Scope offset while in AC coupling mode?

Thread Starter

Elerion

Joined Sep 11, 2017
117
Hi everyone.

Is there any reason why an (almost new) oscilloscope in AC coupling mode shows a small offset (a couple of milli volts)?
Example (using 1x probe):
scope ac error_.png
Is this normal for relatively cheap new oscillosopes?
I didn't realize this behaviour with anolog scopes.
The offset spoils the (AC) RMS measurements, as it takes into account a mean value, and it is annoying.

Thank you!
 

andrewmm

Joined Feb 25, 2011
574
I dont know, I've not ussed a scope to measure RMS,
but
I'd have thought the RMS needs to know the DC offset of the signal,
so can you take an RMS measurement if the scope is AC coupled ?
 

Ian0

Joined Aug 7, 2020
455
It's because the frequency of the signal is too low to fit on the screen.
I'm guessing that you have about 500us per division so the screen is about 7ms wide, and that the signal you are measuring is perhaps 50Hz, so that the negative-going half is off the screen to the right.
 

dl324

Joined Mar 30, 2015
11,506
AC coupling mode shows a small offset (a couple of milli volts)? ...
Is this normal for relatively cheap new oscillosopes?
What is the vertical sensitivity? DSO's are infamous for their fuzzy traces.
I didn't realize this behaviour with anolog scopes.
Analog scopes can also have an offset. Unlike DSO's, you don't need to go through a menu to remove the offset; you just turn the knob.
The offset spoils the (AC) RMS measurements, as it takes into account a mean value, and it is annoying.
Just measure the peak value and divide by 1.414 to get RMS.
 

Ian0

Joined Aug 7, 2020
455

tautech

Joined Oct 8, 2019
124
Hi everyone.

Is there any reason why an (almost new) oscilloscope in AC coupling mode shows a small offset (a couple of milli volts)?
Example (using 1x probe):
View attachment 215759
Is this normal for relatively cheap new oscillosopes?
I didn't realize this behaviour with anolog scopes.
The offset spoils the (AC) RMS measurements, as it takes into account a mean value, and it is annoying.

Thank you!
Check the manufacturers websites for new firmware versions and install them as necessary.
After which and the scope has been running for some while so good and warm, run the Self Cal utility which should address AC coupling channel offset.
Better measurement accuracy can sometimes be got using measurement Statistics if your scope has such.
 

Thread Starter

Elerion

Joined Sep 11, 2017
117
Check the operating manual. There is usually a way to adjust the zero offset even in the most inexpensive scopes..
Unfortunatly no. The only reference in the whole manual is: "AC: block all the DC components and attenuate signals lower than 8 Hz. Use AC coupling to get a stable edge trigger when your waveform has a large DC offset."
Of course I can move the trace up and down (it is called offet too), but that doesn't fix the issue. The trace mean value is over the offset level.

I'd have thought the RMS needs to know the DC offset of the signal,
so can you take an RMS measurement if the scope is AC coupled ?
RMS can be measured for AC only, or for AC+DC. In my case, it takes into account DC (and a wrong DC!) in the AC mode, which is not as expected.

It's because the frequency of the signal is too low to fit on the screen.
No. I can set a 1 s/div and take a long capture, and the trace mean value is always above.
 

KeithWalker

Joined Jul 10, 2017
1,216
If you can move the trace up and down in AC mode then I don't understand your problem. Does the mean value shown change with the position of the trace?
 

Thread Starter

Elerion

Joined Sep 11, 2017
117

kaindub

Joined Oct 28, 2019
24
The signal is AC coupled. Even if its AC coupled there will be a DC offset if the area under the positive peaks does not equal the area under the negative peaks (its called integration).
I believe what you are seeing is correct
Prove it yourself by coupling in a clsoe to perfect sine wave and see if the offset still exists.
 

Thread Starter

Elerion

Joined Sep 11, 2017
117
The signal is AC coupled. Even if its AC coupled there will be a DC offset if the area under the positive peaks does not equal the area under the negative peaks (its called integration).
I believe what you are seeing is correct
Prove it yourself by coupling in a clsoe to perfect sine wave and see if the offset still exists.
Yes, there is. If I disconnect the probe (nothing connected to the BNC input), there's an offset too. If I set the time base to something like 10 or 20 ms (that's already pretty slow), there's still a non-zero mean value.

I updated firmware and did a self callibration successfully. Still, the offset is there.
Could it be a fault? Sometimes is larger, sometimes is lower (same setup).
Today, while working on a real circuit, I got zero offset for a couple of minutes. Then, the offset came back (same circuit, same probe, same attenuation, etc.)
Any idea?
 
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