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
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.
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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