I am trying to design a picoammeter with AD549K, in Integration mode opamp. I have used a reed relay to manually discharge the capacitor through a 100K resistor. a null pot is used in the in noninverting input. bypass capacitor of 100nf is used at power supply and input 2and 3 connected with 10Mresisitor.
What I find is after I reset the capacitor there is a drift even after nulling. it is about 2mV per and settles at 30 mV after 15 hours and then the output drifts up and down about 2mV. output is grounded via 100K resistor and input is given directly to a DPM.
I am not able to understand the drift every time I reset, and expect it to be much lower given the drift and offset specs of the AD549K