Electronic Circuits and Projects Forum

Originally Posted by shosh

Is there a low voltage signal emitted by your circulatory system, or do you measure the resistance, therefore you need a voltage source.

There is low voltage signal. Do not try to apply voltage or current to measure human body resistance.. that is lethal.
Google goes a long way on this one: http://www.swharden.com/blog/2009-08...-on-the-cheap/

I've done a few and have had the best luck with three electrodes: one left, one right, and one ground in between.

If memory is serving me correctly: an instrumentation amp with a gain of 50, a band pass filter with 12dB/octave roll-off outside of 45-290 beats per minute, and a final output amp/buffer with a gain of 10. Strong hearts can send the output from positive saturation to negative saturation with something like an LM324 connected to 5V supply. Rail to rail op-amps, lower gain, or higher supply voltages can be used to eliminate most of the clipping but that is unnecessary if you're just detecting and counting heart beats. Weaker hearts will end up with about 1.5 Volt peak to peak signals.

Three properly placed electrodes allows you to more effectively cancel out common mode noise and concentrate on the differential mode signal. The result is a S/N ratio that's a few decibels higher and with these circuits those few decibels can be the difference between working well and not working at all. Even with the third electrode, the processor will still be analyzing a noisy signal.

Two electrodes "might" get you a usable ECG signal, but the third "common" electrode makes the differential inputs of the instrumentation amp do what it's supposed to do...sense small signals in the presence of large common-mode signals...60Hz power...muscle artifact...RF. "12-lead diagnostic" ECGs can use up to 10 electrodes, but ECG "monitoring" uses only three or four. The combinations of three or four or 10 electrodes are still only using the two differential inputs and one common of the instrumentation amp, through a switch and resistive matrix.

Yes, if it is a multi-channel diagnostic ECG machine, they will have multiple differential amps so they can capture several different electrode combinations simultaneously.

Ken

You probably cannot use opamps and resistors to make an instrumentation amplifier. The instrumentation amplifier IC already has its opamps and resistors very well matched. Most datasheets show an ECG circuit with a common-mode feed into the right leg to cancel common-mode signals.