It's a simple Jack 3.5 TS cable that I use to connect the eurorack to the Bela. The signal then goes to a voltage divider, is added to a voltage reference from Analog Out 0, goes through a Low Pass Filter centred around 15kHz, then into Analog In. That's it. Something like this:
I might add that the first screenshot of the scope has been made while holding the cable in my hand, plugged into this circuitry above but not into the Eurorack. Dead flat.
The third one, the gif, is when the unplugged connection of going NEAR the Eurorack! Not plugged into a module, but just in my hand facing the synth.
I tried some things this morning. First, I don't have to plug a cable at all. Without any cable, just the circuitry, I get the noise by just having the PSU block powered and close enough to the Bela. If I unplug the PSU, the signal slowly gets back to flat. So i guess this is it, but why is it so bad on the Bela ?
I'll try to find an other PSU in the meantime, see if it changes something. The one I am using: http://www.4mspedals.com/powerbrick90.php
Edit: So i guess that solves the problem with the simple circuitry I got, where I got spikes for doing nothing, as the Bela was so close the the Eurorack world at the time...