I decided to try the instrumentation amplifier again, doing an electromyograph, rather than an electrocardiogram. The principle is the same, but you get the signal from a muscle other than the heart. I chose to use my left biceps, because it can be conveniently close to the breadboard to the left of my laptop.
The circuit is a very simple one, taking advantage of the off-the-shelf ina126 instrumentation amplifier:
With my arm relaxed, I get about 40mV of noise at the amplifier—mainly 60Hz and 1.080 MHz, the frequency of the strongest local AM radio station. I can get rid of the AM radio signal by putting a 4.7 µF capacitor on the output of the instrumentation amplifier, but this does not remove the 60Hz noise.When I tense the biceps, I get low frequency signals—spikes about 10 msec long and about 0.2v high. Given that the gain is about a hundred, the signals from the electrodes must be about 2 mV. I ran off of batteries initially, to avoid problems with noise from the wall wart, but after I got it working, I tried a 5v wall wart (with a 100Ω resistor and 470µF capacitor, to remove the worst of the ripple from the power supply), and it worked fine also.
Having had this success with EMG signals, I decides to try electrocardiogram signals also (ECG or EKG). Most of the info on the web is about 12-lead EKG systems, but I want to use only 3 wires. I found a nice reference intended for nurses that describes where to place electrodes for 3-lead, 5-lead, and 12-lead systems. I tried using the MCL1 setup in Figure 4.3
Interestingly, the ECG Primer refers to any pair of electrodes as a “lead”, which does not match usual electrical engineering usage (in which a “lead” is a wire and corresponds to a single electrode).
With the MCL1 placement of the electrodes (in the hollow below the left shoulder and between the ribs just to the right of the sternum), I was able to get EKG signals. The depolarization pulses are about 50mV (which means about 500 µV at the electrodes), but the background noise is still bothersome. I added a very simple RC low-pass filter between Vout and the oscilloscope (3.9kΩ and 4.7µF), and reduced the noise to less than 5mV, while still getting 40–50mV depolarization peaks. Replacing Rgain with a 100Ω resistor increase the spikes to about 300 mV.
With a little more amplification and a slightly lower-pass filter, this should be good enough to feed into an Arduino ADC, and test out the data logging program my son has been writing.
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