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?

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  • Gary,

      What does "typical" mean?  Is it an average?  A mean?  An average plus one standard deviation?

    I've always considered "typical"s to be a waste of ink.  If it is an average, then 50% are higher

    and 50% are lower.  If a big guy pays for a special selection and sets his limit at typical, then

    ALL of the parts you get will be above typical.  For a worst case design, you have to use the max

    number, which is 50nA.

    Harry

Reply
  • Gary,

      What does "typical" mean?  Is it an average?  A mean?  An average plus one standard deviation?

    I've always considered "typical"s to be a waste of ink.  If it is an average, then 50% are higher

    and 50% are lower.  If a big guy pays for a special selection and sets his limit at typical, then

    ALL of the parts you get will be above typical.  For a worst case design, you have to use the max

    number, which is 50nA.

    Harry

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