Problem with digital noise entering analog output

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Wes Brenner
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Joined: Tue Jun 25, 2019 4:47 am

Problem with digital noise entering analog output

Post by Wes Brenner » Tue Jun 25, 2019 5:26 am

I have tested the Ultra2 (from my kickstarter participation) with two files generated with Audacity. The first was generated with "silence" at 192k signed 32bit. The second was generated with 440hz max amplitude at 192k signed 32bit. I have narrowed the search for the noise to the output terminals from the ADC/DAC board to eliminate any possible sources from the pre-amplifier board. I put those signals through an external amplifier to get the levels high enough to make oscilloscope comparisons possible. I amplified them to get a 10 volt p-p from the 440hz signal. I then measured the silent signal and found about a 10 millivolt signal at about 200khz which I assume is actually something based on 192Khz. This translates to just 60 db of signal to noise ratio. Since even 16 bits has a signal to noise ratio of 96 db something isn't working right. To see if it was somehow the sample rate, I generated and tested the 440hz signal again at 96k and 48k with signed 32bit and it was still the same amount of noise in the 192k region.

My question is, how does one get rid the digital signal that has infiltrated the analog output. Is there a way to shield the parts of the board, trap it with a capacitor without hurting the analog signal, etc.

The testing was done with only the ADC/DAC board mounted to a raspberry pi zero (not a W).

Thanks for any insights anyone can provide,
Wes Brenner

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