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.