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

[rsdio at audiobanshee.com]

Thanks, everyone, for the great suggestions.


I like the LC filter idea, and I assume that I can experiment with this by replacing the existing resistor in my design with the appropriate inductor. My prototypes tend to start as through-hole, because I don’t want to wait for third-party assembly, but if the circuit will be manufactured then I prefer surface mount. I suppose that SMD inductors may very in shape more than through hole.

One question that pops up is, “How do you determine the inductance value for those ferrite beads that don’t have anything specified?” I always check the OEM data sheet, but a few ferrite beads literally don’t say what their inductance is. Hmm, maybe that was resistance that was missing, though.

I’m new to modular, so I don’t know what kind of power supply noise is most common. I see your point about “it depends.”

For my digital modules, I try to focus on isolating my circuit from the supply so that module noise doesn’t end up on the common supply rails. For analog modules, though, I assume there won’t be any local noise, and thus it’s only necessary to filter out incoming noise from random assorted other modules.

Brian


On Jan 6, 2020, at 11:19 AM, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
> 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