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

On Sun, 02 Aug 2009 14:04:31 -0600, Nicholas Kinar <n.kinar@xxxxxxxx>
wrote:

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.

Which is why continuous auto-zeroing is good; it keeps adjusting for
any drifts.


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

You can do that. The az filter only makes sense if the autozeroing is
done as part of a continous repeated process.

In general, one prefers to not have the noise of the zero offset add
to the noise of each data measurement. So signal-average or filter a
stream of zero measurements.


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.

No, just a short. But a short that's not trashed by ground loops or
thermals. We did one thermocouple-based temperature controller where
the short was a couple of inches away - on a solid ground plane - from
where the t/c was grounded. Not far away was a 150 watt PWM heater
driver. Voltage gradients in the ground plane made big temperature
errors. Moving the ground sense point about an inch fixed it.



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?

Yes. Check it for reasonableness.



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?

Do the zero experiment at each gain setting and keep multiple ZOFF
things.

John

.

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