Getting an extra digit from a 5.5 digit multimeter - Page 1
Forum section: electronics / general

author: xerbo
author_rank: Contributor
timestamp: October 13, 2022, 07:42:56 pm
content: I have a Fluke 8840A (Dave has a video on it, linked below) which has uses the overkill LTFLU-1 as its reference, it also has a "medium" sample rate which provides full resolution (albeit noisier) data at ~17sps (4 sample average).My question is, with further processing (maybe just a moving average?) how practical would it be to get an extra digit of resolution at a lower sample rate, I know the Solartron 7081 used this trick.I would try, but I don't have a GPIB card and have had no luck finding one.

author: sahko123
author_rank: Frequent Contributor
timestamp: October 14, 2022, 12:03:51 am
content: Depending on the inherit noise it might take so long to average that the temperature drift, other random drifts, popcorn noise and random crap. Meaning that your gained digit is virtually meaningless

author: alm
author_rank: Super Contributor
timestamp: October 14, 2022, 12:10:19 am
content: In theory averaging (=over-sampling) should give you extra resolution, though not necessarily precision or accuracy.But this assumes there is some noise in the signal, because if the true value is 1.0004, but your instrument is just reading 1.000 all the time, then no amount of averaging is going to get you closer to 1.0004. But if the instrument is flipping between 1.000 and 1.001, then you might be able to get an extra digit of resolution by averaging the signal. Datron in their 1081 7.5 digit multimeter actually introduced a small amount of noise in the signal (dithering) to guarantee that there would be some variation in the least significant bits of the ADC so averaging would yield extra resolution.Without GPIB I think you might have to get out pen and paper and a calculator, or maybe record a video and run character recognition of the video. I might have a GPIB card that I can sell, DM me if you're interested and then I'll check.

author: bdunham7
author_rank: Super Contributor
timestamp: October 14, 2022, 01:01:09 am
content: I think it might work. There should be enough sample noise at the medium rate on an 8840A to give you something to average. I don't know how meaningful that extra digit will be, but I would guess more than the last digit of some of the budget 6.5-digit DMMs we discuss here. Keep in mind that it is already averaging, but unlike many meters that calculate more digits internally and round them off, this meter has an unusual type of ADC and there aren't any more internal digits. The exception would be fast mode, which I think is just raw ADC output truncated to 4.5 digits. Perhaps the GPIB output would have the full 5.5 digits in fast mode and you could average that.I just tried an 8842A and in the slow mode with a 10V reference it is either completely steady or flickers between two consecutive digits. In the medium mode it flickers +/- 2 digits or so. I'm thinking you'll need to average at least 32 to 64 medium readings to get an extra digit that isn't completely noise. So your update rate will be slow.As for GPIB cards, AFAIK they aren't all the same or interchangeable in this series, so look into that in detail before you go trying to obtain one. The CPU/memory configuration and firmware vary on these.

author: xerbo
author_rank: Contributor
timestamp: October 14, 2022, 07:17:06 pm
content: Thanks to the work of Burner_357 inhttps://eevblog.com/forum/projects/fluke8840a-ieee-488-redesign-(ethernet-replacement-for-gpib-option-05)/I have bodged together an interface card, and yes, the last digit does get truncated in fast mode.

author: xerbo
author_rank: Contributor
timestamp: October 15, 2022, 07:56:17 am
content: Measuring the output voltage of a AD580 (don't have a anything better at hand). Results aren't particularly good but its something, 128 reading average so about 8s for one measurement.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 15, 2022, 08:38:13 am
content: The noise does not look that bad: The drift and lower frequency part are likely from the reference. The short term noise is in the 30 µV peak to peak range, which is OK for 10 µV resolution.With the lower resolution ( 3 and 4 digits) meters it is common to expect an essentially stable last digit, about peak to peak noise of about 1 display step. With the high resolution meters (6-8 digits) this shifts a bit toward considering a digit usefull if the RMS noise is less than 1 display step.Instead of the AD580 one could consider some alkaline batteries in a quiet environment as a low noise, maybe slowly drifting (e.g. with temperature) voltage source.As averaging helps with noise, but only very little with linearity, it may be worth checking the linearity. A relatively simple test would be the histogram for the raw data before averaging.