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Jeff,

that's a how long is a piece of string type question. But it's still a good one. How accurate a reference needs to be rather depends on the purpose to which the DVMs which might be checked against that reference will be put. A good rule of thumb might be to aim for a reference whose inaccuracy is 10 times less than the accuracy that is being aimed for. For example, for a meter that is accurate to 1% of measured voltage, then a reference that is accurate to 0.1% would be good. For a meter accurate to 0.3%, then a 0.1% accurate reference would be tolerable, but could do better.

But that then raises the question as to just how accurate that reference is on the day when it is used. How much does it drift with time? What is the temperature coefficient? What can one check it against? Does that checking reference have any traceability?

The AD584K is, according to the spec sheet, good to, or better than, ±0.1% on the 10?V output.

Thinking of a 3? digit DVM, that is a resolution of 0.05% at FSD, or for a 4? digit DVM 0.005% at FSD. The claimed accuracy is quite another matter, but is probably specified for the meters that you own. E.G. for a 4? digit meter of specified DC accuracy ±(0.05% + 3 LSDs) that works out, at 10?V DC to ±8?mV, or ± 0.08%. Now, in regards to the reference, the question becomes who is validating whom!

My own experience with one of the Chinese suppliers of AD854 units including a built-in Li-ion cell is that the measured voltage from the 10?V output is very consistent across my small fleet of DVMs, ranging from a few years old to 30 and more years. That's consistency is very encouraging and a tribute to the basic pocket multimeter design of PCB switchery and precision resistors. (And the utility of end-of-service pulls—best sort of recycling!)

HTH, 73, good question BTW,

Robin, G8DQX

On Sun, Dec 11, 2022 at 07:10 PM, Robin wrote:

A good rule of thumb might be to aim for a reference whose inaccuracy is 10 times less than the accuracy that is being aimed for

I do not maintain any references to check my instruments and test equipment.. I have multiple quality variable power supplies and a number of GR L, C, and R decades plus a couple of GR L and C standards, so I depend upon a 4-1/2 digit true RMS Keithly as my standard, which goes to the cal lab annually. The accuracy of the Keithly is a factor of 10 better than any of my other instruments. My need for accuracy in L and C measurements can be met by checking their accuracy with my GR elements, and the Keithly is a sufficient reference for everything else which I own.

Dale H. Cook, Retired AM/FM/TV Chief Engineer, Roanoke/Lynchburg, VA
https://plymouthcolony.net/starcityeng/index.html

I also have the dmmplus … very pleased with it.

73,

Frank
K4FMH

---
Frank M. Howell, PhD
Ridgeland, MS
frankmhowell (at) hotmail.com
---

Voltage references and calibration techniques for the home lab:

See:

Jacques?? VE2AZX

The one Goldmine sells is +/- 75mV so that is why the cost is so low.
--
Dave, AA6RE

Jeff, some advice from a recovering calibrataholic.

It is good to know how to calibrate instruments, know their tolerances and how much you can trust what is displayed. However, in the quest for ever greater accuracy and precision, you can fall into the vortex of chasing calibration till it prevents actually practicing the hobby you want to use the equipment for.

How much accuracy and precision is needed for hobby work? It really depends on what you hobby is. If you rebuild tube amplifiers and vintage radios, 6.5 digits of accuracy is not very useful (and probably detracts from getting anything done) and one decimal digit will probably be enough. If you fall into the average hobbyist needs, probably 3.5 digits are fine.? If you design high accuracy test equipment, measure strain gauges, or A/D or D/A converters then more digits can be very helpful.

You need to realistically look at what you are going to need for the hobby and invest at that level. You generally need some calibration standards 10X better than what the accuracy is you are calibrating for (your actual needs). Because you have a 8.5 digit DMM (great deal or good price), does not mean you need to calibrate it to that level, if that is not what your needs are. Focus on what you need for your hobby. You don't need a micrometer to frame a wall when a measuring tape is plenty accurate.

For what I do, 3.5 digit DMMs or the ~2% accuracy I can get from a well calibrated analog meter is fine 99% of the time. I typically use a 500Mhz HP scope but often don't need anywhere near that bandwidth. I have a bunch of much higher accuracy/precision instruments I can use if I ever need them.

For calibration I have settled on a few things. I splurged and have a calibrated HP 3456A. I decided I wanted one device that was much more accurate than any other I have, to calibrate against. I have a GPS disciplined oven based oscillator with 1pps, 1Mhz and 10Mhz outputs. I also have some known cap, resistor and inductor standards I have collected over the years that are occasionally re-characterized. I only calibrate to one standard source for voltage, current, resistance, capacitance and frequency.? This removes the lingering doubt that the standards might be off from each other and assures all my equipment reads the same in relation to each other. Remember the old saying about clocks and time; if you have one clock you always know what time it is, but if you have two clocks you can never be sure.

Once your equipment is calibrated it can still seem to read wrong if you are not intimately familiar with the operation and quirks of the equipment itself, and its connectivity to what you are trying to measure/display. Frequency limitations, impedance mismatches, input capacitance, aliasing and other things need to be accounted for, or corrected if possible. The presentation mentioned above is a great source for info.

Uncertainty calculations are important and bring another level of realization that equipment published specs and displayed values aren't always what they seem.

Sorry for the long winded answer but have fun with you calibration and hobby adventures.

> Jeff, some advice from a recovering calibrataholic.

I didn't realise recovering was even possible !

It's not, he's in denial. We should probably get him a nice precision resistor.

Vince - K8ZW.

--
Michigan VHF Corporation -- nobucks dot net

K8ZW -
Etsy Shop:

I am not in denial...?? I can quit calibrating equipment any time I want.???????? I just don't want to right now...

...and don't worry about sending me any nice precision resistor, just a plain junkbox 0.001%? resistor with a ±0.2ppm/°C tempco would be fine. I can get by with that.

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Not going for anything with precision then.... ???Ho Ho, Merry Christmas.

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You need a JVS:

-- 
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
420 Via Palou Mall
Stanford University
Stanford, CA 94305-4070

Typo, obviously "you need a calibration". Not an intervention ?

Well,
in the past there was only one reasonable 10V voltage reference from Joe Geller, the SVR-T.
It was very stable over time, temperature and humidity and could be used as a starting reference point when you also had a transfer standard available.
It was stable to about 5ppm in all parameters, because it used a hermetic, ceramic package, based on the AD587UQ or LQ, if I remember correctly, and it was calibrated against a 3458A.
In this sense, it could really be used for metrology / calibration purposes. But these hermetic ICs are obsolete, as well as Joe Geller does not offer these references any more.
I attach a picture of a re-build, w/o the T.C. compensation by Lars Walenius.

All other such references are simply toys, I guess, and some are even hoaxes, so please pay attention.

Maybe the CheckDMM can be regarded as a reasonably stable reference, but he himself declares it as a (sanity) checker only, not as device for calibrating anything.?
It covers a few modes / ranges only, so it's not very handy, I think. Another problem for you is, that a resistive 10:1 divider wouldn't help you, as your DMMs probably have 10MOhm input resistance only, spoiling the divider ratio.

In general, calibrating any DMM requires to compare all ranges and all modes to a known good and stable reference standard, anything else is useless.

If you want to fully check or even adjust your DMM at home, I fear that you'll directly fall for volt-nuts passion.
There are very good DIY 10V references out there, on eevblog, based on either the much more stable LM399, or even based on the LTZ1000, all have to be calibrated against a good other reference.
As said, you would also need transfer standards for 1V, 100mV and 100 / 1000V to make a proper calibration of DCV alone.

Frank??