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I am trying to measure the leakage current of a SMF36CA zener diode. I am doing this with a transimpedance op amp (LMC6062IM/NOPB).enter image description here

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?

enter image description here

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Tried with Zener 3.3 V.

See what happens at opamp output. Should see also the negative input of the opamp.
It is "saturated" so don't measure anyway ...

enter image description here

Be aware that you should not apply an input voltage higher than Vsupply/2 (V1/2).

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