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offset voltage

Discussion in 'Electronic Design' started by Reg, Oct 28, 2007.

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  1. Reg

    Reg Guest

    I have a DC op-amp application requiring ultra low quiescent current (about
    2uA) and a gain of about 100.
    The offset voltage means trimming would be required but this is not
    practical.
    I cannot AC couple it.
    If I had two amps in series with x10 gain on both, would the offset
    (magnified) be x10, x20 or x100 ?
    Anyone?
    Thanks
    Reg
     
  2. Winfield

    Winfield Guest

    That's the output offset voltage you're calculating.
    Shouldn't that be Vos(1)*100 + Vos(2)*10, or Vos*110
    for identical offsets. That's an effective input
    offset* of 1.1 Vos, showing 10% from the 2nd stage.
    Reg, it's generally useful to refer all errors back
    to the input, that way it's clear what's going on.

    Reg, have you considered zeroing out the offset at
    the second stage, where it's larger? Actually, I'm
    curious why you think two stages are necessary. Are
    you trying to achieve a "wide" bandwidth? One thing
    to consider, when evaluating error budget, we often
    worry first about drift budgets, rather than offsets,
    because simple offsets can be removed, but drifts
    are another matter entirely. Once again, it's input
    drifts we're concerned about, not only voltage drift,
    but current drift times your DC signal impedance.
     
  3. PeteS

    PeteS Guest

    The offset voltage would be Vos(1) x 10 x [(Vos(2) x 10)]. Assuming an
    identical offset voltage, it would be Vos * 100.

    There are amps with ultra low Vos, incidentally, specifically for
    situations like this - and don't forget Ios.

    Cheers

    PeteS
     
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