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Old 27th November 2002, 09:42 PM   #1
jwb is offline jwb  United States
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Default My ignorance knows no bounds

In my CS43122 DAC I'm picking up a lot of noise via the BNC connector. If I remove the cable altogether, there is a huge amount of audible crap coming from the analog outputs. Presumably this is because the connector, the digital circuit, and the analog circuit all share the same ground plane, but I really have no idea. What should I try?
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Old 27th November 2002, 09:55 PM   #2
halojoy is offline halojoy  Sweden
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Default Put a dummy resistor in the input

Or try to connect a large value resistor
across the input, near the connector..
I am not sure this would cure it. Maybe.
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Old 27th November 2002, 10:06 PM   #3
jwb is offline jwb  United States
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There is already a 300Ω resistor to terminate the input. A parallel resistor would have little impact.

Also the ground (shield) of the connector is attached to the circuit ground through 100pF ceramic capacitor.
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Old 27th November 2002, 10:11 PM   #4
halojoy is offline halojoy  Sweden
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Okay. We will have to wait for
our friends with greater experiences in this thing.
Like Jocko and Elso.
But I would also think that the ground is involved in some way.
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Old 28th November 2002, 03:37 AM   #5
jwb is offline jwb  United States
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I cut the ground plane down on a line down the center of the pulse transformer, completely severing the input from the rest of the circuit. I connected this separate ground with its own wire back to the star ground at the power supply. This cut the noise significantly, and now there is no difference between the noise with the cable plugged or unplugged.

I'm starting to believe that my Revision B with three ground planes and four power entry points will be significantly better.
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Old 28th November 2002, 08:44 AM   #6
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Quote:
Originally posted by halojoy
Okay. We will have to wait for
our friends with greater experiences in this thing.
Like Jocko and Elso.
But I would also think that the ground is involved in some way.
Very high performance DAC's are like wild beasts. The pcb layout is essential (I mentioned it earlier). You can get real scared if you read the datasheets or the application notes. The evaluation board from Crystal is a 4-layer pcb.

I did also fail but my failure was not to get approved for CE marking which is much harder than just get descent sound out. I think also that the CS43122 maybe can be even harder to connect proberly.

jwb, I think your main trouble is ground, supply voltage and location of the parts. MuRata-filters are good in order to filter out RF-noise.

Good luck with your revision B and keep us posted about your progress.
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Old 29th November 2002, 01:14 AM   #7
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Hello Per,

I am afraid that you said the forbidden words "CE marking".
Now you opened this can of worms this thread will be perpetual...

Regards,

Jean-Paul
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