AGND DGND audio Noise Problem

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rotabox

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hello guys
i have a an issue with a project im building with digital noise that coming from encoder and button signals to agnd.
i m using tracos ac dc convertor and from there i have 3 dc dc convertors to supply 12v 9v and 5v all commons of the convertors is connected acesept the 5v common out thats is for DGND all convertors connected without caps for this test after i try simple psu regulators.
12v is analog voltage for relays 9v is analog voltage for electert mic preamp with switch on preamp output to disconnect.
5v supply digital voltage to: teensy 3.5 , optical encoder, arduino nano , i2c screen. and buttons.
every thing is working preaty wall except...
when i connect power amp to test mic preamp when the mic is on all is ok but when i switch it off and move the encoder i ear in the analog line some low digital noise swing
related to the encoder motion. allso if i push some button i hear very low digital noise spike in the speaker.
i try measure it with oscilloscope but i cant realy figure out where this noise leaking from. i see it in few places most between Agnd to Ddgn or any close place to digital pins or traces on the board.
the only place that Agnd and dgnd r connected is in the teeensy. anyone is familiar with that problem? should i use inductors or diodes somewhere in the dc dc psu to avoid this noise return?
any seggestions?
Thanks
Rota
 
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Using the ADC differential inputs and providing matched input paths (same routing, same filtering, etc for both lines) is helpful.
 
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