AC RMS DMM tests

[mawyatt]

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.

If you have the "B" version of the CD4047 this supports a 20 volt Vdd, so you would have a 10 volt ACRMS reading!! The CD series of CMOS doesn't have a low output impedance (high n and p FET Rds_on), so a buffer may be in order (why we used a discrete inverter/buffer on ours). We used the 4060 version so we could use a crystal for more precise frequencies.

If you really want to have some fun, try this use with the CD4047 FF out. Precise exact low ripple/noise voltage division without precision components using CMOS FlipFlop, won EDNs Idea of the Year way back

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

Reviewing this link (see post #6) brings up a very interesting thought. In the thread there is a reference to a CMOS circuit that EDN used as the front cover of the Ideas For Design for 1992 and 1993, "CMOS Logic Create Precision Waveforms". This technique is capable of producing very accurate and precise Sawtooth and Ramp Waveforms, the waveform amplitude peak is set by the CMOS Vdd, so could produce a very accurate result similar to Square Wave except of course the "Scaling Factor" is not 1/2 but 1/(sqrt(3)).

Maybe some will want to play around with this, also note the ramp and triangle waveforms have the same RMS value for the same peak to peak range,.

Anyway, just some thoughts

Best,

[Majorassburn]

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.

A few months ago, I tested this concept on Zoyi ZT219's and Aneng 870's? (both TRMS) as well as a couple of Harbor Freight throwaways (probably average responding) along with a half-dozen Fluke 87V's.

The TRMS meters all liked the DC square wave calibration checks and displayed expected results but the Harbor Freight clunkers' resolution was so bad you couldn't draw any real conclusions from them.

Interestingly, the Zoyi ZT219's proved to be more accurate when checking with the square wave than the Fluke 87V's (the 87V's read about 8LSD's higher at 100Hz) but the Flukes began to far outshine the others when the frequency was kicked up toward 1-2KHz.

NOTE: Both the 87V's and the Zoyi & Aneng's displayed similarly accurate TRMS results when tested on my sine wave modules.

SUMMARY: The square wave checkers work! Maybe not quite as good as a nice sine wave but sure as hell good enough, as mawyatt has indicated.

EDIT: Just dug up some old measurements I referred to above:

CD4047 Source         Fluke 87V           Fluke 87V                  Zoyi ZT219
100Hz SqWv             10.86VAC        10.87VAC         10.810VAC
21.63VDC
Unipolar

[mawyatt]

Even 40+ year old Fluke 77 we haven't used in a couple years is with 2.5%, this is an Average Responding DMM.

Also, added DSOs DHO814 and SDS814X just for fun.

                                          DCV Ref                      SW DC Avg                        SW ACRMS

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.

Edit: Added to pervious post of results.

Best,