Multimeter last digit flicker - Page 1
Forum section: electronics / testgear

author: Hobby73
author_rank: Regular Contributor
timestamp: March 28, 2016, 06:02:41 am
content: When working with a low power DC circuit, I measure voltage and current in the range of millivolts and milliamps. Many times the last (highest precision) digit on my DMM will flicker between two adjacent values.Is there any way I can determine if this instability is simply the result of the contact surface varying by minute amounts between my DMM probe and the powered contact, or is it an indication of noise in the signal?

author: rs20
author_rank: Super Contributor
timestamp: March 28, 2016, 06:05:59 am
content: A third possibility is that the noise is inside the multimeter. Regardless, there is absolutely no reason to pursue this thought; the last digit on a multimeter is always nonsense anyway.

author: amspire
author_rank: Super Contributor
timestamp: March 28, 2016, 06:11:01 am
content: If it always flickers on every reading, then yes there is noise somewhere, but every multimeter will always have the last digit flickering sometimes. If you have a 2000 count multimeter and you apply 1.0005V, then even the most perfectly accurately calibrated and low-noise multimeter will flicker between 1.000 and 1.001, It is basically saying "the value I am reading is somewhere between 1.000 and 1.001".

author: jitter
author_rank: Frequent Contributor
timestamp: March 28, 2016, 06:11:57 am
content: Quote from: rs20 on March 28, 2016, 06:05:59 amAgreed, the specifications are always given as a percentage of reading + a couple of digits. The latter will be the least significant digit.A fourth possibility is that the actual value is somewhere in between and noise makes it "hesitate" between the one and the other digit. Edit: amspire beat me to it...That leads me to this question: if I turn a HP 34401A into a 4 digit readout, it looks like it rounds the values up or down. But would there also be meters that don't do any rounding and just truncate the digits?

author: rs20
author_rank: Super Contributor
timestamp: March 28, 2016, 06:28:19 am
content: Quote from: jitter on March 28, 2016, 06:11:57 amRounding and truncation have exactly the same behaviour with respect to oscillating digits -- a rounding meter will oscillate if the real value is close to 1.0005V; a truncating meter will oscillate if the real value is close to 1.0000V. Because round(x) = floor(x + 0.5).If you meant to ask, are there any multimeters out there that have 8.5-digit grade innards and take long enough readings to get 8.5 digits worth of accuracy, and then only display 4 digits? No, that'd be a waste of money and good components. Even 8.5 digit multimeters speed up and become less accurate when you select 7 or 6 digit modes on them, because it's just ridiculous to read any significance into the final digit.

author: amspire
author_rank: Super Contributor
timestamp: March 28, 2016, 06:32:56 am
content: Quote from: rs20 on March 28, 2016, 06:28:19 amDon't get accuracy and resolution confused. If the last digit is stable, then it is extremely useful, even if it is not accurate. If you are matching two voltages or resistors, accuracy does not matter in the slightest.

author: rs20
author_rank: Super Contributor
timestamp: March 28, 2016, 06:51:29 am
content: Quote from: amspire on March 28, 2016, 06:32:56 amIndeed, I should have said "the LSB".

author: Hobby73
author_rank: Regular Contributor
timestamp: March 28, 2016, 07:08:49 am
content: Quote from: rs20 on March 28, 2016, 06:05:59 amOK, I won't be bothered by it and will chalk it up to normal operation.

author: jitter
author_rank: Frequent Contributor
timestamp: March 28, 2016, 10:29:33 am
content: Quote from: rs20 on March 28, 2016, 06:28:19 amWhat I meant was only rounding or truncating. Are there meters that display, say, 10.499 as 10.49 (i.e. truncating) instead of 10.50 (i.e. rounding) when you choose to loose a digit in favour of speed?

author: rs20
author_rank: Super Contributor
timestamp: March 28, 2016, 10:49:19 am
content: Quote from: jitter on March 28, 2016, 10:29:33 amWho cares!? The difference between those two options is literally the same as a half-count offset, which is vastly cast into irrelevance by the normal 2-to-3 count errors in multimeters. It's an irrelevant implementation detail; the answer is probably "yes" because someone forgot or (rightly) didn't care to add the 0.5 before taking the floor.