Hi everyone , I'm having trouble resolving a noise issue with a project I'm working on. Essentially, my project involves acquiring a photoplethysmograph (PPG) signal and displaying this on an OLED screen. The PPG signal involves a photodiode (in the NELLCOR DS-100) connected to a transimpedance amplifier, followed by filters and a gain stage on my PCB. The OLED screen is the 2.42" OLED screen from Adafruit which uses the SSD1305 driver chip.
Until yesterday, both the signal acquisition/circuitry and the OLED screen were working well together on my PCB. Yesterday afternoon, I plugged in everything as per usual and strangely there was a lot of 93Hz noise on my +3.3V power rail (when there was not previously). I am really confused as this has never been an issue before. When I remove the screen from its PCB socket, the noise seems to disappear, so I'm guessing something to do with the screen could be the culprit. I have since tried multiple PCBs, multiple screens and adding capacitors (various values) between the +3.3V and GND rails near the screen. Nothing seems to make a difference. I already have an LC filter on my power rails (as suggested by the microcontroller datasheet).
I've attached schematics for my power supply/firmware/PPG circuitry as well as some images of what the noise looks like (zoomed in and out).
It would be really appreciated if anyone had any suggestions for how to approach this problem/ideas of things to look at. Thank you very much in advance!