[sdiy] Will LCD backlight PWM cause audio noise?

Olivier Gillet ol.gillet at gmail.com
Fri Aug 17 18:25:50 CEST 2012


I've encountered problems like that in the past and it's hard to
answer without looking at an actual layout and without knowing the
full circuit.

Factors that might come into play:
- Whether the analog section is powered from the same rails as the
digital section, and if so, how sensitive your analog circuitry is to
power supply noise (things to look for: signals coming straight from
the rails into CV scaler circuits or pots).
- How much decoupling you have in place.
- Grounding and layout - whether the return currents for your audio
section or maybe for some DACs hooked to your MCU will share the
return path of the LCD control. Good partitioning and contiguous
ground planes help.
http://www.hottconsultants.com/techtips/split-gnd-plane.html
- Consider how this signal might affect other signals coming out of
you MCU - a trace carrying a high-frequency PWM signal will bleed on
nearby parallel traces through capacitive coupling for example. I've
seen such bleed happen in other situations which did not involve
parallel traces, so I think - but I'm not really sure - that there's
also a share of bleeding internal to the MCU. Not a problem in the
digital domain, but this might become a problem if you inject signals
out of the MCU straight into your audio path (eg using a timer
peripheral as a square audio source, using a PWM generator on the MCU
as a ). In such situations, it's better to put logic buffers
inbetween.

Olivier


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