Multimeter 1/2 digits... Why? - Page 1
Forum section: electronics / beginners

author: signals
author_rank: Contributor
timestamp: February 18, 2015, 10:08:49 pm
content: I'm fairly certain that I have a good grasp on what a 1/2 digit is, and what it means in terms of resolution on a multimeter. What I want to know is WHY do most multimeters have 1/2 (or 3/4) digits instead of just having X number of full digits?I've been Googling this off and on for a couple of days, and I have yet to find a good answer. Most of what I find is a description of what a half digit is, not why it exists. Maybe I'm not asking Google the right question, so I'm asking this forum. The best answer I've been able to find so far is that it's a cost saving measure; that way they don't need to have so many segments in the most significant digit, but I don't buy it. If it was a cost saving measure, why do the 3458A and 2002 have 8-1/2 digits? I suspect "cost" was far down on the list of design criteria for those instruments.Maybe it's obvious if you understand the theory of operation of a digital multimeter, but I guess I don't have a deep enough knowlege of how they work to see it.Anybody know?Thanks.Edit: Grammar.

author: Wytnucls
author_rank: Super Contributor
timestamp: February 18, 2015, 10:11:55 pm
content: Dave did a video on the subject. Try finding it.Never mind, I found it:

author: signals
author_rank: Contributor
timestamp: February 18, 2015, 10:26:38 pm
content: Quote from: Wytnucls on February 18, 2015, 10:11:55 pmThanks for the link. I've seen it, and Dave does a great job of explaining WHAT 1/2 digits are, and how they relate to resolution, but I don't think he addresses the reasons that multimeters have 1/2 digits in the first place.I'm watching it again anyway. Maybe I wasn't paying attention.

author: AG6QR
author_rank: Frequent Contributor
timestamp: February 18, 2015, 10:44:52 pm
content: Maybe it's because a lot of common supply voltages begin with a "1" or "2"?120V or 240V mains, 1.5V alkaline cells, 12V automotive batteries, etc.For example, with a 1000 count meter (three full digits, no half digits), you can measure a 12V automotive electrical system to a resolution of the nearest tenth of a volt, or 12.0V. With an extra half digit, a 2000 count meter can measure that same 12V electrical system to a resolution of the nearest hundredth of a volt, 12.00V. A 10,000 count meter does no better than a 2000 count meter for this particular case, both getting to the nearest hundredth of a volt. But a 10,000 count meter is more expensive to make.I don't know, the true reason may be somewhat lost in the mists of history and tradition. I just observe that a DMM has its best resolution (that is, a change of one count represents the smallest percentage of full scale) when the voltage being measured is close to, but a little bit below, full scale. If it's possible to choose full scale so that many common measurements will be a bit below that, it would be a good choice to make.

author: IanB
author_rank: Super Contributor
timestamp: February 18, 2015, 10:49:59 pm
content: I think it is related to logarithmic scales and Benford's law. It turns out that the leading digit "1" occurs far more frequently than any other leading digit, and so adding a leading "1" to the display almost doubles the resolution of the meter.Benford's law

author: electr_peter
author_rank: Unknown
timestamp: February 18, 2015, 10:50:52 pm
content: The main reason is that the left most digit has incomplete set of 7 segments (and accompanying circuitry to drive that). In a typical meter with 7-segment LCD/LED/VFD... display with all turned on digits you see "1.8.8.8" or "2.8.8.8". It is designed in HW, it is not just FW. Thus, one digit is only "half".

author: c4757p
author_rank: Super Contributor
timestamp: February 18, 2015, 10:56:05 pm
content: Quote from: electr_peter on February 18, 2015, 10:50:52 pmI'm going to guess that the extra segments on the display are literallytheleast expensive thing in one of these meters, or at least very close to it (less a few GP decoupling caps and pullup resistors).More likely IanB has the right idea. Adding a 1 is a significantly larger proportional expansion of the range than adding a 2 or 3, so if you're going to pick a nice, wide range it's likely to start with a 1.If the "saving segments" idea is correct, I can only imagine itwascorrect back when digital numeric displays were expensive, and now we're just following the tradition.Also, fwiw, my "good" meter goes to 301000 counts (yes, just over, it has a bit of overrange capability). HP 3468A.

author: Maxlor
author_rank: Frequent Contributor
timestamp: February 18, 2015, 11:14:42 pm
content: I've seen 2000 count, 3400 count, 4000 count, 5000 count, 6000 count and 10000 count meters among other higher res ones, and the 2000 count ones aren't even the most common, only the cheapest.What I'm wondering then is why the el cheapos aren't 1000 count and make do with just 3 decimal digits. It's not like they have the accuracy or precision for more anyway.

author: Matje
author_rank: Regular Contributor
timestamp: February 18, 2015, 11:27:39 pm
content: Quote from: signals on February 18, 2015, 10:26:38 pmDon't know whether Dave explains it in the video.But the reason is most likely cost. Cost for an ADC does not go up linearly with resolution, an order of magnitude more resolution may more (at high resolutions much more!) than increase the cost tenfold, also other parameters need to scale with resolution for the increase to be useful, think accuracy, drift, speed of conversion should not drop, etc.So for a given price point a compromise has to be made. Just check out the differences in specs between some cheap handheld DMMs, some good handhelds and some benchtop devices. Then check the prices, there should be a nonlinear correlation (until the manufacturer just lies of course).

author: Matje
author_rank: Regular Contributor
timestamp: February 18, 2015, 11:33:53 pm
content: Quote from: Maxlor on February 18, 2015, 11:14:42 pmMarketing. At such low resolutions there is likely no meaningful difference in price between 1000 and 2000 counts, if a 1000 count DMM chipset is even available anymore. So everybody just uses 2000 as the lowest. With technology getting better, in 10 years time, the lowest may well be 6000 counts, just because it doesn't make any sense to go lower - and the competitors all start at 6000 too.

author: signals
author_rank: Contributor
timestamp: February 19, 2015, 12:08:50 am
content: Quote from: Matje on February 18, 2015, 11:27:39 pmNow you've got me thinking... I think what I was missing was the relationship between ADC resolution and the size of a range. If you have an ADC with X number of bits you may find that it's too much resolution for 1000 count, but not enough resolution for 10,000 count. So the work-around is to adjust your ranges so that you use all of those bits in 2000 counts. (Or 4000 counts, or whatever.)Sound about right? It certainly would explain why even high priced instruments do the 1/2 digit thing, too.

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 19, 2015, 08:26:14 am
content: Instruments, which are designed for +/-12.0000000V like all bench DMM from HP, especially the 3458A, have a historical / metrological background.(*)Historically, voltage or ohm references have mostly plain decimal values, or nearby ones, as 10.0000V, 1.0186.. for Weston cells, 1.000000 Ohm or 10.00000k Ohm, and so forth..These are the so called Cardinal Points, if a series of 100mV, 1V, +/-10V, 100V and 1000V is used for the calibration of digital voltmeters.To measure, calibrate or make a transfer to highest precision around or near these plain decimal values, you need some "overrange" capability above 1V or 10V.Until 1972, that was the need especially for the 1,0186..V Weston cell metrology... Please study the manual of the Fluke 750A Reference Voltage Divider.Therefore all such instruments had about 10% over range, i.e. 11.111111 for Flukes calibrators and transfer standards like 332D, 334A, 335D, 720A, 5440A, 5200A, 5720, and similar ones from other companies.For that reason, DMM later were also designed to have an over range of 20%, e.g. "11999999" maximum display, as this was more feasible in digital design.All other end-range limits as 20000, 30000, 50000, and so on, were designed to have better minimum resolution at 10% reading, or due to operation supply limitations (specially on battery powered systems).. as all internal OpAmps have to cope with the same voltage range also..In the 3458A, its A/D converter is designed to always digitize a +/-12.0000000V input, and it uses two +/- 12.0000V sources as internal run-up and run-down references, and all OpAmp in the front end were supplied from +/- 15 or 18V to handle that input range.A +/- 19.999 000 0V instrument (with > 10GOhm input resistance) like the Fluke 8508A, is very uncomfortable to design, in contrast to that..Frank(*) You may download an HP catalogue from 1965, or 1966.Visithttp://www.hpmemoryproject.orgorhttp://www.hparchive.comThere you will already find DMM with 9.9999 or 11.9999 V range. Latter type survived until today.HP produced DCV calibrators also, like the 740A, or the 3420A differential VM, having these 11.11111 ranges.

author: Matje
author_rank: Regular Contributor
timestamp: February 19, 2015, 11:13:43 pm
content: Quote from: signals on February 19, 2015, 12:08:50 amIt is not really a workaround, but yes, it is what technically happens.Since there is no point in having more (decimal) digits than one has equivalent (binary) resolution steps, one ends up with somewhat strange decimal digit counts, like a 11 bit ADC (2047 steps) leading to 2000 counts (3 1/4 digits?), 12 bit ADC to 4000 counts (3 1/2 digits?) and so on. Marketroids have their fingers in it all too, so beware.One could of course use a 11 bit ADC but declare, say, 10000 counts or 4 digits. Nobody sane would buy such a contraption, though.

author: EEVblog
author_rank: Unknown
timestamp: February 20, 2015, 12:49:20 am
content: I'm not sure if this is the real reason, but it's likely a tradeoff of update time vs clock rate on the dual slope integration ADC.3.5 digit ADC's usually work in 1000 count cycles, and 4.5 digit meters 10,000 count cycles.It's a 3 step process, so you have the Auto-zero count (1000, or 10,000 cycles), signal integration (1000, or 10,000 cycles), and then the reference integrate count that determines the measured count. This reference integrate cycles is usually twice the signal integration time, so 2000 counts for 3.5 digit, or 20,000 counts for 4.5 digit.So there are 4,000/40,000 cycles all up, and that takes time.So perhaps someone chose those nice round numbers "back in the day" and they kinda stuck?

author: signals
author_rank: Contributor
timestamp: February 20, 2015, 02:56:11 pm
content: Thanks for all of the responses everybody. I'm not sure if I understand this any better, or if I'm more confused now, but I learned a lot about digital multimeters and their history from all of your answers.

author: opty
author_rank: Regular Contributor
timestamp: February 20, 2015, 03:24:12 pm
content: If we look at voltage meters only then it looks like a cost saving exercise. I mean if your voltage ref is 5v then whatever divider(but powers of 10) you use will leave you with numbers 000... to 1999... or 2999... or 4999 or 5999... and so on *; then there is no need to implement those upper digits.* highest value can not be used as it could mean over-voltageI know it would save probably only a tiny amount but multiplied by 1000x...? Then it means that in DMM all other measurements are limited to that particular digit count but perhaps there is another technical/economical explanation.Opty