Attached is the circuit I am using. The amp and diode in the circuit are not the ones I am using, just the ones Circuit lab had. The diode is a bi-directional one so I put 2 zeners in the circuit to mimic that. The gain resistor is 1G ohms, although I have used a variety of different values. I am trying to measure the leakage current through several different values, from 1v to 40v. The breakdown voltage of that zener is 36. My problem lies, in the fact that when I approach 36 volts the output voltage starts to ramp at a way that is inconsistent with the leakage current graph of that diode. What could I do to fix this?