[volt-nuts] Unable to zero 3420B on 10**5 sensitivity

Dr. Frank frank.stellmach at freenet.de
Mon Nov 13 17:14:09 EST 2017


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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