data induced noise on audio

[cjupiter]

Hi,

I have a PCB that I am fault finding at the moment. The board contains a RS485 data line and an audio line. There is a speaker attached to the circuit. There are LEDs on the board that flash on and off when data is being transmitted and received. When data is being transmitted and received, you can hear a noise coming out of the speaker that is in sync with the flashing LEDs. This has led me to believe that the data line is inducing noise onto the audio line. Has anybody has a similar circuit and experienced this before and if so how did they go about alleviating it.

THanks

 

[IanP]

Audio and video signals as analog signals are very susceptible to digital noise.
It is quite possible that this noise is imposed on your audio signal through sharing common ground by audio and digital (RS-485 + LEDs) circuits ..
Try to separate these grounds (and possibly supply) by using RF-chokes (inductors) of mHs values together with additional decoupling caps ..
Another thing is shielding of audio signal especially the input stages ..
Regards,
IanP