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I've made a AC voltage measurement board, the structure is simply:

input -> gain -> block capacitor - ADC.

Now I want to calibrate it. Because the input may be sub mVs, so I'm considering calibrate it by using a DC input (sure, I need bypass the block capacitors). And I've heard someone said it's convenient to calibrate AC channel using DC input, because a DC can be measured more precisely. Is it true? How the commercial DMM's AC measurement channels are calibrated, particularly at small signal input?

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Professional voltmeters are calibrated with precision AC sources (if you bypass a component, you are changing the circuit, and if you change the circuit your initial calibration is not guaranteed). That being said, I'd be surprised if any sub $30 meter had any kind of serious calibration (anything that's can't be automated is unlikely to happen in a low cost mass production factory).

But for your application, it depends on how accurate you want to be, if you're not looking for super high precision (you're not trying for microvolt precision are you?), then bypassing the cap and checking the gain with a DC signal sounds like a rough and ready, but still valid, test to perform. If you can, get a function generator/signal generator/Arbitrary waveform generator, that way you can dial up whatever AC frequency and amplitude you want and use that to test your setup (if it's a function generator , you can probably generate all kinds of different AC waveforms too).

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