AD8253 excessive bias current

Using AD8253 in classic scope-type DAQ front end. Both inputs have 1 Mohm to ground. In an AC-coupled mode, the amp Ib develops offset voltage across the input resistor. Data sheet specs indicate 5 nA typical, 50 nA max over temp, further supported by Figures 8, 9, 18 and 19. I am seeing consistent 30-50 nA = 30-50 mV on tens of samples with all my other circuitry isolated. Any hints?

  • Paul,

    Thank you for the feedback. I see the same amount of offset when I

    remove all ancillary components and have only a 1M resistor from AD8253

    pins 1 and 10 to AGND, so the Ib and Ios both seem to be running toward

    the maximum spec on almost all the chips I've tried.

    The ideal DC reading is as close to zero millivolts as possible, so the

    target has to be pragmatic. In practice users have found 5-10 mV

    acceptable but 30-60 mV is a  noticeable and troublesome offset for the

    typical AC-Coupled IEPE accelerometer application. I don't have room or

    power for a discrete FET-input in-amp topology in this design. If that

    is looking like the norm for the AD8253 part at +/-7V I'll have to

    figure out some other mitigation strategy. I have to say it makes the

    "typical" numbers published in the datasheet not only a waste of ink as

    Harry pointed out but actively misleading. I appreciate that you folks

    have always been helpful and frank even when the answer is not what I

    would have wanted.


  • 0
    •  Analog Employees 
    on Aug 2, 2018 3:45 PM
    This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

    Thank you,
    EZ Admin