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Op Amps (AD620, AD820) cutting out during Electrophysiology experiment

Question asked by doyle.benj on Apr 29, 2015
Latest reply on May 4, 2015 by doyle.benj

I have a small differential amplifier board that uses both an AD620 and AD820 op amp.  I have the total gain set to x5000, and have been testing the amplifier by touching the two input electrodes onto my tongue to get a simple EMG recording.


I've discovered that whenever I use two different wires (a fine silver wire electrode and an alligator clip), I get a bit of signal for a second or two before then the amplifier completely cuts out.  Touching the electrodes together makes the amplifier active again.


What sort of issues could be causing the amplifier board to cut out?  I know electrodes are supposed to be close in resistance, but I've only measured a difference of ~1 Ohm between the wires which I'm told should be negligible.  Also, if resistance in electrodes was the issue I would expect the signal to be more saturated, not cut out completely.  The tongue EMG signal is never cut off or shunted before the amplifier cuts out, it just diminishes until the amplifier isn't responding any more.  I do not have this issue when I use two of the same wires, so it seems to be an issue stemming from the difference in electrodes (using silver wire + alligator clip versus both silver wire or both alligator clip).


Does anyone have any advice?  I'm mostly curious as to what causes an amplifier to cut out under these sort of circumstances, and only respond once the electrodes are touched together.


Please let me know if you need any additional details.