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Pretty much what the title says. I'm fairly confused with this simple question. Shouldn't waveforms which change amplitude be AC?

EDIT: So this is about a RAMP Type DVM (Digital Voltmeter). Can the input signal be both DC and sawtooth is my question?

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Just don't get too hung-up on calling something ac or dc. If there is an ac signal superimposed on a DC level then that is exactly what it is, an ac signal + a dc level.

Calling that composite waveform either dc or ac is missing the point.

You wouldn't call a battery an ac source even though it gradually discharges and then possibly is recharged - that would be what could be described as a signal with dc and a sawtooth.

Give it a full description is my take on things.

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  • \$\begingroup\$ It's about a RAMP Type DVM which operates on DC. In that case, can the input be a sawtooth signal? \$\endgroup\$ – user3484582 Jul 1 '15 at 19:11
  • \$\begingroup\$ I have no idea what you mean. please explain \$\endgroup\$ – Andy aka Jul 1 '15 at 19:25
  • \$\begingroup\$ A Digital Voltmeter with a ramp voltage. Link \$\endgroup\$ – user3484582 Jul 1 '15 at 19:57
  • \$\begingroup\$ Are you perhaps asking what the reading DVM might display with this sort of composite waveform? If you are then it depends on what sort of DVM you have and some only measure the average voltage on DC whilst others will give the true RMS irrespective of waveform. \$\endgroup\$ – Andy aka Jul 1 '15 at 20:12
  • \$\begingroup\$ I'm asking if it's possible actually. To clarify, I have the following question and have to chose the correct answer: Ramp-type DVM accept as input: a) AC voltage b) DC voltage c) Sawtooth d) Triangle \$\endgroup\$ – user3484582 Jul 1 '15 at 20:13
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A dual (or single) ramp DVM works by integrating the input voltage over a (usually fixed) period of time. A dual slope DVM then integrates a reference in the opposite direction and measures the time. The voltage measurement is the time divided by the integration period times the reference voltage.

The integral of a fixed voltage is a ramp.

The integral of a sine wave on top of a fixed voltage is a -cosine on top of a ramp (and if the period is chosen to be an integral number of cycles, the effect on the end voltage is nil.

To measure an AC voltage with a slope DVM, the AC voltage is converted to a DC voltage with an RMS to DC converter or a precision rectifier and then measured as above (usually with some scaling factor to give equivalent-to-RMS reading for sinusoidal inputs in the latter case). It is not measured directly.

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