Electronics > Beginners
Why difference between scope measured voltage and DMM voltage
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vk6zgo:
To start with, forget about the displayed voltage on the 'scope.

Vertically centre the trace with the input signal removed, then on "DC coupled", apply the signal & see whether the ac waveform is symmetrical about the zero line you just adjusted for.

If it is not, you have a DC component to the signal, so RMS will not be simply 0.707 times Vp, & needs to be calculated differently.

If you have such a complex waveform, but want to look at the ac component only, switch your 'scope to "ac coupled".
Your voltage readings should now be what you expect.

If not, as a sanity check go "retro"----- count the squares, then knowing the volts/div, you can determine Vpp.

Half of Vpp is Vp, which you then multiply by 0.707, giving you volts RMS.

I would suggest that you go up a bit in frequency to say, 1kHz.
50 or so Hz may be getting near the bottom of the 'scope pass band in "ac coupled" mode.
prwiley:
Thanks again for the helpful info.

As will come as a surprise to no one, probably, it turns out that there is nothing wrong with the scope of the meter, but only with my understanding of and expectations about what I was looking at. Exe's comments re possible problems with Brymen meters go me to search and I came across this very helpful video:



"RMS Voltage for Sine and square waves, and why your DMM might not work right!" in which vk6zgo's 0.707 factor is explained.

Since I could reproduce the results in his first example for sine waves, I realized there was nothing wrong with my equipment.

I didn't really understand that BM235, when in AC mode, is always displaying RMS voltage. So, the Vrms in on the scope and the Vin measured on the meter are only going to match when I am sending Vrms out of the signal generator -- and they almost do at 1.000 Vrms out of the DG1022 the scope says 1.0-1.02 Vrms, the meter says 0.986V.

Sending 1.000 Vpp out of the generator, the scope sees 1.20 Vpp and 363 mVrms, which is as it should be and the meter sees 349 mVrms

Somehow I was expecting Vpp in on the scope to match V on the meter, and of course that's the totally wrong expectation. :-[ I think I know better now.
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