OPA928EVM: The actual bias current is much different from the specification

Hello,

The OPA928EVM must be kept clean and free of contaminants in order to maintain femtoampere performance. This board is highly sensitive to contamination from dust, fingerprints, and solder flux which will reduce the measured bias current performance. This is because these contaminants provide current leakage paths to the input pins. 

Please refer to section 1.1 "EVM Cleanliness Guidelines" of the OPA928EVM User's Guide. 

If the instructions in 1.1 were not observed during your initial handing and testing of the board, it is likely that board was contaminated resulting in the reduced performance of 50-100fA that was measured according to your document.

I see that you then replaced the integrating capacitor CF with another SMD device. This process introduced an extreme amount of contamination in the form of solder flux and other oils that severely degraded the performance to 10pA according to your document. The integrating node is the mode sensitive area of the board and as you have seen, it is imperative that this node is kept clean and free of contaminants.

It is possible for the femptoampere performance to be recovered by following the cleaning protocol described in section 5 of the OPA928EVM User's Guide. For extreme amounts of contamination in the sensitive nodes, it may be necessary to do multiple cycles of the cleaning procedure to recover femtoampere performance. Note that if the PCB traces were burnt or damaged during the soldering process, the leakage performance may be permanently compromised.

Thanks,

Zach