I'm using an AD549L in a transimpedance amplifier configuration. I'm trying to measure currents down to 1 pA.
I'm using a 1 Gig ohm feedback resistor. The pc board is an 8 layer board with 2 ground planes. The non-inverting input is at circuit common. I have a guard ring around the circuitry associated with Pin 2.
I am using very low noise plus and minus 15 volt power supplies.
I'm seeing very little high frequency noise, However, I'm getting about 3 mV of 60 Hz interference at the output.
Does anyone have suggestions for handling this?