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AD620 and AD820, underwater 60Hz noise ("clicking" or clipping)

I have two differential amplifier boards that each use AD620 and AD820 op amps.  I'm using the boards for electrophysiology recordings from lobsters.  The boards are in a waterproof case and attached to the lobster's back to allow the animal to move around a tank.  Outside of water the boards work fine when taking bipolar EMG recordings with both electrodes placed in target muscle tissue.  However, once the lobster and the case are submerged underwater the amplifiers pick up significantly more 60Hz noise that drowns out the signal.  This noise presents itself as both sinusoidal 60Hz noise and, once fully submerged, a "clicking" or clipping 60Hz noise (see attached image).

Is this sort of noise unavoidable due to the conductive nature of salt water?  Also, has anyone encountered 60Hz noise that looks like this before?  Please let me know if you have any advice.

  • Hi Harry,

     

    Here's the schematic.  A few components have been switched out:

    C6 is 0.270uF

    C11 is 560pF

    C1 and C3 are 5600pF

    C2 is 0.056uF

    R3 is 1k Ohm

    These changes were made to give the circuit a bandwidth of approximately 200-1000Hz, and a total gain of x5000.  The amplifier receives power from a LiPo battery, providing the +/- 12V of power.  Two boards are stacked on top of each other to create two channels.  The two channels are transmitted through a basic A/V FM radio transmitter as left and right audio with a video component, then sent from the receiver to the computer and recorded on the computer as a video file.

     

  • The board is in a waterproof case.  I hadn't tried adding any shielding because of how tight the space is.  I'll test lining the inside using aluminum foil since that will fit easiest, let me know if you would suggest any other materials.

    Thanks for your help,

    Ben