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Old 02-01-2011, 02:10 PM
mendiola_loyola mendiola_loyola is offline
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Question Difference between Oscilloscope and Multimeter voltages

Dear All:

One question, I have a multimeter that shows 24.0 volts DC but when I measure the voltage on the DSO-2090 Oscilloscope the Vrms voltage is higher.

The oscilloscope Vrms is 25Volts

What is the true measure the oscilloscope or the multimeter (Brand Nitsuko, Model UR-3203).

What Kind of Voltage is showing the multimeter, the v Base?

I'm attaching a picture with the oscilloscope voltage measures.

The vertical Division is 10V

I'm using a 10x probe.

Alfredo Mendiola Loyola
Lima, Perú

Last edited by mendiola_loyola; 02-01-2011 at 02:27 PM.
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Old 02-01-2011, 03:38 PM
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Adjuster Adjuster is offline
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You mention attaching a picture, but I cannot see it.

In general a multimeter is likely to be more accurate than an oscilloscope when measuring DC voltage. Errors of a few per cent plus an LSB or two may be acceptable when using a scope (check the specs of your instrument). Further error may result from using a probe.

Another issue when using a Vrms measurement from a scope is that it may respond to higher frequency signals or noise which may be ignored by the multimeter. If you really want to measure the DC component of the signal, you would be better to use an average voltage function (Vavg?) assuming that the scope has one.

Of course, either instrument could be suffering from calibration drift or damage, but that's another story.
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Old 02-01-2011, 04:16 PM
someonesdad someonesdad is offline
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You're learning about the old saw "The man with one watch knows what time it is. The man with two watches is never quite sure."

Without calibrated standards to compare to, you simply don't know which instrument is giving you a correct reading (and maybe both of them are lying).

The short answer is you'll need to either make some calibrated sources and send them to a cal lab or send one or both of your meters to be calibrated. Don't be surprised if the services cost more than the instrument.

A number of years ago I made a simple voltage reference from some AAA batteries, a resistor, and an LM285Z integrated circuit. I measured its voltage output over 24 days and found that it was a nice, inexpensive voltage reference (see attached graph). You could easily build yourself one for a few dollars -- follow the instructions in the data sheet. But you'll need to get it calibrated if you don't have a lab-standard voltmeter.

A standard source for an AC voltage is more work, but you can find things on the web if you look. As Adjuster said, you also need these standards to determine whether your scope's RMS measurement is calibrated to DC levels. One of my scopes, for example, reads RMS down to DC, but is off by 10% for a DC voltage.
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Old 02-01-2011, 04:33 PM
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kubeek kubeek is offline
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I suggest building a true RMS meter if you really need RMS figures. Your oscilloscope is calculating the RMS from its 8bit samples, which will never be very accurate. I couldn´t find anything on the DMM, so the oscillo may be the better one. Check the accuracy rating of both and see which one should give you more accurate readings.
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Old 02-01-2011, 08:08 PM
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nomurphy nomurphy is offline
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Most scope manufacturers do not guarantee vertical accuracy, I think LeCroy may on some of their higher-end scopes.
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