# Scope: RMS and AC vs DC coupling

#### ApacheKid

Joined Jan 12, 2015
1,273
I was looking at a sine signal from a signal generator, 60Hz 4v pk-pk.

The scope offers detailed stats and I noticed that when DC coupled the displayed RMS value (1.38v) differs slightly from when its AC coupled (1.41v)

Why?

#### crutschow

Joined Mar 14, 2008
33,369
I was looking at a sine signal from a signal generator, 60Hz 4v pk-pk.

The scope offers detailed stats and I noticed that when DC coupled the displayed RMS value (1.38v) differs slightly from when its AC coupled (1.41v)

Why?
Good question.
If there is a difference, I would expect the AC coupled measurement to be the smaller one.

#### MrChips

Joined Oct 2, 2009
29,868
All signals consist of DC + AC.
When you AC couple the signal you are removing the DC component.
When the signal is DC coupled you are viewing DC + AC.

Hence DC coupled RMS should be higher or same as AC coupled RMS.

4V pk-pk is 2V amplitude.
The RMS value = 2/√2 = 1.414V

#### crutschow

Joined Mar 14, 2008
33,369
So neither of us ha a plausible explanation for the difference you see.

#### ApacheKid

Joined Jan 12, 2015
1,273
I have a new hand held multimeter you see. That has true RMS and was reading differently to the scope. But when scope is AC coupled the two readings become very close. I guess the scope is more precise and in DC mode even a small DC bias either +ve or -ve would skew the data it uses to perform the RMS calculation.

#### Marley

Joined Apr 4, 2016
501
All measuring equipment has a degree of accuracy. Look at the specs of the multimeter and the 'scope. This may explain the difference in results.
Also, how "pure" is the sine wave you are measuring? This will affect the peak v. RMS relationship.