Oscilloscope and Dmm doesn't match

Hello everyone,

I was testing some circuit and realise that my scope and dmm doesn't match. My oscilloscope always shows more voltage than the dmm. What can I do to fix it?
Here is an example.

,
Oscilloscope shows 3.476 while the DMM shows 3.37. I think the problem is in the oscilloscope because I made this test with a fluke DMM some time ago and the result was the same.

Thanks.

What are the error bounds on the two measurements?

Do you have a good reason to believe either one, like a carefully calibrated voltage source for testing?

Send them to a calibration facility, have them calibrated.

Before you assume there is a problem, you need to know what the voltage your measuring actually is.

They only differ by 0.1V so it is it much of a problem?
Are you needing critical measurements like this?

Your problem is you don’t know which one has changed and without a standard you can’t tell.

Thanks for the answer.

I don't have a calibrated voltage source but I did the same experiment again with 2 DMM's this time.

Scope 3.47
Chinese DMM 3.36
Fluke DMM 3.32

The device manuals will specify the error bounds.

Thanks for the answer.

No.It just makes me frustrated.

Thanks for the answer.

I made a second experiment.

Scope 3.47
Chinese DMM 3.36
Fluke DMM 3.32

Give a person a clock, and they know what time it is.

Give a person two clocks, and they aren't sure.

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Thanks for the answer.

It seems like I'm going to send it for calibration.

Thanks for the answer.

So, I need a third clock. Right?

Rather more expensive than reading the manual, and discovering that "it" is within spec. You will be charged regardless.

How would you decide which instrument to send for calibration? If you pick one and send that off, then you could still end up with different readings to the other instruments.

Just because 2 instruments give the same reading, does not mean that they are right. They could both be equally wrong.

Rigol scopes have a built in Self Calibration feature.

I think it’s under the Utility menu.

All probes are to removed during this process and the scope should be turned on for 30 minutes to warm up.

Search your manual to see how to accomplish this calibration.

Thanks for the answer.

I will try it.

I am looking now.

In the OP, the difference is 3% of the DMM measurement. Assuming the DMM value is correct (a poor assumption), the scope value is within error bounds of the Rigol DS1054Z.

Don't know about the DS2202A.

Thanks for the answer.

I will write again after I complete the calibration process.

Tolerance is something you have to get used-to... I used to drive my dad crazy when I'd say "close enough" :smiley:

Nothing in analog is perfect. If you send your 'scope and meter out for calibration they probably still won't measure exactly the same but they will be within their allowed tolerance (or the calibration lab will tell you they can't meet the specs, etc.).

Sometimes we'll have a plus-or-minus spec and sometimes there will be a maximum or minimum but every thing has a variation/tolerance and every measurement has an error/ tolerance.

Oscilloscopes usually aren't as accurate as a meter. Oscilloscopes are for "looking at waveforms." The display is approximate and the displayed measurements are an "added feature". But of course it depends on the particular piece of equipment so your 'scope may be more accurate than my meter, etc.

...Eggs are digital - There is exactly one dozen eggs in a dozen eggs. Milk is analog - You'll never get exactly one gallon of milk but there are legal requirements so it can't be too much less.

I noticed that on the oscilloscope you have the zero level on the centre line of the graticule - that means that you are only using half the range of the oscilloscope's ADC.

Try moving the zero level to near the bottom of the screen and go on to the next more sensitive range (500mV/div). See if that makes a difference.