Electronics > Beginners

Power supply multi-rail noise separation?

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David Hess:
A pi filter for each separate output is the way to go.  Since the filters are all starting at the same distribution point, the first capacitors are all in close proximity and in parallel so really an LC filter for each output.  A low value of resistance is placed across the inductor to quash its Q in place of a lossy ferrite core.  In the past the inductor might be wound around the low value resistor.

So... in the meantime (until I redesign a whole board) I have the battery and I have loads... and I have protoboard.

First test shows without filters of any kind, save an LM7815 regulator, the battery powers the current 16V single supply headphone amp... silently.  Of interest, it's also silent when connected to the rest of the apparatus.  Confirming the noise I am annoyed with is coming from/caused by/influenced by the power supply.

Next I need to see if I can power the digital audio box (STM32H7 based) from a separate LM7824.  It has it's own buck module capable of 24V and I don't want to drop too much linearly if I can avoid it.  The box pulls 200mA ish.

I might not need any filtering...  I doubt it though.  Something is going to pull all that USB ground garbage into the audio somewhere, I just know it.

Does this look sane? 

The right hand side I'm fine with, even without a darlington it will work.  It's only temporarily / testing.

On the CLC/PI filter.  I'm out of my depth.  If a capacitor multiplier is a "High school" level thing, inductors are definitely one level higher up from where I am.  I can't seem to get a straight answer it's always a cyclic this depends on that, which depends on that which depends on the original thing.  It seems to make sense of this, mathematically, we get into greek and I go blank.

It's not just impedence at a rising frequency, it's proper understanding on the impedence of the L and both Cs and how they interact.... with the variable load R throw in.  If you can get it to attenuate rising frequencies from as low as you dare, you are also likely to have to deal with the resoance of some random (calculable) higher frequency.

It's beyond me :(  I feel religated to stealing an example and YOLO or buying some cheap AliExpress module and hopeing it's worth bothering.

I'm presently looking for examples to pillage instead.

Is that a common-ground (non isolated) module?  I guess being that it says "buck", it is.  This can introduce noise in the common-mode (between grounds) loop between equipment, and tends to be rather pernicious.  The C-mult filter can help, but you might still need a common mode choke and whatnot to keep the noise confined locally.

(Normally a buck module should be common ground, as in identical ground pins, but the fact that they often put connections on opposite ends of the board, and may have a DC offset between them besides (e.g. if low-side current sensing is used), means ground-loop EMI can be picked up, i.e. the ground path crosses the main switching loop.)

As for the filter, you typically want to design it for some cutoff frequency far enough below Fsw to be useful, and Zo a modest fraction of the DC operating conditions.  Notice that a filter only works when both ports are terminated into a resistance (or at least one for suitable prototypes), but we have no resistance in the circuit: the battery is as good as a short, or still reactive (consider stray inductance of the connecting cables), and the supply is negative-resistance if anything (current draw decreases as supply voltage increases).  We can't match to that at all, or, if we did we'd get an oscillator not a supply.

The solution is to use enough ESR (= Zo) in the capacitor(s) to dampen the filter; this necessarily costs HF response (adds zeroes to the transfer function), but stabilizes it by introducing resistance to dampen the system.  Losses can be added anywhere, really, but a deadass shunt resistor at the input or output is obviously out of the question (it would sink way more DC than the load proper!), but we can basically cap-couple it in place, blocking DC while still keeping it effective at the transition frequency (where the filter is most reactive, and therefore in need of damping).

The preferable way is to design the LC filter, and wire additional R+Cs in parallel with capacitor C, with Cs > 2.5 C and R = Zo.  This gives a zero in the transfer function, right in the middle where it's needed (for damping), then the hard-C-to-GND still gives good HF performance.

If electrolytics are used, the ESR likely comes along for free.


I have managed to get the noise on the scope and get some basic FFTs.

It's a right mess.  There are many audio band harmonics from the bottom up, almost equally spaced suggesting they are harmonics of the same signal?

So, while I have it on the breadboard (well when I finish work), I'm going to see if I can "fumble" a filter together.

Showing how little I have used the scope in anger in the past 6 months it took me 30 minutes of wondering why everything had a -14.50V offset and then realised you have to set the "channel" to AC coupling, not just the trigger!

Oh... and thanks for the reminder on putting a load on circuits.  I did chase the AC Coupled amplifier output up and down the DC range earlier before remembering that.  47Ohm on both output and it stopped migrating around.


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