AC RMS DMM tests

[mawyatt]

We developed a “Scope & DMM Calibrator” some time ago to “see” if the two 34401A DMMs and two Tektronix 2465 scopes we had purchased off eBay were somewhat reliable for measurements. Later we got a new KS3465A and knew which measurements were accurate. This home brew calibrator utilizes a technique where the True RMS is “inferred” by design to a DC measurement and the DC and RMS values are theoritcally identical. This simple technique is just a perfect squarewave of peak amplitude V and zero-volt referenced.

To approximate this ideal squarewave with perfect symmetry a CMOS Flip Flop was utilized and followed by a discrete CMOS inverter with moderate Rdson P and N channel FETs. The clock frequency is low to keep the edge artifacts from seriously influencing the measurement. VDD for the discrete CMOS inverter is supplied by a precision 5.000V reference.

If things work as expected, the ACRMS value should be VDD/2 or 2.500V RMS and 2.500V DC. Here's some results with various DMMs we now have.

KS34465A           VDD 5.0001076 VDC         VAC 2.4998394 VRMS                VAC    2.5000144 VDC
DMM6500                   5.000098 VDC                  2.499396 VRMS                           2.500009 VDC
AG34401A                  5.00012 VDC                    2.50043 VRMS                             2.50000 VDC
HP34401A                  5.00012 VDC                    2.49934 VRMS                              2.50002 VDC

Anyway, certainly not a replacement for proper calibration equipment and certification, but might help folks get an idea for an accurate AC RMS waveform that can be somewhat verified with a DC measurement. The image shown the Home-Brew device which uses a CMOS divider chain (CD4060) with a 4.096MHZ crystal and ADR4550 Reference, a 2N7002 NMOS and BSS84 PMOS for the inverter.

Edit: Added a 1second TC prefilter for the DC meaurements.

Best,

[Kleinstein]

The rectangular waveform can behave quite a bit different for the AC circuit. One point is a possble slew rate limit that may apply.
The other point can be a time delay in detecting the polarity reversal in some analog RMS converters.
A 3rd point is a possibly limited bandwidth - some of the power will be outside the BW of the DMM. This would not be very much for the higher end bench DMMs, but it can be significant for handheld ones.

So the rectangulator wareform is a relatively special case. It is still one of the easier ones to generate.
A little low pass filtering may be a good idea to get at least rid of the slew rate limit and reduce the delay effect. If only filtering out the higher frequencies (e.g. > 50 kHz) the effect of the filter should not depend that much in the parts accuracy and effect of the DMM input.

[mawyatt]

The rectangular waveform can behave quite a bit different for the AC circuit. One point is a possble slew rate limit that may apply.

That's why we are using a fast edge and low frequency 

The other point can be a time delay in detecting the polarity reversal in some analog RMS converters.

That's why we use a unipolar squarewave

A 3rd point is a possibly limited bandwidth - some of the power will be outside the BW of the DMM. This would not be very much for the higher end bench DMMs, but it can be significant for handheld ones.

That's again why we are using a low frequency

So the rectangulator wareform is a relatively special case. It is still one of the easier ones to generate.

Exactly, and verifiable by DC measurement as mentioned. The only waveform that has the same RMS and DC value except, well, obviously DC

A little low pass filtering may be a good idea to get at least rid of the slew rate limit and reduce the delay effect. If only filtering out the higher frequencies (e.g. > 50 kHz) the effect of the filter should not depend that much in the parts accuracy and effect of the DMM input.

This is exactly what you DON'T WANT To DO with the low frequency squarewave since you are now placing an additional uncertainty on the slow edge of the LPF result and the source squarewave source impedance. If the slower edges aren't very very close in rise and fall and shape they will introduce and error in both RMS and DC readings, whereas with a much faster unfiltered edges the result is much less influenced by the edge since the edge period is so much smaller than the squarewave period. 

Edit: A quick experiment or a little Fourier Analysis can verify the LPF is NOT what you want to do. Just ran a quick setup and added a simple 5us (32KHz) RC (500 ohm and 10nF (9.96nF actually)) low pass and the result was additional error of  -2.5mv!!! Kept the series 500 ohm R in place since it's forming a low pass with the DMM leads and input capacitance, then just added the shunt C . So keeping those edges as fast as possible is what you want to do


Best,

[alm]

This is exactly what you DON'T WANT To DO with the low frequency squarewave since you are now placing an additional uncertainty on the slow edge of the LPF result and the source squarewave source impedance. If the slower edges aren't very very close in rise and fall and shape they will introduce and error in both RMS and DC readings, whereas with a much faster unfiltered edges the result is much less influenced by the edge since the edge period is so much smaller than the squarewave period. 

Edit: A quick experiment or a little Fourier Analysis can verify the LPF is NOT what you want to do. Just ran a quick setup and added a simple 5us (32KHz) RC (500 ohm and 10nF (9.96nF actually)) low pass and the result was additional error of  -2.5mv!!! Kept the series 500 ohm R in place since it's forming a low pass with the DMM leads and input capacitance, then just added the shunt C . So keeping those edges as fast as possible is what you want to do

That's a great illustration why I think this method of comparing DMMs is problematic: if DMM A has a bandwidth or slew-rate limited front-end that affects the edges of the signal the way you simulated, while DMM B does not, then DMM A would read lower with the chopped DC while it might read the same with a 1 kHz sine wave.

I've seen this technique used for scope calibration, where only the amplitude is important (Tektronix PG506), but calibration of DMMs and thermal RMS conversion is always done with low-distortion sine waves, so bandwidth of the signal can be strictly controlled.

[mawyatt]

Agree that an accurate low distortion sine wave is the best RMS source, however this is very difficult to create and quite expensive equipment. Creating a low distortion sine wave is one thing, but how do you verify RMS levels without some "reference"?

With the squarewave mentioned the verifiable "reference" is easily done with a high resolution DMM by measuring the CMOS squarewave reference VDD DC voltage. You can also measure the Average DC Waveform content of the squarewave with the same DMM, and know by design it should read VDD/2. If one is concerned about the DMM when measuring the DC waveform content of a squarewave, then a simple RC low pass works well to squash the waveform into a low frequency average moving DC term for measurement. With the DMM set to high input impedance Giga-ohm mode, then a high series R can be used.

For the True RMS AC measurement you don't want to slow down the edges but keep them crisp. Why? Because the two known waveform states are ground (zero) and VDD, both of which are easily verifiable by DMM DC measurements. Only during transition from low to high and back are the waveform characteristics uncertain and likley not precisely equal. By using a low frequency waveform not only are the squarewave odd harmonics lower in frequency, but the edges are a very small % of the waveform period, and also by using a very fast Flip-Flop with a low frequency to create the squarewave period the nearly perfect symmetry is guaranteed by design and thus contribute a very small error in the measured result.

Of course not saying this is a replacement for a proper sine wave calibration source, it's just a means to a somewhat "verifiable" waveform that is easy to create, verify and because of the waveform uniqueness has identical RMS, and Average DC values. Sinewaves can not do this nor any other waveform I'm aware of other than simple "DC". Also note that a Squarewave by definition has a "Crest Factor" of 1 and a Sinewave is 1.414!!

So rather than speculate like many tend to do, we simply verified this concept with multiple measurements with multiple quality True RMS DMMs. The Keysight KS34465A and Keithley DMM6500, both of which use computational RMS methods, and a pair of the highly trusted 34401As both of which use the analog RMS chip method.

These are our "go to" instruments at the Labs for precision waveform measurements and we will be augmenting soon with another DMM6500 and possibly another KS34465A to support the present ongoing Project where these instruments are employed.

So for now, we'll let these results speak for themselves

If you question this method, then please give it a try with your own True RMS DMMs. It's simple enough and a breadboard doesn't cost much, less than $15 total

If interested, we can send the geber files & partial BOM for the shown PCB, if I can find them   Not sure how to post them here tho, so PM.

BTW the earlier comment about the decision delay regarding the polarity under AC True RMS, a slow waveform edge speed would introduce more uncertainty and thus more potential error.

Best,

[Kleinstein]

BTW the earlier comment about the decision delay regarding the polarity under AC True RMS, a slow waveform edge speed would introduce more uncertainty and thus more potential error.

With a slower slope the timing error from noise will be larger, but there is less consequence, as the wrong sign for a signal near zero has essentially no effect. This also still applies to a longer delay from slower reaction. With the square wave the timing error is multiplied with the full voltage.


There is nothing wrong with filtering the square wave waveform a little to round the edges - the effect of the fitler is not easy, but it can be calculated and if not so low, the exact value of the capacitor is also not that critical. With the RMS part it is only about the amplitudes, the phase shifts can distort the waveform, but it does not effect the RMS.

If a meter fails on the square wave test, it can still work pretty well with a sine. An even if a meter reads fine with the square, this does not guarantee that it works OK with a sine. It is a different and relatively extreme test pattern. With the usualy AD637 or similar chips it would not detect a failing averaging capacitor, or the effect at low frequencies where the capacitor is at the low end.

[mawyatt]

With a slower slope the timing error from noise will be larger, but there is less consequence, as the wrong sign for a signal near zero has essentially no effect. This also still applies to a longer delay from slower reaction. With the square wave the timing error is multiplied with the full voltage.

There are more sources of timing errors than just noise, offset voltage and hysteresis for example, and all timing errors are improved with a crisp edge to detect.

There is nothing wrong with filtering the square wave waveform a little to round the edges - the effect of the fitler is not easy, but it can be calculated and if not so low, the exact value of the capacitor is also not that critical. With the RMS part it is only about the amplitudes, the phase shifts can distort the waveform, but it does not effect the RMS.

A simple thought experiment will show this is thinking is flawed. If you consider slowing the edges down, the extreme case shows the eventual waveform will approach a triangle wave which we all know has an RMS value of 1/(sqrt(3)) not unity as the normalized squarewave. That's a significant reduction in RMS value and shows a strong relationship between RMS and edge slowing the edges down reduces the squarewave harmonic content which reduces the energy available from the waveform and thus reducing the RMS reading. Capturing the harmonic content is another reason to keep the squarewave frequency low as well as a fast edge.

If you assume a trapezoidal waveform a little trig and algebra will show that for a peak amplitude of unity:

Vrms = sqrt(1-4*(tr/3)), where tr is the rise and fall time normalized to a waveform period of 1 second. For different rise and falls time it gets a little more complicated.

For example: Assume a rise or fall time of 10% of the period, the normalized Vrms ~ 0.931, for tr =1% then Vrms ~0.993, for tr = 0.1% then Vrms ~0.99933. Or for a 1000ppm Vrms error due to rise fall time then the fall time must be less than 0.145% of the waveform period.

So if the waveform has a 250Hz frequency (where we measured the DMM RMS values), then the rise time needs to be less than ~6us. These results make sense and tend to agree with what we experienced with our DMM squarewave measurement tests, when we introduced a ~5us time constant LPF and saw a -2.5mv error in ~2.5Vrms reading!

If a meter fails on the square wave test, it can still work pretty well with a sine. An even if a meter reads fine with the square, this does not guarantee that it works OK with a sine. It is a different and relatively extreme test pattern. With the usualy AD637 or similar chips it would not detect a failing averaging capacitor, or the effect at low frequencies where the capacitor is at the low end.

Agree, the effects on an unknown meter would be questionable until evaluating such, too many unknowns. However we now know how the KS34465A, DMM6500 and 34401As behave wrt each other with the squarewave tests we've done

Best,

[tautech]

If interested, we can send the geber files & partial BOM for the shown PCB, if I can find them   Not sure how to post them here tho, so PM.

When posting attach them by expanding the Attachments and other options link below the text window.

Note the allowable file types of doc, gif, jpg, jpeg, pdf, png, txt, zip, tar, c, h, hex, bas, xls, odt, asm, wav, aiff, wma, mp3, flac, asc, ods, xlsx, py and 7z however the server can be fooled to accept other types by just adding .txt to the filename however if done always add a note to what the correct file extension is.

[alm]

A simple thought experiment will show this is thinking is flawed. If you consider slowing the edges down, the extreme case shows the eventual waveform will approach a triangle wave which we all know has an RMS value of 1/(sqrt(3)) not unity as the normalized squarewave. That's a significant reduction in RMS value and shows a strong relationship between RMS and edge slowing the edges down reduces the squarewave harmonic content which reduces the energy available from the waveform and thus reducing the RMS reading.

What's the problem? Say you built a circuit to produce a triangle wave with an amplitude that's very close to a DC level Vdc. You can compare the RMS reading of the triangle wave to Vdc/sqrt(3). Why is this worse than comparing the reading to Vdc/1?

Capturing the harmonic content is another reason to keep the squarewave frequency low as well as a fast edge.

That makes a lot of sense when testing instruments made for wide-band signals like a scope. I'm not sure if it's the most realistic test for DMMs. Generally the configuration of a DMM, like filter settings and reading rates, are tuned to measure signals with a particular, known, frequency and quite limited bandwidth.

[mawyatt]

Here's the schematic, BOM and gerber files with tautech's help

Edit: This includes a DC reference, AC variable frequency RMS reference, resistor reference, and thermistor, all jumper selectable. Some notes for those interested to improve the AC RMS precision would be to use a CMOS 74AC74 FF after the 4060, and CMOS buffer to the discrete output inverter. Or just use the CMOS buffer directly, parallel up all the outputs for a lower impedance. The 74AC74 FF is much faster than the 4060, and the CMOS buffer isolates the output inverter from the FF.

Best,

[mawyatt]

A simple thought experiment will show this is thinking is flawed. If you consider slowing the edges down, the extreme case shows the eventual waveform will approach a triangle wave which we all know has an RMS value of 1/(sqrt(3)) not unity as the normalized squarewave. That's a significant reduction in RMS value and shows a strong relationship between RMS and edge slowing the edges down reduces the squarewave harmonic content which reduces the energy available from the waveform and thus reducing the RMS reading.

What's the problem? Say you built a circuit to produce a triangle wave with an amplitude that's very close to a DC level Vdc. You can compare the RMS reading of the triangle wave to Vdc/sqrt(3). Why is this worse than comparing the reading to Vdc/1?

Sure you can use a triangle, however you'll need a very accurate bipolar reference, good low offset high gain comparator, precision integrator with good capacitor (low dielectric absorption, leakage), good switches, and so on, and you'll need to change R or C values for different frequencies.

Can you be sure your comparator is switching at the precise peak, equal switching delays going up and down, linearity in both directions, all guaranteed by design?? With the squarewave just measure the DC voltage, that's it, no precision parts required, just some common CMOS logic. No problems with drift, offsets, waveform linearity and so on. As an added benefit the AC RMS and DC Average are identical, no other waveform has this feature I'm aware of.

One can use any waveform they desire for RMS measurements, but doubt that anything other than DC is as simple to create and easy to verify, and has identical AC RMS and Average DC levels!! Please if you know of any waveform other than DC that has these properties please enlighten us!!!

Best,

[SilverSolder]

Kind of cool!   

Just thinking out loud and imagining a little here:   One could try measuring signals with a duty cycle other than 50% with this kind of precise amplitude and see how the meters deal with it...  so, for example, the error at e.g. 1% PWM might tell you something about how a particular meter responds to fast edges, which could then possibly be subtracted out from the 50% PWM test later...  hmmm...

[mawyatt]

That makes a lot of sense when testing instruments made for wide-band signals like a scope. I'm not sure if it's the most realistic test for DMMs. Generally the configuration of a DMM, like filter settings and reading rates, are tuned to measure signals with a particular, known, frequency and quite limited bandwidth.

Think you are missing the point of using the squarewave, never said it was the most realistic or best test, just simply the easiest to implement and verify. Doesn't cost much either

Also mentioned much earlier it's not a replacement for a proper RMS source, but certainly has been handy for our use

Regarding particular frequencies DMMs are "tuned" too, this would be the mains and maybe a few harmonics that come to mind for obvious reasons. Regarding reading rates, some of the newer DMMs like the KS34465A and DMM6500 seem to have higher digitizing rates, although the old HP3458 also had high digitizing rates I believe.

Best,

[mawyatt]

Kind of cool!   

Just thinking out loud and imagining a little here:   One could try measuring signals with a duty cycle other than 50% with this kind of precise amplitude and see how the meters deal with it...  so, for example, the error at e.g. 1% PWM might tell you something about how a particular meter responds to fast edges, which could then possibly be subtracted out from the 50% PWM test later...  hmmm...

You could easily create waveforms with 25% and 75% duty cycles with some simple logic. A little math should show what the True RMS values and average DC values should be.

Edit: If you like this stuff here's another discussion about using this CMOS FF concept.

https://www.eevblog.com/forum/projects/fun-circuit-to-play-with/msg3109636/#msg3109636

Best,

[David Hess]

I have gotten similarly good results using precision square waves for verifying the performance of and calibrating the average AC and RMS AC measurements of my various meters to their accuracy limit.  A precision sine wave source would be better of course, but the coat has to be cut to fit the cloth.

[alm]

One can use any waveform they desire for RMS measurements, but doubt that anything other than DC is as simple to create and easy to verify, and has identical AC RMS and Average DC levels!! Please if you know of any waveform other than DC that has these properties please enlighten us!!!

You keep talking about this unity factor as if it's something magical that is exact by definition. Say you take your chopped DC circuit, run in through a slew-rate limiter. Now say you do the math, and figure out that for the slew rate, frequency and amplitude, the RMS value is (0.95 +/- 0.01)*Vdc. How is this any less accurate than comparing to (1.00 +/- 0.01)*Vdc? Are you doing a bridge circuit with a thermal RMS converter where you are directly comparing the analogue RMS value of two signals without digitizing them, like with the old Fluke 540B? Or are we not talking about a unity factor, but about a very precisely defined factor that may be pretty much any value within a factor of two or so of unity, just to stay in the same DMM range?

Think you are missing the point of using the squarewave, never said it was the most realistic or best test, just simply the easiest to implement and verify. Doesn't cost much either

That's fair, but you made it sound like the higher harmonics content was an advantage of this technique, while I think it's a distinct disadvantage, though maybe an acceptable ones given the advantages you mention. I know it's easy, that's why Tektronix used it. But I think you should be very careful to use it on a not previously tested DMM, and might need to characterize the DMM before you can reliably use this method. Especially DMMs with a bandwidth less than the audio frequency range would be a concern. Or if you would want to use a similar technique for current. Many DMMs, like the 34401A, have a much narrower bandwidth for current (5 kHz in the case of the 34401A). But for DMMs with a wide bandwidth it's a neat solution indeed!

Regarding particular frequencies DMMs are "tuned" too, this would be the mains and maybe a few harmonics that come to mind for obvious reasons. Regarding reading rates, some of the newer DMMs like the KS34465A and DMM6500 seem to have higher digitizing rates, although the old HP3458 also had high digitizing rates I believe.

In the case of the 3458A, you have the choice between three different built-in AC modes that change the low frequency and high frequency cut-off and may make assumptions about the repetitiveness of the signal, and in addition there's Swerlein's algorithm which assumes the signal is a low distortion sine-wave with a frequency up to 1 KHz or so. In addition, you could digitize the input signal at up to 100 kS/s (depending on the desired resolution). So before you would do an accurate ACV measurement, you would identify which kind of signal it is. Presenting such an instrument with a wide-band signal would be pretty much a worst-case requiring use of the analog True RMS converter.

[mawyatt]

One can use any waveform they desire for RMS measurements, but doubt that anything other than DC is as simple to create and easy to verify, and has identical AC RMS and Average DC levels!! Please if you know of any waveform other than DC that has these properties please enlighten us!!!

You keep talking about this unity factor as if it's something magical that is exact by definition. Say you take your chopped DC circuit, run in through a slew-rate limiter. Now say you do the math, and figure out that for the slew rate, frequency and amplitude, the RMS value is (0.95 +/- 0.01)*Vdc. How is this any less accurate than comparing to (1.00 +/- 0.01)*Vdc? Are you doing a bridge circuit with a thermal RMS converter where you are directly comparing the analogue RMS value of two signals without digitizing them, like with the old Fluke 540B? Or are we not talking about a unity factor, but about a very precisely defined factor that may be pretty much any value within a factor of two or so of unity, just to stay in the same DMM range?

Having a waveform that has the exact same RMS and Average DC value is a big advantage. One can take a DMM, measure the DC Reference voltage, then measure the Average DC value of the waveform using a simple RC LP using the same DMM. This ratio should be very close to 1/2 or somethings wrong!! Then measure the AC RMS waveform value, it should also be very close to the Average DC value and 1/2 the DC Reference measurement, unless something is wrong or the DMM can't handle the waveform.

This is exactly why we wanted to compare the RMS computational DMMs like the KS3465A and DMM6500 and the RMS analog chip DMMS like the 34401A. The fact that they all behaved quite well it a tribute to the design and implementation of these fine instruments, so hat's off to Keysight, Keithley and the older Agilent and HP

Best,

[David Hess]

One can use any waveform they desire for RMS measurements, but doubt that anything other than DC is as simple to create and easy to verify, and has identical AC RMS and Average DC levels!! Please if you know of any waveform other than DC that has these properties please enlighten us!!!

Precision square waves, pulses, and triangle waves are the simplest to create from a DC reference without leveling.

[David Hess]

Having a waveform that has the exact same RMS and Average DC value is a big advantage. One can take a DMM, measure the DC Reference voltage, then measure the Average DC value of the waveform using a simple RC LP using the same DMM. This ratio should be very close to 1/2 or somethings wrong!! Then measure the AC RMS waveform value, it should also be very close to the Average DC value and 1/2 the DC Reference measurement, unless something is wrong or the DMM can't handle the waveform.

And an average response calibrated for sine waves should read Pi/(2*Sqrt(2)) high.

[mawyatt]

Early this morning freed up the KS34465A from the on-going System testing. Forced the CMOS 4060 2^14 divider on the PCB using a 2^14 * 60 signal from an AWG to get a 60Hz Squarewave waveform. Here's the results.

Vdd Reference     =5.0001813VDC
Vsqwave Average = 2.5000648 VDC
Vsqwave RMS      = 2.5000922 VRMS

Set the DMM to  mx+b Scaling, Slow Smoothing Filter & Statistics, Auto Zero On, 100 PLC and Input Z Auto.

No attempts were made do anything other than hook up the DMM and force the Squarewave to ~60Hz, although the AC wasn't on as things were warming up from the colder night and a storm just passed through.

So you could say maybe, the temp change, humidity, phase of the moon or whatever influenced the measurements, but think these are quite good results for the KS34465A IMO, certainly better than expected

Now back to some testing so we can afford another DMM

YMMV

Best,

[mawyatt]

Somewhere/how we've damaged the device. Was going to take some measurement late yesterday and getting significant errors when using the Squarewave. A scope trace showed the squarewave hosed up, so first thought was blown output P & NMOS. These were replaced, same result, then replaced CD4060, same result. Did more troubleshooting and found the ADR 4550 gave proper 5 volts when in DC mode, but output dropped during AC mode.

Figured P and/or NMOS was still bad, and drawing too much current and pulling the ref down, so replaced both again with same result. Then figured the ADR4550 couldn't supply the current of the CD4060 (~10ma at 4MHz clock), so added a bypass resistor from U1 8 V regulator feeding the ref to the ref output to help supply additional current. This helped but didn't achieve a precise 5 volt ref output. We didn't have another ADR4550 handy so used a ADR435 (same pinout) which has a higher output capability, and problem solved!!

We must have momentary applied an over voltage to one of the exposed Banana jacks and blew the MOS drive device in the ADR4550 output, it could supply an almost no-load condition for DC measurements, but no additional current under AC mode. If we ever do a PCB respin will add some small series protection resistors and diodes.

We updated the schematic and everything is back to working altho we've lost the couple years burn-in of the ADR4550 reference 

A couple measurements with a new (few months) KS34465A revels everything is back in order!!

VDC 5.0007415V
250Hz Squarewave
VRMS (SD) 2.5000447V


Anyway, updates in case anyone is considering one of these.


Best,

[mawyatt]

Updated measurements 4/15/24, setting for 250Hz Squarewave.

@ 42% RH 26.3C

Model                                  Ref DCV                   SW DCV Avg                   SW ACV (RMS)   

KS34465A Grey (new)         5.0008727                2.5004551                       2.499903           
KS34465A Grey (~1.5y)      5.0008817                2.5004559                       2.500101
DMM6500                           5.000880                  2.500445                         2.499920           
~25y old Fluke 87               5.00                         2.500                               2.506                 
KS U1233A                         4.999                       2.499                               2.509                 

Few hours later @ 41% RH 25.6C Only 1/2 to 1hr Warmup

KS34465A Tan                    5.0008538                 2.500445                         2.500140
HP34401A                          5.00080                     ---                                  2.49970
AG34401A                         5.00080                      ---                                  2.50004
SDM3065X                         5.00081                     2.50039                          2.49800

The 34401As Square Wave DCV Avg bounced around some, needed some averaging so not recorded.

Edit: Added UT210E 4/17    4.99                           2.49                                2.45

Additional results added:
Fluke 77                              5.00                          2.497                                2.706 (2.4363 RMS eqv)*
DHO814                              5.0282                       2.5178                              2.5086
SDS814X                             5.0135                       2.5036                              2.4926

* Average Responded AC which (pi/2)/rt(2) times RMS, so RMS is 0.9003163*reading.

Anyway, seems the new ADR435 reference (see post above) is settling in, and all the meters agreeing within reason. Also the questionable "Square Wave" seems to produce acceptable results with all our DMMs, handheld and lab bench types, analog or digital RMS, so will continue to rely on this for AC RMS sanity checks

Of course, as always YMMV.

Best,

[Majorassburn]

Those are terrific results from a wide variety of DMM's. You really have that nailed as proof of the DC square wave as viable and accurate for AC range DMM checking. Of course, you are doing it much more professionally than I have done with my eBay device. I used a CD4047 as the basis for my quick checker only because I had dozens of them laying around. Here's a link if you're interested.

https://www.ebay.com/itm/285781922404

Please keep updating your test results because you have some of the most popular DMM's at your lab and it's nice to know that the AC functions are displaying within their specs, albeit at very low voltages.

One of my future experiments will be to boost the DC square wave voltage to see if your great results hold on higher AC ranges.

[alm]

I think it would be interesting to test on lower end DMMs, like average responding meters, or meters with just 50-400 Hz bandwidth. These are all pretty high end meters.

[mawyatt]

Lowest end DMM we have is UT210E, added see above.

Best,