# Difference between AC and DC signal

Discussion in 'Analog & Mixed-Signal Design' started by kalindiberad, Dec 26, 2018.

Dec 25, 2018
6
0
What is the difference between AC and DC signal?

2. ### crutschow Expert

Mar 14, 2008
23,121
6,856
AC (Alternating Current) voltage varies with time.
DC (Direct Current) voltage is steady and does not vary with time.

Jul 18, 2013
18,847
5,890
AC also reverses direction of current (alternates polarity each cycle).
Max.

4. ### MrChips Moderator

Oct 2, 2009
19,132
6,148
The meaning of AC and DC depends on the context. Either explanation given above can be applied depending on the context of usage.

1) From a frequency domain perspective, DC is zero frequency, AC is non-zero frequency

2) From a time domain perspective, DC never changes sign. AC has sign reversal.

5. ### recklessrog Well-Known Member

May 23, 2013
988
387
A DC signal, can mean a varying DC level, such as the output from an analogue sensor. For example, the output from a thermo-couple will vary with temperature etc.

Last edited: Dec 26, 2018
6. ### wayneh Expert

Sep 9, 2010
16,102
6,219
Bingo.

For instance the power coming off a full wave rectifier can be called DC even though it has a huge voltage swing. But the current does not reverse direction.

Pure DC is almost an abstract concept, since any DC source will likely have some noise in it and that noise can be called the AC component. Likewise an AC signal might contain an offset, a DC component.

Jul 18, 2013
18,847
5,890
My personal perception and the one I have always adhered to has always been the 'Alternating' refers to the current change in Direction through the conductor.
Max.

8. ### MrChips Moderator

Oct 2, 2009
19,132
6,148
Here is an example of where AC may or may not change direction.
When analyzing an amplifier circuit, there are two types of analyses, DC and AC.

In DC analysis, one studies the current and voltages of the steady-state conditions, quiescent or non-quiescent.
In AC analysis, one studies the signal amplification. The input and output signals need not change sign.

9. ### KeepItSimpleStupid AAC Fanatic!

Mar 4, 2014
3,557
671
Need to add a distinction between DC and AC coupling for a scope.

DC includes the non-time varying part. AC just shows the time varying part of the signal.

10. ### crutschow Expert

Mar 14, 2008
23,121
6,856
That seems too restrictive.
It means you can't call a varying signal riding on a DC level, such as in an AC amplifier "AC" whereas in practice it is always an AC signal.
To me, if you can see it using the AC coupling on an oscilloscope, it's "AC".

Jul 18, 2013
18,847
5,890
I agree, the AC coupling removes the DC reference point and uses the non-offset trace datum as the zero point where the reversal occurs.
Max.

12. ### wayneh Expert

Sep 9, 2010
16,102
6,219
That feels right, but the problem is that a varying voltage can very easily be explored to produce an alternating current, for instance with a single capacitor to couple.