I am working with a device. Block diagram looks like this:
I was facing a noise challenge and asked a question here. I implemented the RC filter design suggested by @analogsystemsrf and got good results for 7 V line. I cut the Vin trace on my PCB and introduced the filter using external components to check if everything works alright.
To my surprise, the filter was working earlier but I was getting a lot of noise on 3.3 V line. Then I realized that noise is getting coupled through UART lines to the VCC.
Noise signature is exactly same (in waveform shape and magnitude) on UART lines and VCC.
PCB design details: 2 layer board (30 mm X 40 mm) with GND plane covering entire PCB on both sides. GND plane is shared by device as well as zigbee.
I am thinking whether putting a similar filter on UART lines (baud rate - 115200) will mitigate the issue. Or should I put a series resistor on both UART lines? I am clueless in this regard about exact circuit and values. Since both lines are pulled up, I think this circuit should work:
Can someone please help me decide whether I am on right path?