Author Topic: AC and DC Coupling Mode in An Oscilloscope  (Read 12294 times)

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Offline wuiven64Topic starter

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AC and DC Coupling Mode in An Oscilloscope
« on: November 02, 2018, 11:13:27 am »
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.
 

Online Brumby

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Re: AC and DC Coupling Mode in An Oscilloscope
« Reply #1 on: November 02, 2018, 11:49:23 am »
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.
« Last Edit: November 02, 2018, 11:54:29 am by Brumby »
 

Online Brumby

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Re: AC and DC Coupling Mode in An Oscilloscope
« Reply #2 on: November 02, 2018, 02:01:05 pm »
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.
« Last Edit: November 04, 2018, 10:46:22 pm by Brumby »
 
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Offline vk6zgo

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Re: AC and DC Coupling Mode in An Oscilloscope
« Reply #3 on: November 04, 2018, 02:18:20 am »
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".
 
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Offline JustMeHere

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Re: AC and DC Coupling Mode in An Oscilloscope
« Reply #4 on: November 04, 2018, 03:03:38 am »
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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Offline wuiven64Topic starter

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Re: AC and DC Coupling Mode in An Oscilloscope
« Reply #5 on: November 07, 2018, 10:08:33 am »
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
« Last Edit: November 07, 2018, 10:11:35 am by wuiven64 »
 

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Re: AC and DC Coupling Mode in An Oscilloscope
« Reply #6 on: November 08, 2018, 07:55:03 pm »
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?

AC 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.
Avid Rabid Hobbyist.
Some stuff seen @ Siglent HQ cannot be shared.
 
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Offline alsetalokin4017

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Re: AC and DC Coupling Mode in An Oscilloscope
« Reply #7 on: November 08, 2018, 08:59:11 pm »
Perhaps this video may be helpful.

The easiest person to fool is yourself. -- Richard Feynman
 
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Offline TimFox

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Re: AC and DC Coupling Mode in An Oscilloscope
« Reply #8 on: November 10, 2018, 10:03:47 pm »
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.
 
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Offline David Hess

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Re: AC and DC Coupling Mode in An Oscilloscope
« Reply #9 on: November 11, 2018, 11:35:22 pm »
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; some include a special precharge setting or multiple ground settings.
 
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Offline alsetalokin4017

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Re: AC and DC Coupling Mode in An Oscilloscope
« Reply #10 on: November 12, 2018, 09:45:03 pm »
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; some include a special precharge setting or multiple ground settings.

That'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.

The easiest person to fool is yourself. -- Richard Feynman
 

Offline w2aew

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Re: AC and DC Coupling Mode in An Oscilloscope
« Reply #11 on: November 12, 2018, 10:11:42 pm »
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.

I did a video on this about 7 years ago:

YouTube channel: https://www.youtube.com/w2aew
FAE for Tektronix
Technical Coordinator for the ARRL Northern NJ Section
 
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Offline David Hess

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Re: AC and DC Coupling Mode in An Oscilloscope
« Reply #12 on: November 13, 2018, 01:00:38 am »
That'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.

The 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.


 

Offline vk6zgo

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Re: AC and DC Coupling Mode in An Oscilloscope
« Reply #13 on: November 13, 2018, 04:59:12 am »
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; some include a special precharge setting or multiple ground settings.

Oscilloscopes 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.
 

Offline David Hess

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Re: AC and DC Coupling Mode in An Oscilloscope
« Reply #14 on: November 21, 2018, 06:49:06 pm »
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; some include a special precharge setting or multiple ground settings.

Oscilloscopes 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.

Offset 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.
 

Offline IanMacdonald

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Re: AC and DC Coupling Mode in An Oscilloscope
« Reply #15 on: November 22, 2018, 09:00:27 pm »
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.
 

Offline alsetalokin4017

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Re: AC and DC Coupling Mode in An Oscilloscope
« Reply #16 on: November 22, 2018, 11:58:54 pm »
The AC coupling guts of two oscilloscopes:

The easiest person to fool is yourself. -- Richard Feynman
 


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