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Audio DACs and some odd frequency goings on.
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hamster_nz:

--- Quote from: SiliconWizard on December 02, 2019, 02:40:05 am ---Have you tried something very simple: sending all zeroes to the DAC and look at the output?

--- End quote ---

Yes, all zeros will give just a flat spectrum of the background noise.

When fed from the ADCs with the signal generator switched off (which should be randomly flapping around somewhere near +/- 0) gives about the same level of noise on the outputs.
magic:
I wonder if a constant 8kHz jitter component could intermodulate with your signal to produce similar output?
hamster_nz:
It took a while to do some testing...  Tested using a 12MHz clock, and now running at ~96kS/s, then had to add logic to turn things off and on, and mute channels.

The problem is 99% between the chair and keyboard.

The biggest problem is that my signal generator is a 14-bit DDS - so when connected direct to the input I get the fundamental and then an harmonic at -72db or so. Not really good enough for testing audio. Image 1 is the frequency of the test signal.

With the test PCB in the loop, but powered off I get image 2.

With just the signal generator turned on (and everything else turned off) I get Image 3 - which is crosstalk in the testing setup -  it was much worse until I moved things around on the bench. This also explains why I thought I had really poor channel separation or bits were somehow leaking between channels - it was external crosstalk.

With the FPGA, ADCs and DACs turn on, but the FPGA sending zeros to the DAC (and the signal generator off) I get Image 4. I think this is pretty good - about -100db, just inside what I can measure.

With the ADCs in the loop, but receiving no signal I get image 5. The noise on one channel is a PCB issue, I can change the level of noise by moving my finger around (but not touching) the ADC. Still, it is about -96db so perfectly adequate for my first hack. Be interesting to see how it looks when I build the next PCB that is better laid out.

And finally with the signal generator turned on, I get image 6. I get some unexpected signal at 15kHz (-72 db) and 16.25kHz (-78 db). The spike at 20 kHz is present in the input from signal generator.
SiliconWizard:
For testing audio circuits, I would suggest using a good sound card instead of a typical lab signal gen (unless it's extremely good.)
macboy:

--- Quote from: hamster_nz on December 01, 2019, 08:24:03 pm ---...
The DAC being used is a Cirrus Logic CS4344 - https://statics.cirrus.com/pubs/proDatasheet/CS4344-45-48_F2.pdf

MCLK is as smidgen off of 36.864MHz (like 0.01% off), due to not being able to nail it with the valid FPGA's PLL multipler/divider combinations. Maybe I should try clocking directly from the 12MHz oscillator?
...

--- End quote ---
Actual precise frequency of MCLK doesn't matter. But make sure that it is exactly 768x (in your case) the sample rate (LRCLK). They must be synchronous. Close isn't close enough.  You said about 0.01% off... well that's in the single digit kHz territory so it would explain your spurious sidebands if you are using asynchronous clocks. Page 12 of datasheet: "MCLK/LRCK must be an integer ratio".
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