I have been working on a small differential amplifier that will ultimately be used for whole nerve recording from lobster connectives. Throughout the process I've been testing the amplifier using tongue EMG signals (taking the two input electrodes, putting them on my tongue).
Recently I discovered that the amplifier only seems to work properly when I use two identical electrodes. In this case either two fine silver wire electrodes, or two wired alligator clips.
When identical electrodes, I get an amplified tongue EMG signal and everything appears to work great. BUT, when I mix different wires for electrodes (using a fine silver wire and an alligator clip), I briefly get the signal before the amplifier goes silent. Once the amplifier goes silent, the individual electrodes stop reacting to touch unless I touch the wires together.
I initially thought electrode wire resistance may have been the issue (the silver wire electrodes seem to have a resistance of ~1 Ohm, while the alligator clips register at 0 Ohms), but I've been told a difference that small should be negligible.
Is it possible that the differences in surface area of what is touching my tongue (larger piece of metal vs. fine wire) is causing the problem? The lab I work in traditionally uses alligator clips as grounds, so I hadn't expected this to be an issue.
In general I'm curious how differences between the electrodes being used could cause an amplifier to go silent and/or not function properly. I appreciate any advice someone has on the matter.
Best Answer
When you have two different metals, you are making a battery - also called galvanic cell: http://en.wikipedia.org/wiki/Galvanic_cell
If your electrodes are silver and zinc (assuming alligator clips a zinc covered), we can get the resulting voltage using "standard electrode potential" table. In your case, the amplifier gets 0.7996 - (-0.7618) = almost 1.5 volts at input. Given that it is designed for millivolts, it overloads and shuts down.