Monday, July 18, 2011

That's my heart, people.

Some time ago I bought an excellent toy - Xprotolab mini-scope/logic analyzer by Gabriel Anzziani.
I touched it signal legs, and immediately saw that it picks up 60Hz through my body. I looked around and found couple of simple and inexpensive ECG projects, which definitely could use such a nice mini-scope. In this case the task is just the opposite - reduce the mains pick up as much as possible, because cardio signals are very weak - up to 5mV (1mV is more typical). These schemes I found use classical 3 Op Amp setup to enhance so called Common Mode Reject Ratio (CMRR). The Op Amp I used is TI LM324 - probably one of the cheapest choices. I did not use instrumentation amplifier - I did not have one in my parts box, and I thought it would be much funnier to make one. Admittedly, you probably will not achieve such spectacular low noise performance, but at least there is a space for experimentation.
I tried several schematics (with middle voltage right leg and with active negative feedback right leg), different Op Amps (mentioned very cheap LM324N, and low noise and 8 times more expensive MCP604) and various setups - on a solderless breadboard and on a soldered proto board. The results are still puzzling me - theoretically better schematics is not always better on practice. I did not set a formal experiment with only one variable changing, but active right leg driver on a soldered proto board was so noisy that I still prefer breadboarded middle voltage right leg schematics. The presented shots are actually the best I achieved, more typical result looks like this:
I used pennies soldered to thin coaxial cable as my electrodes. They are not on these photos, unfortunately. When I have a chance I'll make a photo of the electrodes also.
There are problems with different impedance of the electrode contacts, mentioned in the literature. I solved (better word "avoided") this by using decoupling high pass filter just before the oscilloscope.
This is still work in progress, and I plan to move it to the "real" PCB (made as usual by excellent Laen's PCB group order service).

If you need further links to learn a bit more about ECG and DIY efforts, is a good place to start.
I currently study TI's Getting the most out of your instrumentation amplifier design which touches problems which plague my current design - large electrode DC offset and high AC common mode noise.

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