DAS/ECG board during a squeeze is displayed in this oscilloscope image. The upper trace is the response with a +5V bridge excitation, while directly below it is the response with a +2.5V bridge excitation. Notice that the amplitude change during the squeezing event is about half with +2.5V excitation as compared to that with +5V excitation. This is as expected. Also observe that the event had a duration of about 5 seconds. This translates to a frequency of about 0.2Hz. This is still within the board s AC passband when the high-pass filter is set to a cut-off frequency such as 0.05Hz. Setting the board for DC coupling may be the best option for use at even lower frequencies. Some examples of low frequency uses are geophysical, mechanical and industrial process control applications. 39

40 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; an instrument used for measuring changes in volume within an organ, body members or the whole body. An LED is positioned so that its light output is directed through the finger. 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. Notice the connection of the photo diode and the 3 series-connected, 499k resistors across the photodiode. The cathode end of the diode is referenced to +2.5V. This same common-mode voltage appears at both of the INA326 inputs through the resistors. When light shines on the photodiode, photo generated current flows through the diode and through the 3 resistors. One 499k resistor is connected directly across the INA326 differential inputs. 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. 40

41 The DAS/ECG board is shown outfitted with the monolithic OPT101 photodiode/transimpedance amplifier. The three, 499k bias resistors have been added to the board. An overall gain of 6kV/V is used with the application and the bandwidth has been set from 2Hz to 17Hz. 41

42 This oscilloscope display provides the output traces when the DAS/ECG is connected in the plethysmograph application. The upper trace tracks the changing blood volume within the finger indicating the blood pressure level. A 700mV P-P output amplitude results when the overall gain is set to 6kV/V. The middle two traces are the 1 st timer s input and output pulses. The output pulse corresponds with the peak blood pressure. This pulse is used to key the output burst oscillator. 42

43 In summary, the DAS/ECG board is useful for demonstrating the ability of highperformance analog circuits in low signal level, front-end applications. The board s versatility allows one to experiment, evaluate and optimize circuit performance in medical and non-medical sensor applications. 43