Electronics > Beginners
Why difference between scope measured voltage and DMM voltage
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prwiley:
I recently purchased a RIGOL DS1054Z and have been having fun using it to look at a little CanaKit amplifier that I built awhile ago (https://www.canakit.com/20w-bridged-audio-amplifier-kit-ck193-uk193.html).

I am looking at the voltage across the output (speaker connection) with the scope and my EEVBlog BM235 at various volume levels. One example: With a 60Hz signal a 1.0Vrms from my Rigol DG1022 the scope measures 6.22Vpp and 6.06Vrms. Screen shot of scope is here: https://photos.app.goo.gl/i63DGD5UYSyQN9sz9

My BM235 DMM, EEVBlog edition, is set on V auto is measuring 5.339V.

Seems like a big difference between the two to me, but what do I know, I'm coming back to all this after many years.

Could someone suggest what the source of the difference might be? I not that at any setting of the pot, the DMM is generally between 70 and 75 percent of the scope value.



Benta:
Try AC coupling your 'scope input.
It's just a switch.
exe:
yep, I think it's coupling "issue". I see sine amplitude is 7.68-3.18=4.5V. The AC RMS value should be 4.5/sqrt(2) ~= 3.2V. Not sure why it says RMS is 6.06V, I calculate it to be Vac_rms + Vdc = 3.18 + 3.2 = 6.38V :).

Anyway, suggestions: try to make waveform taking the whole screen and display more periods if possible. This makes reading more stable. But always use sanity check, often automatically calculated numbers are "incorrect" due to many reasons (e.g., coupling, too low amplitude, noise, incorrect probe attenuation, not enough bandwidth, loading effects, etc).

UP: formula I used is incorrect, see https://electronics.stackexchange.com/questions/240242/using-a-voltmeter-to-measure-rms-value-of-a-sine-signal-with-a-dc-offset
rstofer:
Unless you have terminated the cable at the scope with a 50 Ohm terminator, make sure that your signal generator is set for Hi-Z output.  If you have terminated the cable in 50 Ohms, make sure the signal generator is set for 50 Ohm output.  This causes a x2 error.

Next, make sure that your scope probe setting (x1-x10) agrees with the scope input setting.  Press the CH1 button briefly and there is a button for Probe 1X versus 10X on the menu.

As discussed above, make sure you use AC coupling.  It is also available on the menu with the Probe setting.
prwiley:
Thanks everyone,

55Hz, 1Vpp. scope sees 4.48Vpp, DMM 3.544V

Probe agrees with probe attenuation software setting: check
More wave on screen: check
Signal gen output "High-Z": check
Tried AC coupling: check, but:

https://photos.app.goo.gl/awCr2RLSar5pxijB9 
https://photos.app.goo.gl/Hwap1VX3tEU45TDK7

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