Electronics > Metrology

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Rax:
This may be the dumbest question ever asked on this forum. But here it goes.

Assume I'd be adjusting a reference 7.00000V (the digits represent my ability to read it) in a bench DMM. This needs to be adjusted to +-.0002V of that value.

Do I adjust to the cusp of the flicker between 6.99999V and 7.00000V? Do I adjust to a solid 7.00000V? (assuming the latter is possible, which depends on a few factors, one being whether I have so many digits of reading that I can't have a perfectly solid, immutable reading, and the last count will likely have fluctuations).

I feel the first choice above is closer to the dead center of the range (evenly distanced between 6.99980V and 7.00020V). A truer 7.00000V. The second choice may be right at the center of the 7.00000V and 7.00001V range.

guenthert:
Are you telling me that your meter has a, even just 20m, short-term stability better than one least digit?  Not even speaking of the stability during the calibration period.

And the current value of the source is known to better than 1ppm (given that you apparently intend to adjust a 6.5 digit meter)?

There's no point to fret about fractions of the last digit, if the confidence interval of the actual value spans many counts.

Kleinstein:
Higher resolution meters usually have noise of more 1 of the last digits.  A common situation is about a RMS value of about 1 of the last digits. So something like 6 steps peak to peak, maybe a little better.

Noise less than 1  digit step does happen with low resolution meters at the 2-3.5 digit level, but there it is usually advised to get a high resolution meter insted of trying to get better than 1 LSB.

The idea target is usually such that the average over a little longer time is at the right value. Still with limited stability and accurace of the ref. elemt to adjust too there is no need to overdo it. The good enough point is often more than 1 step off nominal.

iMo:
In case the OP owned a 34401A (as a typical 6.5digits bench dmm) and the DUT reference was an LM399 the OP would see a "rock stable" 5th digit after the decimal point on the display most of the time (ie his/her 7.00000V).

bdunham7:

--- Quote from: Rax on February 04, 2023, 01:57:23 pm ---What's the correct adjustment methodology?

--- End quote ---

Assuming your meter is somehow absolutely perfectly calibrated, linear and stable, what is the threshold for it to transition from 6.99999V to 7.00000V?