[sdiy] Module power - regulated or filtered (passively)?

[Richie Burnett]

Whether or not a passive RC filter solves the problem really depends on what 
sort of interference you're trying to guard against (hum, buzz, fizz, hiss, 
crackle, RF noise, etc...) and the current draw of your system.

For something that draws very little current like the front end of a 
microphone preamp, an RC filter might make sense to ensure the supply to 
that part of the circuit is as clean as possible.  (The R can be made 
relatively large because the current drawn at this point is likely tiny. 
Likewise for the RC filters that people often make across the potential 
dividers used to generate the bias level for op-amp circuits running from a 
single supply rail.  Op-amp inputs draw very little current.)

For something that draws more current like a power amplifier, or something 
that has an erratic current draw like an oscillator, an RC filter in series 
with the supply is much less desirable because it causes significant power 
dissipation and wrecks the steady-state regulation of the power supply rail 
downstream of the filter!

Also a simple first-order RC filter is a pretty crap filter so it doesn't 
really give you much attenuation until you get up to some pretty high 
frequencies for a given choice of components due to the gentle 6dB/oct 
roll-off characteristic.

I personally don't find myself using RC filters to clean up power supply 
rails much, but have used LC filters quite frequently.  Particularly for 
things like preventing noise from a digital 3V3 rail from getting onto an 
analogue 3V3 supply rail on mixed-signal PCBs.  Some advantages of the LC 
filter are that it has a much steeper 12dB/oct roll-off slope (can be made 
steeper) and that it doesn't cause a voltage drop like the resistance in the 
RC filter does (okay the L has a little bit of series resistance if you 
choose a physically small part, so you do get a little bit of voltage drop.) 
Even just a ferrite bead and a capacitor can make an LC filter sufficient to 
attenuate RF noise up in the MHz if that is what you're trying to protect 
against.  The only thing to watch out for with LC supply filters is the 
damping.  Since it's a second order filter it can be resonant if the 
downstream load or the ESR of the inductor aren;t sufficient to damp the 
circuit, and if the Q is too high you can actually end up boosting supply 
noise around the filter's resonant frequency! :-O

-Richie,