AC and DC Coupling Mode in An Oscilloscope - Page 1
Forum section: electronics / beginners

author: wuiven64
author_rank: Newbie
timestamp: November 02, 2018, 11:13:27 am
content: 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.

author: Brumby
author_rank: Unknown
timestamp: November 02, 2018, 11:49:23 am
content: 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 oneand 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.

author: Brumby
author_rank: Unknown
timestamp: November 02, 2018, 02:01:05 pm
content: 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.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 04, 2018, 02:18:20 am
content: 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".

author: JustMeHere
author_rank: Frequent Contributor
timestamp: November 04, 2018, 03:03:38 am
content: 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.

author: wuiven64
author_rank: Newbie
timestamp: November 07, 2018, 10:08:33 am
content: Hi.Thanks for your reply. I would like to ask how to measure ripples in AC coupling mode. Can share an example on how to measure ripple in AC coupling mode?Thanks

author: tautech
author_rank: Super Contributor
timestamp: November 08, 2018, 07:55:03 pm
content: Quote from: wuiven64 on November 07, 2018, 10:08:33 amAC coupled input ripple measurements.1x probe and increase channel sensitivity into the low mV ranges.Timebase should be set to ~5ms for linear PSU's (2x mains frequency) and low in the us range for SMPS.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: November 08, 2018, 08:59:11 pm
content: Perhaps this video may be helpful.

author: TimFox
author_rank: Super Contributor
timestamp: November 10, 2018, 10:03:47 pm
content: Another use for AC/DC coupling (especially for analog CROs) is to switch between AC and DC on a complex waveform. The vertical shift in the displayed signal is the DC (mean) value of the waveform.

author: David Hess
author_rank: Super Contributor
timestamp: November 11, 2018, 11:35:22 pm
content: The ground setting on good oscilloscopes also precharges the internal AC coupling capacitor to the average DC level without blowing out the input protection or front end circuits when switching coupling modes or moving the probe. That is why old oscilloscopes controls have the ground selection between the DC and AC selections;someinclude a special precharge setting or multiple ground settings.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: November 12, 2018, 09:45:03 pm
content: Quote from: David Hess on November 11, 2018, 11:35:22 pmThat's right and is an excellent point. My old Tek and HP scopes have input coupling switches that do just that, by switching through the "ground" position when changing from AC to DC or vice versa.But what does the DS1054z do? I seem to recall some instructions somewhere about precharging input caps on the Rigol but I can't find any mention of it in the User's Manual at the moment. In fact I don't even know if the Rigol actually uses an input cap for AC coupling or if it is done somehow in software. In my ancient Link parallel port DSO you can actually hear a relay click when switching coupling, plus one of its coupling caps is shorted so has no effect.

author: w2aew
author_rank: Super Contributor
timestamp: November 12, 2018, 10:11:42 pm
content: Quote from: wuiven64 on November 02, 2018, 11:13:27 amI did a video on this about 7 years ago:

author: David Hess
author_rank: Super Contributor
timestamp: November 13, 2018, 01:00:38 am
content: Quote from: alsetalokin4017 on November 12, 2018, 09:45:03 pmThe Rigol DS1000Z series implements "ground" in a later stage so it has no way to precharge its input AC coupling capacitor. Its input protection is no better and actually worse so it is likely more susceptible to damage under conditions where this would matter.Implementing ground at a later stage also means that the Rigol DS1000Z series does not correctly report its own input noise when ground coupling is selected. An external short is required. Arguably this is irrelevant though since it does not perform RMS noise measurements correctly anyway.

author: vk6zgo
author_rank: Super Contributor
timestamp: November 13, 2018, 04:59:12 am
content: Quote from: David Hess on November 11, 2018, 11:35:22 pmOscilloscopes with a "DC offset" control are often used to look at an AC component on a DC voltage level, without switching them to AC coupling.Many of the 'scopes I used over the years were like that.

author: David Hess
author_rank: Super Contributor
timestamp: November 21, 2018, 06:49:06 pm
content: Quote from: vk6zgo on November 13, 2018, 04:59:12 amOffset controls are nice. Most oscilloscope vertical position controls function as limited range offset controls.In the extreme case are oscilloscopes which have differential comparator inputs. A Tektronix 7A13 can operate at 1mV/div over +/-10 volt offset range which is like having a position control with a 20,000 division range.

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: November 22, 2018, 09:00:27 pm
content: Point to note is that the series capacitor's value is inevitably a compromise between LF cutoff and taking too long to settle. Thus using AC coupling on very low frequency signals may give considerable waveform distortion.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: November 22, 2018, 11:58:54 pm
content: The AC coupling guts of two oscilloscopes: