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Test Equipment Anonymous (TEA) group therapy thread

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bd139:
Lets just say we need 6.5 digits for dynamic range AND resolution at the same time.

A contrived example without going into offset drift, picoamp bias, chopper amps, regulator standby currents and all the cool stuff that needs a 6.5 digit meter (or differential voltmeter but bugger that thread fork):

Consider a 20mV signal on a DC bias of 20V. What resolution can you measure that to on a 4.5 digit meter (assume 22 based count on both)?

4.5 digits: 20.020V     -> 20.??mV
6.5 digits: 20.02005V -> 20.05mV

Adding count uncertainty or say +/- 2 counts:

4.5 digits: 18 - 22mV
6.5 digits: 20.03-20.07mV

Specmaster:

--- Quote from: bd139 on June 26, 2018, 10:13:28 am ---Lets just say we need 6.5 digits for dynamic range AND resolution at the same time.

A contrived example without going into offset drift, picoamp bias, chopper amps, regulator standby currents and all the cool stuff that needs a 6.5 digit meter (or differential voltmeter but bugger that thread fork):

Consider a 20mV signal on a DC bias of 20V. What resolution can you measure that to on a 4.5 digit meter (assume 22 based count on both)?

4.5 digits: 20.020V     -> 20.??mV
6.5 digits: 20.02005V -> 20.05mV

Adding count uncertainty or say +/- 2 counts:

4.5 digits: 18 - 22mV
6.5 digits: 20.03-20.07mV

--- End quote ---
Fair points, I hadn't considered the higher voltage ranges, perhaps it had never occured to me that there ever would be a need to be that precise (assuming the meter is able to stop bouncing about on the last few digits) on say 20v or 100v ranges etc as the actual percentage difference is so small in comparison to the really low voltage ranges. For example ,amy low powered devices have a cutoff voltage below which they don't function, such as handheld DMM's where as little .1v can be the difference between running and switch off. Somehow I just couldn't see that level of value being as critical at the higher end of the scale.

djos:
I finally found a half decent DSO, that I could afford, 2nd hand - it was half the cost of a new 1054z so I'm happy ... anyhoo, it's a UNI-T UTD2102CEL 100MHz, 1GS/s unit in retina searing Red and Black!  It's a huge upgrade from my virtually useless Velleman WFS210 Wifi POS.  :-DD



 :-+

Specmaster:
Nice unit, why not add a splash of colour to the lab?  :-+

Cerebus:

--- Quote from: Specmaster on June 26, 2018, 09:39:05 am ---There was a time when everyone knew what the voltage, current or resistance was and there was never any question about it, it was what it was precisely. When was that, it was when everybody could afford to relinquish their analogue meters and go digital and we all had 3.5 digits and that new meter also gave us other things to worry about. That was because most of the service manuals/charts had their voltage references taken and stated as being measured with a meter @ 20,000Oopv and voltages we were reading were much higher because the new meters had an input impedance of around 10,000Mopv had a significantly lower loading on the circuits under test.

Then someone came along with a 4.5 digit meter, which is not too bad as it happens, then came a 5.5 digit, a 6.5, 7.5 and 8.5 digits and suddenly no one really knows what the voltage is anymore. A case in point, I'm sitting here looking at my 3.5digit DMM's connected to a voltage source, I've connected 10 up and they are agree with each other, 1.948V fine. Then I connect the same voltage source (untouched) to my 4.5 digit DMM's and now that extra digit is constantly changing, its now 1.9485, no wait its 1.9482 no wait its 1.9488 and so one.

Next I swap out the 4.5 digit meters for my 5.5 digit ones and WTF, I now have the last 2 digits constantly changing so what is the actual voltage that I'm reading?? The true constant here is the 1.943v which remain steadfast anything after the 3 just really cannot be relied upon as it only takes a gnat to fart and the reading changing again and again and again.

Its much the same story when switched to measuring resistance but this time the only digit that changes is the last on the 5.5 meters.

So the results that I'm getting is that the more digits the meter has, the more unsure of the actual value you become but tend to to go with either the first 3 or 4 digits after the decimal point so it would seem that the extra digits become just points in a game of brag or top trumps rather than serve a useful function. How many times have I either read it or heard people say that that they would have to have their meter professionally calibrated, especially so if it were to be 6.5 or higher digits because it cause them to worry about those last digits really were? :palm: As if they really matter in the grand scale of things  :popcorn:

Now then the real big question is if there was to be a 6.5 digit DMM that came within my price budget would I go for it? You'd better believe I would so I can join in the game of top trumps and besides there is something hypnotic about seeing all of those digits light up, even if they aren't giving me or the equipment I'm working on any real advantage  :-DD

--- End quote ---

That sounds more like something wrong rather than an intrinsic characteristic of using a high resolution/long scale meter. I'd take seeing that as an indication of either a horrifically noisy environment, poor measurement technique or something wrong with the meter.

For what it's worth, I rarely see last digit instability on a 5 1/2 digit meter unless it represents a genuine feature of the signal and my measurement technique is generally quite sloppy with the exception that I do use 4 wire techniques for resistance measurements as a matter of course as I have kelvin clips and keep them handy.

If the last digit or two is changing for me on something that ought to be rock solid I take that as an indicator that I need to change my measurement technique or hook the scope up to make sure that what I think ought to be present actually is (e.g. HF instability showing up on a signal that ought to be DC). If I'm on 2 wire ohms, switch to four wire, if I'm measuring voltage, add a guard connection to suck up common mode currents and so on. I'd suggest a read of the Keithley "Low Level Handbook" to brush up on measurement artefacts and their sources and see if that isn't your problem - the name's a bit of a misnomer, as the issues and techniques covered apply as much to precision measurements as low level ones.

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