I'm trying to build an analog frontend with a high input impedance (1MOhm). I thought the OPA1662 would be a good candidate, so I built a prototype (see schematic below). The problem is that, with the input (P4) left unconnected, I see a 150mV drop across R12. I've checked the circuit for shorts or leaks, but it seems fine. The only explanation I can think of is that there is a 300nA current leaking out of the noninverting input.
I'm trying to figure out if that is the case, and if so, if there is any way to fix it or if I need to change the opamp in my design. What parameter should I look for in the datasheet to tell if an opamp is suited for such high input impedance setup?
