ADC readout comb structure

[vikulin]

Crossposted from vikulin/OpenGammaKit#5: Visible gaps in raw background spectra every 16th channel

See the screenshot of a 1h30m background spectrum:

The issue appears unrelated to any real physical processes in ambient radiation and is possibly caused by a hardware artifact in the ADC.
The data was collected using a slightly modified version of @Open-Gamma-Detector's firmware, with no critical changes.

@NuclearPhoenixx, can you confirm whether you're encountering the same issue on your hardware/software? The issue seems to become more apparent with longer recording times.

[NuclearPhoenixx]

Yes, although it is slightly less pronounced on the Pico 2 than the Pico 1 as far as I remember. And from what I can tell it's every 8th channel, and it's obviously not related to the background spectrum itself, because it's some electronics issue.

In the future, please post complete issues and not just one single link to your own repository. You also don't need to @ me necessarily. I will read the issue regardless.

[vikulin]

I suggest adding an issue template to help users understand what information is required when creating a new issue. This could simplify the process and ensure more consistent and useful submissions.

Any thoughts on how to fix this issue? So far, replacing the RP's ADC with an external chip seems like the only option.

[NuclearPhoenixx]

I'm not sure. If you ask me, it's probably a noise issue, not immediately related to the ADC itself. Probably the surrounding electronics, maybe even the peak and hold circuitry. If you have the same issue with a different layout, it can only be related to the circuitry itself and not the PCB picking up noise or something.

[vikulin]

It's quite unlikely that a noise signal would naturally exhibit a digital pattern with an 8(64) period. I’m fairly certain this is an issue with the built-in Raspberry Pi ADC. For reference, see the second diagram here, which shows a pattern very similar to what I’m observing.

[NuclearPhoenixx]

You're right about that, that's highly unlikely. Definitely an interesting insight in the Pico 2's ADC.

[vikulin]

Please review the latest updated schematic incorporating the external ADC. It should address the issue.

opengammakit-v0.2.0.pdf