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Average measuring multimeters more precise than TrueRMS counterparts?

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David Hess:
For what it is worth, the RMS meters that I have which also support average AC return identical results with calibration waveforms.  In cases where I have a choice, I usually select average reading mode because it settles so much faster even though I then have to apply a correction for the waveform type.


--- Quote from: MadTux on January 21, 2023, 03:11:33 pm ---0.5% is plenty enough for any electrical work, for precicion electronics I'd rather get something like a 12bit DSO, where I can measure RMS signal a a waveform and do the math afterwards....
--- End quote ---

A DSO, even a 12 bit one, is unlikely to deliver better results.  Input offset, gain, and linearity will usually be worse, and the RMS conversion will include broadband noise, although averaging will remove that error term.  Older DSOs removed excess significant digits but I think unscrupulous companies stopped doing that to make their DSOs seem more accurate and precise than they really are.

Some digital voltmeters, even 20 years ago, used high resolution sampling and then DSP for RMS measurements like Keysight's "Truevolt" does.


--- Quote from: bdunham7 on January 21, 2023, 03:59:11 pm ---The Fluke models you mentioned actually use the same AD737 for the averaging and TRMS models as the chip has both modes.  I don't know if using it in the averaging mode solves the dynamic range issue and I suspect it doesn't.
--- End quote ---

From the looks of the application notes, average computation mode on the AD737 disables the RMS averaging part so the average of the square root of the square is calculated, instead of the average of the square root of the average of the square, resulting in the same dynamic range.  This would also preserve the same offset and gain errors, and the same frequency response.

In my opinion switching between RMS and average reading mode to produce inconsistent measurements would be worse than limiting the performance of average reading mode, although I have the same complaint about multimeters which produce inconsistent results because their input resistance changes with the range selected; that wasted hours to days of my time before I got into the habit of looking for that particular malady.

mawyatt:
This gets into an interesting area regarding Analog RMS computation vs. Digital.

With Analog you have the limited dynamic range due to the non-linear log type conversions for the square and square-root functions, as well as the scaling and offsets of these conversions.

With Digital you have a sampling system will all the effects and such, but also the benefits of directly doing the computations in the digital domain.

The net result IMO is the Digital solution is superior in most cases, and lends itself to the constant CMOS improvements with feature size reductions. Also, the better DMMs from Keysight and Keithley use Digital computation methods for RMS and those folks certainly know what they are doing, which alone would tilt towards the Digital solution.

Best,

Kleinstein:
Digital RMS is not just used in higher end DMMs. There are also lower end handheld DMMs that use digital RMS as part of some of the chip sets. These cheap solutions usually have a rather limited bandwidth (e.g. 1 - 5 kHz), but otherwise they work quite well. Especially the essentially instant response (similar to the DC part) is very conventient. By nature the analog RMS part is slow from the averaing filter. The filter also limits the accuracy below some 20 Hz.

I see a chance that the DMM chips get better (faster) and replace the old analog RMS solution at least for most of the hand helds. The power metering chips show that it is possible at a low price and good performance.

mawyatt:
Certainly makes sense wrt the CMOS trend of more & faster computational power with smaller chips, which yields lower recurring chip cost!!

Works with the high resolution SD ADCs also, perfect example of how the "analog" world has benefited from the CMOS digital revolution.

Best,

David Hess:
The digital computation might as well be error free, but it is at the mercy of the high resolution sampling, which is no trivial task.  We now have inexpensive integrated sampling ADCs with incredible performance.

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