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Old 9th March 2017, 04:31 PM   #1
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Default DAC noise observation on Oscilloscope

Hi,

I am trying to observe some DAC points on an oscilloscope.

It is a sigilent 200MHz scope.
Probe is correctly compensated and set at 1x.

CH1(yellow) is connected to the GND stub on the scope
CH2(pink) is connected on the DAC XLR output (between pins2,1)

The DAC is powered on but there is no signal being fed to it.

The noise levels seem very high.

Am i doing to wrong? What could be the source of this crud?

thanks in advance for comments and help.

I apologize for the poor contrast in the image, but thats the image saved by the scope. Will need to investigate how to save better screenshots.
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Old 9th March 2017, 04:36 PM   #2
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Sorry. Meant to post this to the 'Equipment & Tools' forum, but in a hurry got it wrong.
Can one of the mods help move this.

thanks
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Old 9th March 2017, 04:41 PM   #3
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Location: Silicon Valley
When I connect my scope probe tip to probe ground (crocodile clip), I see 10 millivolts of high frequency noise. The only way to make this disappear, on my scope anyway, is to use a better grounding scheme. Either the pigtail ground (red) or the BNC female adapter (blue) works well.

_
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Old 9th March 2017, 04:42 PM   #4
dc3 is offline dc3  United States
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I have never found scopes to be great tools for viewing low-level audio signals in the single digit millivolt range. First, the scope itself has a high noise floor (relative to audio work). Second, the scope is showing you all sorts of high frequency digital-related noise in the signal out of the audio band that you may not care about even if it is really there.

I have had some luck with adding a low pass filter and preamp as a front end to the scope. Better yet, use a USB sound interface and a PC (but still you have to mind grounding, balanced input, noise from USB, etc).
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Old 9th March 2017, 06:24 PM   #5
sesebe is offline sesebe  Romania
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The oscilloscope is not good for this type of measurements.
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Old 9th March 2017, 09:06 PM   #6
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It looks like your DAC has some clock feedthrough and otherwise you are mainly measuring your scope's noise.

Last year I tried to measure the noise floor of a home-made DAC using a preamplifier, A-weighting filter and oscilloscope, but that didn't work out well either. It is a noise-shaping DAC and the out-of-band noise rises faster with frequency than the A-weighting filter rolls off. In fact I could switch the DAC between three noise-shaping algorithms and the algorithm that produced the least audible noise gave the largest signal on the scope. I then switched to using an audio recorder and an audio editing program.
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Old 9th March 2017, 10:13 PM   #7
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Is this an ARC DAC 8?
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Old 10th March 2017, 06:09 PM   #8
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Quote:
Originally Posted by eriksquires View Post
Is this an ARC DAC 8?
no this is not ARC DAC 8.
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Old 10th March 2017, 06:19 PM   #9
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Thanks for all the comments and pointers.

The scope supports down to 500μV/div. So a noise floor significantly higher than that is a bit of a surprise for me.

I agree, a sound card analyzer is better for low level measurements.

I was investigating some gross audible issues from the DAC.

Hearing strange distortions

Can one of the moderators please move this to "Tools and Equipment". This is really out of place in multi-way loudspeakers.

thanks
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Old 10th March 2017, 06:22 PM   #10
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i am seeing all kinds of noise and even some 60Hz components through the scope.
At this point, not sure what is from the circuit under test and whats from the scope/probe.
Is the scope input/probe that sensitive and unreliable for low level signals?
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