Re: Noise created by resistor used to reduce op amp input offset

Thanks, John!


Sure. Short the input with your cmos switch and digitize the output of
the entire chain. We call this "software autozero."


It's much, much better than trying to reduce offset by adjusting trim pots! This could literally take *forever* to achieve. And if a change in temperature occurs after you've finally done the adjustment, you would have to repeat the process again, and again, and again.

In some of our products, like thermcocouple scanners, we have, say, an
8-input mux, and one of the inputs is a high-quality ground. All 8
inputs are scanned on a rotating basis. The "ground" value is
regularly digitized in its turn and software lowpass filtered to make
the internal variable ZOFF that's subtracted from all other channel
measurements. The filtering removes most of the noise from the
ground-measurement data.


If the measurement time takes only 1 second (max), then would it be safe to assume that the offset is the same over the time of measurement? I would imagine the following steps of the sampling process:

(1) close switch and measure offset voltage;
(2) open switch and take measurement of signal from transducer;
(3) apply software autozero filter

I would suppose that with a MUX, you would not have to worry about this, since for every sample that you take, you also have a ZOFF value.

What's a high quality ground? Could this be created using an RC filter? I'm thinking of using an RC filter tied to GND.


The ZOFF value is also useful as a gross error check.


So the ZOFF value will show the maximum error in the analog signal processing chain?


If you have multiple gain ranges, it may be prudent to have a zero
factor for each.

That's a really good idea!
How would I separate out the contributions from each gain range?




.

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