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input bias current of AD8638


during evaluation of AD8638 I experienced a higher than expected biascurrent.

The device was (accidently) out of regulation with:

  - inverting input at 0V

  - noninverting input at -2V

  - Powersupply was +-6.7V

In this situation there was a current of approx 10uA flowing into the noninverting input (and appearingly out of the inverting input)

I reproduced the behaviour twice. The pcb board is reasonably clean.

Possibly this large input current is related to some kind of autozero switching. Can someone elaborate on this or point to reference material on this subject? Under which conditions is the datasheet specification of the bias current valid? How should it be measured?

Best Regards

Norman Werner