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DC offset....sometimes

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DC offset....sometimes

Postby jaray » Wed Dec 09, 2015 7:47 pm

I've always notice, but didn't pay much attention to, that there is a DC offset associated with the capture data block when my scope A input is AC coupled.

Some details about this are that the signal I am testing with has a DC offset of about 325 mV with a Pk-Pk value of 600 mV.

The issue is that sometimes when I capture the data, my signal has a perfectly 0 DC offset. Other times there is anywhere between .5-2 mV with I have seen at times upwards of 10 mV. It almost seems random what offset value I will have, however I will always have that offset between cycling my software.

I made a previous post on this forum about how there is a decaying DC offset associated with the first capture after changing from DC coupling to AC coupling, but this does not seem related.

The system I am testing on is powered through USB along with a DDS device. The scope is not tied to earth ground.

EDIT: I should note that I have tested the same signal using the PicoScope 6 software and the DC offset is not present at all.
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Re: DC offset....sometimes

Postby AndrewA » Mon Dec 14, 2015 2:12 pm

Hi Jaray,

Could you post some details of the capture. Scope used, capture settings etc.
Which driver and version number are you using?
Could post a psdata file?
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Re: DC offset....sometimes

Postby jaray » Wed Dec 16, 2015 11:30 pm

Sorry for the delayed response. As it is currently 5:30 PM over here, I will post the captures in the morning.
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Re: DC offset....sometimes

Postby jaray » Mon Jan 04, 2016 5:57 pm

baselinenoise.PNG
The Holidays got the best of me.

Here is what I was able to capture. This is a mix of the software I wrote along with using The PicoScope 6 software. The sampling rate I am using in my software is the max allowed with a signal channel at 16 bit resolution, 62.5 Mhz.

The signal looks a bit strange in these images because I am just plotting the data points and not drawing lines between them.

Here is a capture I obtained of what I would hope to see 100% of the time
0dc.PNG


The signal I am using is the output from an AD5930 DDS evaluation board. This signal burst was at 1 Mhz. The signal from the board looks like this without AC coupling the signal.
1VADCDC.PNG


Most of the time however, when I have the PicoScope set for AC Coupling, the signal looks like this.
ACcoupled, 1VADC.PNG


With the baseline noise looking like this
baselinenoise.PNG


As you can see there is a few mV of DC on this signal. This is what I typically see though there are times where the offset is as much as 10 mV.



This is the same signal recorded using the PicoScope 6 software.

DC Coupling
picodc.PNG


AC Coupling
picoac.PNG
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Re: DC offset....sometimes

Postby AndrewA » Thu Jan 07, 2016 12:38 pm

Hi Jaray,

Have you tried zeroing the channel in your software.
I would setup the channel- AC coupling, voltage range, then short out the channel input and then take so values from the channel. Average these and use them for offsetting your values, in future.

The other reason maybe the time constant response of the channels input when AC coupled. What is the frequency of your signal? It just looks like white noise?
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Re: DC offset....sometimes

Postby jaray » Fri Jan 08, 2016 4:25 pm

I should have noted the white noise. That is in fact white noise, but it is because I am doing burst frequency testing. So for this particular test I am sending out 100 cycles of a particular frequency, anywhere from 1-3 Mhz. The images I posted are just the baseline noise after the burst is complete. For my actual testing I will be performing a burst and listen test, so I need to extend my capture time. These images are just the burst running straight into the scope for debugging purposes.

I'll try the suggestion out.
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