I need to pass a signal from a geophone (essentially a moving coil in a magnetic field with ac output) through an amplifier to a processor whose A/D inputs are 3.3 volts tolerant. As far as I'm aware, there are no zener diodes I could place back to back in front of any op amp between the geophone and the A/D circuitry which can limit the input to the op amp (or instrumentation amp) to 3.3 volts max (I would be adding 1.6 volts dc offset at the op amp stage).
So my first question is: what kind of clipping circuit could I assemble to clip the input the to the amplifier to say, +/- 3.3 volts, if any ?
I have also thought about introducing an INA 121 with a single-sided 3.3 volt supply as the input stage (again with a 1.6V offset), relying (if I am correct) on its high input voltage protection features to clip its output to the ADC to the 0 to 3.3 volt range in the presence of possible input voltages exceeding +/- 3.3 volts. I don't know if this would work, so I'd appreciate feedback on whether this is another viable option.
Finally, if I do use the instrumentation amplifier as the input to the ADC circuitry, given the nature of the geophone output, would I need to provide a high resistance path to ground for both INA inputs as input protection for the instrumentation amplifier ?
I hope I haven't muddled this question too much !
Thanks !