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I got a bit confused about how AC voltmeters on digital multi-meters work. I don't want to know about for example sampling method or conversion of analog to digital data. I need to know whether it can only measure the sinusoidal wave with only zero offsets.

I tried to compare the AC voltmeter and oscilloscope responses together in one of the electronic simulators programs (Proteus.) I know about the difference between the Vrms and Vpeak. When the AC signal has the DC offset the AC voltmeter shows an unacceptable value. For example, by using the signal generator, I applied a sine wave by Vp-p= 12 V and DC offset = 0 as you can see in the following picture. It shows the correct answer. Vp-p/2 = 6 V => Vrms= 6/sqrt(2) = 4.24 V

Example01

Now I'm going to add the DC offset to my signal by changing the signal (from "bi to uni" in signal generator settings.) If you check the image you will see that the amplitude of the signal and even the frequency has remained constant while the AC voltmeter shows the value of 7.34 V. How does the voltmeter calculate this value?

Example02

Shouldn't the AC voltmeter remove the DC level of the signal automatically and only calculate the differential voltage between the maximum and minimum voltage?

JRE
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Mohammad Reza Hasanpour
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    There is no standard for multimeter AC voltage measurement design. If you [edit] your question to give the make, model and datasheet link for the meter in question someone may be able to help. What was "unacceptable" about the value the meter displayed? What value *did* it display? What value did you expect? – Transistor Dec 12 '21 at 21:38
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    If you want to measure an AC signal with DC offset, use a series capacitor. – DrMoishe Pippik Dec 12 '21 at 21:49
  • Depends on the implementation of your multimeter. Can you measure the DC value separately and subtract that from your calculations? – winny Dec 12 '21 at 22:59

2 Answers2

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Your meter is actually doing what it's supposed to. It's measuring the RMS value of its input. For the first signal, with no DC offset, the RMS value is for a sinewave of 12 VP-P which is 4.24 V. With a DC offset the RMS value is given by \$ \sqrt{ AC_{RMS}^2 + DC^2} \$ which is \$ \sqrt{4.24^2 + 6^2} \$ which is 7.34 V.

If you don't want the DC offset to be included, add a series capacitor as already suggested. Many meters (especially analog such as the Simpson 260 and Triplett 630) include an internal capacitor for this purpose. This mode is often called AC output as it was intended for measuring the output voltage of an AC amplifier without the DC bias voltage.

Transistor
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Barry
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    Of the answers, I think this is better. It provides a quantitative explanation. And that trumps most things, to me. +1. I'll add, though, that my [Tek DMM916](https://i.stack.imgur.com/8kcCv.png) does both at once, reports them separated if needed, and shows the AC frequency, as well. – jonk Dec 12 '21 at 23:59
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Some general talk only, no comments about the meters in your simulation program.

Most multimeters filter out DC offset and show in AC range only the AC component assuming the offset doesn't lift the peak voltage too high. If you have say 100mV AC with 20V DC offset you must insert a highpass filter to remove the offset - that's said already in a comment which says "use series capacitor".

DC voltage ranges filter out the AC and show the DC-component (=the average of the voltage)

Measured AC voltage in multimeters is most often actually the average of rectified AC-component, only scaled to the right RMS value by assuming the voltage is sinusoidal.

True RMS meters exist, but an oscilloscope gives so much more info of electronic circuits than a multimeter that having one is a superior advantage over the situation where one must guess the waveform. The voltage math and finer signal analyze modes in modern oscilloscopes are a big bonus.

Unfortunately the situation is that if one wants a good and low cost instrument he must buy both of them. That makes an oscilloscope unaffordable for many of us.

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    I like this, too. +1. But no quantitative explanation. Which, given the OP's writing which does include quantitative values in need of explanation, is a lack. But some nicely added thoughts to offer the OP, just the same. As I noted already, my [Tek DMM916](https://i.stack.imgur.com/8kcCv.png) does both AC and DC offset at once, reports them separated if needed, and shows the AC frequency, as well. These details alone make this meter still very useful when a scope isn't handy. You can get a lot out of it without the scope. – jonk Dec 13 '21 at 00:03