I have an OLED display and a high gain op-amp on the same circuit. When OLED is powered on and displaying something, it creates a electrical noise on the output of the opamp. Here is what I tried:
- Opamp and OLED power lines are decoupled with closely-positioned 100nf ceramic caps.
- The display is mounted directly behind of the said opamp. To determine if noise is not caused by radiated EMI by the display itself, I connected the display away from the PCB by running a wire. No change.
- Added an LC filter to op-amp supply (ferrite bead and 100n+10u after). No change.
- If I supply the OLED with a seperate power supply, the problem goes away (output looks same as if OLED disconnected).
Here is a scope capture. Please ignore the rising/falling edges.
The OLED driver internally generates ~9V for driving the display. It uses a capacitance multiplier for that and probably that's my problem.
Any help/idea on filtering this supply noise would be appreciated.
- Current draw of the OLED: 10mA@5V
- What is opamp doing: It is MCP601 that I'm using as a thermocouple frontend. Output goes to an ADC (no load).
- Other noise sources: Circuit is supplied with (clean) 12V, goes through on-board 5V linear regulator (with its input and output caps). Everything (OLED, op-amp and MCU) is on the same supply.
- Routing: Two layer PCB with ground pour on each layer. I did not do star grounding though.
- Decoupling on OLED: I modded the OLED board with its supply passes through a ferrite+100n, no change.
- Ferrite bead: I tried what I have on hand, BLM21PG331SN1D and FBMJ2125HS250NT.