FETS to set DC offset?

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Castillovanv

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What are these FETS doing? why didn't they just use normal fixed resistors instead to set the DC offset for the op amps?

They are using FETs as voltage controlled resistors to set the DC offset for op amps, but why?

What is the difference between using a FET as a voltage controlled resistor VS using a normal resistor?

Adjust the voltage on a FET to the SAME value as a fixed normal resistor, what would be the difference?



 

Looks more to me like these are shorting switches, maybe trying to null
some outer loop or something? If the gate drive signal is "digital" then
that's likely.
 

shorting switches? how would they work?
 

To me it looks like one "throw" shorts the +/- inputs together and the
other attaches the "normal" feedback network.

In the shorted condition you are applying VIN=0, an ideal condition. If
you then "corrected" the amplifier output to null, by some sampled
feedback (summing and closing loop) you could take out all of the
front end error before and including the op amp's input offset. Then
you flip back to "normal" while keeping the null-cal and you have
(potentially) improved the signal chain DC accuracy a lot (at cost of
complexity, additional classes of noise, etc.).
 

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