David,
I've thought a bit further about this excessive offset of at least 8µV.
That could be simply due to excessive offset of the DC amplifier part of
the meter amplifier, caused by ageing, i.e. drifted resistors or faulty
elctrolytics.
The chopper has about 5000x ac gain, due to rectification, that's about
2500x effective dc gain.
So these 8µV translate into 20mV too much of dc offset of the dc
amplifier. How the 1V zero (R16) affects the DC amplifier offset, I
don't comprehend yet.
But the next step is to check the DC amplifier.
Frank
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