Electronics > Beginners
AC and DC Coupling Mode in An Oscilloscope
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wuiven64:
Hi,

I recently using a Tektronix TPS2014 oscilloscope to analyse a square wave.

I would like to ask what is the functions of AC and DC coupling mode in an oscilloscope and how do we choose the suitable coupling mode in an oscilloscope to analyse the waveform.

Thanks.
Brumby:
The DC coupling mode will include the DC offset of a signal.  The AC mode will not.

If you have a 1V AC (peak) + 2V DC offset signal, DC mode will show you the red trace and AC mode will show you the black one and you will not see any indication of DC offset.



What you CAN do, however, is pick a reference point on the waveform (say the top) and check out the trace in DC mode and then AC mode.  The difference in position will be the DC offset.
Brumby:
For completeness, I should add that there is usually a third mode marked as "Gnd".  This shorts the input circuitry of the scope to ground which results in a trace that is a simple straight line - but the important part of this is that this line is at exactly 0V.  (This is the dashed line in the above diagram - and you can move it to whatever level is convenient.)

This allows you to adjust the vertical position of the trace and set a clear 0V reference point.  You can then make accurate observations in AC mode and DC mode.
vk6zgo:
I would suggest that you normally leave the Oscilloscope in "DC coupled".
It will quite happily display ac waveforms in this mode.

If you need to look at an ac waveform superimposed on a DC voltage, & have a particular reason to eliminate the DC, then switch to "ac coupled".

One position will normally become the default, & in my experience, the most versatile position is "DC coupled".
JustMeHere:
I use DC to see voltage levels (signals).
I use AC to measure ripple.

DC will tell me that my power supply is putting out 5v.  AC will me see how well I sit on 5v.
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