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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  • Harry,

    Thank you for the immediate feedback. I'm fairly familiar with "typical"

    specs in a competitive world - in marketing terms it normally means we

    saw one almost this good once, but in engineering terms I have to depend

    on published specs to be somewhat useful for rough guidance. If I get

    worst case Ib at room temperature and if it increases by several times

    again over temperature as datasheet Fig. 19 implies, I could have an

    18-bit DAQ design with a 25% offset error spec. That's a problem for me,

    and would require a few days with a Tenney to find out.

    If a large customer is consuming binned parts we don't have the volume

    to compete with that. Is that a thought example or are you allowed to

    say if it's reality with this part?

    Thanks,

    Gary

Reply
  • Harry,

    Thank you for the immediate feedback. I'm fairly familiar with "typical"

    specs in a competitive world - in marketing terms it normally means we

    saw one almost this good once, but in engineering terms I have to depend

    on published specs to be somewhat useful for rough guidance. If I get

    worst case Ib at room temperature and if it increases by several times

    again over temperature as datasheet Fig. 19 implies, I could have an

    18-bit DAQ design with a 25% offset error spec. That's a problem for me,

    and would require a few days with a Tenney to find out.

    If a large customer is consuming binned parts we don't have the volume

    to compete with that. Is that a thought example or are you allowed to

    say if it's reality with this part?

    Thanks,

    Gary

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