I recently designed a trans-impedance amplifier based on the designs laid out in the datasheet for the OPA818, utilizing the exact capacitor values specified over the power rails and resistor value over the feedback pin designed for a 100-kΩ gain. After testing the resulting circuit I noticed a strange output when I applied 0A, seeing that there was around 50mV being output with no input current.
After checking everything on my circuit I set up one of the OPA818 amplifiers on its own board with nothing attached and see a 10MΩ resistance over the negative input to both the positive and negative input pins. While this is extremely high for the sake of most applications, I am working with input signals on the scale of pA and this is causing a wild amount of input noise on the front end of my amplifying circuit, rendering the amplifier that I designed useless. The OPA818 boasts an extremely low input noise of 2.2 nV/√Hz and 3 fA/√Hz which was my reason for purchasing, however I am seeing input noise on the order of 100pA-1µA.
Is there any way to improve this input noise level? Am I missing something extremely important?