Hi, I am trying to measure the rms voltage for a slightly distorted AC sine waveform. I used a "True RMS" multimeter as well as an oscilloscope. Expecting both rms readings to be the same, but different. Any idea why? Thanks.
They will only be the same if they are a clean sine wave, and both are in calibration.
-- ? Michael A. Terrell
Using the scope, one can only measure the peak or peak-to-peak voltage, then convert to RMS. Not any different than the way AC scales on most DVMs are implimented.
I must be confused...cause he said he used a "True RMS" meter. A True RMS meter, by definition, measures the RMS value of the waveform. If the waveform is only "slightly distorted", a TRUE RMS meter should measure within the specs of the meter.
I don't know about your scope, but mine has a button that calculates the RMS value of the waveform. I didn't bother to look up the accuracy spec.
It would be interesting to know the exact equipment used and the magnitude of the discrepancy.
One common source of such problems is DC offset of the AC waveform.
mike
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What's the frequency? It is within the range of the meter? How different is different? Scopes are typically not better than a few %. My HP 3457A
6.5 digit multimter has an spec of only +/- 10% of reading at 1 MHz.If the difference is very significant, take a known scope and eyeball it.
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It may or may not be your problem, but on my scope (Lecroy) when you use the "rms measurement" function it calculates the rms value pf the signal either through the entire memory or between the two cursors. So the result is NOT the same than an RMS voltmeter except if the two cursors are positionned exactly separated by an integral number of signal periods...
Hope that's helpful...
Friendly,
-- Robert Lacoste ALCIOM - The mixed signal experts
You are fortunate to have the "read in RMS" option on your scope. But, i bet that it works by measuring peak or Peak-to-peak and converting that to RMS.
OK, what are you betting? Send it to me. It's a TEK TDS540. Suppose one could look up the spec. It can do FFT; it's hard to imagine they'd not do a proper RMS calculation. mike
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I think it would be false advertising to call it RMS and then just apply a factor to P-P.
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Try it on a battery.
For sure. But then at least it wouldn't matter what factor you chose -- they'd all be wrong.
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Which waveform has the greatest discrepancy between average and rms values?
Norm Strong
What do you mean by "average?" The average of a sine wave without offset (or any waveform that is symmetric around zero) is zero.
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TEK TDS540 reads peak and RMS as same (within A/D resolution) on a 9V battery. mike
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Hi Dave,
It is a 50Hz 22V rms AC sinewave. The rms reading from my HP54600b can be upto 5% larger comparing with my Fluke 45 meter.
I assumed that a scope would prove a more accurate true RMS reading. Have I made a wrong assumption?
regards
Phillip
NOW i know something i did not previously: what make and model of the scope. Since it digitizes, then it could calculate energy of each sample and thus the equivalent RMS value. If one waveform was converted this way, and if there were (say) 8 bits per sample, then about 3 bits would be used for the + half and 3 bits for the - half and one for zero; this is a bit (no pun inended) better than measuring a peak (or Pk-Pk) and converting. Higher bit resolution would tend toward better accuracy for complex waveforms. Still, no where as good as using the thermal method...
Unless the A/D of the scope has more resolution than the HP or Fluke, expect less accuracy.
Resolution and accuracy are different things.
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This is going from funny to sad. Half of 8 bits is 7 bits.
Nowhere as good as the thermal method at the NIST...but probably on par with a typical cheap commercial thermal rms converter. mike
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Agilent published a few white papers on the subject. In particular they pertained to DMM and RMS issues. The items from Agilent are: "Application Note AN 1389-3" and "Application Note 1392". Hopefully they may explain the difference from the DMM and the oscilloscope.
Tom
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