Products > Test Equipment
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:
--- Quote from: Kleinstein on January 14, 2022, 07:48:24 pm ---The rectangular waveform can behave quite a bit different for the AC circuit. One point is a possble slew rate limit that may apply.
--- End quote ---
That's why we are using a fast edge and low frequency ;)
--- Quote ---The other point can be a time delay in detecting the polarity reversal in some analog RMS converters.
--- End quote ---
That's why we use a unipolar squarewave ;)
--- Quote ---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.
--- End quote ---
That's again why we are using a low frequency ;)
--- Quote ---
So the rectangulator wareform is a relatively special case. It is still one of the easier ones to generate.
--- End quote ---
Exactly, and verifiable by DC measurement as mentioned. The only waveform that has the same RMS and DC value except, well, obviously DC :)
--- Quote ---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.
--- End quote ---
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:
--- Quote from: mawyatt on January 14, 2022, 09:07:38 pm ---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 :-+
--- End quote ---
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,
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