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