I built and tested an EKG circuit today using the protoboard that the students will use on Thursday, to make sure that there are no probems.
I decided to try out a circuit that uses active feedback for the common-mode signal. It turned out to be a bust, having higher noise than just connecting Vref to the body electrode (I provided both options for wiring up the electrodes). I’ve not yet figured out whether the problem is with the details of the design I used, or whether using active feedback to cancel the common-mode signal is a poor idea, despite its appearing in almost all the circuits I’ve found on-line. The instrumentation amp chip has a common-mode rejection ratio of 94dB (1/50,000), so the common-mode noise would have to be huge to be make any difference to the output. As long as the two electrode signals stay well within the power rails of the instrumentation amp, there doesn’t seem to be much point to using feedback to cancel the common mode signal.
I’m also wondering now about some of the other circuit choices that I saw in almost all on-line circuits for EKGs, like putting the high-pass filters to eliminate the DC bias after the instrumentation amp. Maybe after the quarter is over I’ll try another EKG circuit with the high-pass filters before the instrumentation amp, to see if that causes any problems.
I’m glad that I did not recommend active feedback to cancel the common-mode signal to the students, so that they’ll be implementing simpler designs that are likely better.
I still have several things to do today:
- grading redone lab reports—I’d like to get half the stack done.
- trying to catch up in physics with my son.
- designing the T-shirt for the class.
I also have to check with tomorrow’s guest lecturer, to see if she plans to fill the full 70-minute period—if not, I should fill in with a demo of the EKG board I wired up today.