Cmod A7 massive GND noise

[steddyman]

I have created a circuit to replicate the functionality of an old 8-bit chip that I plan to use the Artix-7 on, and for development I am using a Cmod A7 to prototype.

I have found i am getting a lot of noise on the ground plane of the circuit (around 1.2v P-P).  I have decoupling capacitors around my IC's and a voltage regulator with the correct capacitors.

I thought at first the issue was with something I had missed on my circuit, but if I unplug the Cmod A7 from the DIP48 socket, noise drops to around 200 Mv P-P.

Do I need to do extra work to decouple the Cmod A7 itself?  I can see it has a lot of decoupling capacitors on-board anyway.

Thanks

[hamster]

On this topic I've been making an audio DSP board using the CMOD A7, where additional noise is a real pain.

My initial prototype board had some audio noise problems - I couldn't hear it but I could measure it. I initally thought was due to the CMOD-A7 and could not be fixed, but eventually put down to quite a few different causes:

- I had nearly shorted the output of one of the DAC to GND, which as causing spikes on the power rail. Once fixed things were a lot better, but not perfect/

- I had not made any real attempt to stitch the top fill to the ground plain on the bottom - after all it was a hack.

- I didn't have any series resistors in the I2S lines. I added 50 ohm ones (just picked a random value out of the air - might look at this again)

- I had a few capacitor bodges standing up in the air, which could only make things worse

- I was measuring very close to the FPGA, with a high impedance scope probe

So I addressed all of these in the next prototype, and made up a test jig allowing me to measure 30cm from the board and things are much better - to the point I can't reliably measure any additional noise in the audio band.

I guess what I am trying to say is that even with just one GND pin the CMOD-A7 can be part of a low noise audio system, but you have to put some extra thinking and work in to make it happen. 

This may or may not be of use to your use-case.