[AudioI] ground loop on octocard input

[Omar Marcillo]

Hi,

I am trying to connect a geophone (a seismic passive sensor ) to the octocard input. My geophone is a passive sensor with two terminals with variable voltage, with very low voltage signal (+/- 10mV) so I added an instrumentation amplifier  (AD620 board) that is powered with an 9V external battery. If the pi is connected to AC power I get a 60 Hz signal that masks everythign (I think it is ground loop somewhere). If I use a power source that is not connected to AC (a small portable charger) the signals looks fine.

I tried adding a small DC to the output of the instrumentation amplifier board but I still have the same ground loop problem.

about the amplification board, it takes 5V voltage and creates -5V to power the circuit, the board has -/+ inputs and varaible gain,  I connect one terminal of the geophone to + input  and the other to GND. I add 2V offset and  feed the ground and biased output to the octocard.

Any suggestion or ideas to fix the ground loop will be appreciated.

Omar



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