[sdiy] power supply decoupling capacitors

[harrybissell]

Just a quick note (on vacation)

I think it matters a LOT where the current draw is... imho its usually from V+
to
gnd or V- to gnd... with the load ground referenced.  This that for that
half-cycle
of signal... there might as well be NO other supply (simplistic view).

So caps to ground are the right way (imho).

Also the length of the ground trace to the bulk supply does not matter... the
decoupling
cap provides a local shunt path to cover the time until the main trace can
actually supply
current, hence the name "decoupling" cap.

H^) harry

Czech Martin wrote:

> I used to drop caps here and there in the circuit, for each
> active component one or two, depending if it has one or
> two supplies, and I tied the cap to GND, say filter GND.
>
> This was a habit, because this is the way that people handle
> it.
>
> Now I start thinking (which is basically a bad thing, I know).
> Is this necessary, or is it good at all?
>
> ok, what's the difference in putting a cap to V- instead to gnd?
> GND will be a quite long line, maybe a plane on pcb (most diy circuits
> do not have this, they have single layer pcb, wire wrap circuits
> have also usually no ground plane), then comes the connector,
> then the line to PSU. Say 20cm. Makes 200nH (roughly!).
> If the op amp upsets V+ (because the output stage wants to go up)
> V- will not be pulled, so why not tie the capacitor to this V-?
> If think the result will be the same, the *local* voltages of
> V+ and V- are more stable then without cap.
> This should also prevent oscillations via feedback through
> the power lines (which some fast op amps like the 5534 can show).
>
> The PSRR of op amps is poor, if we go higher then 1MHz or so,
> and these are the frequencies where a 100nF cap will get "active"
> (X ~ 1.6 Ohm @ 1MHz). Of course, the cap has also some inductance,
> if the leads are 5mm long (which is quite short for a board
> like I mentioned above) we have > 10nH. So the resonant frequency
> will be 5 MHz (1/sqrt(L*C)/2/pi), above that the cap really
> works like an inductance.
> Maybe this is ok for the circuit, because it does not see 5MHz
> any more. Maybe this is not ok, so a smaller cap  with shorter
> leads, maybe 10nF will be good till 50MHz.
>
> So if we talk about decoupling, we're really talking about
> 1MHz .... 50MHz (for most analog circuits, and some slow CMOS logic).
> Lead inductance is very important. A 200nH trace will have
> X~1.3 Ohm @ 1MHz, and this will linearly scale with frequency.
> Ground cables on my lab bench are 500 mm long...
> So we may have to decide between local ground, and far away ground.
>
> Finally I remember that all this RCL-stuff will create resonant
> circuits with damping. I think it is not too hard to design an example
> where the resonant frequency hits the operating frequency, or some
> harmonic and where the damping is so bad, that things get worse
> with "decoupling" caps.
>
> ???
>
> m.c.
>
> -----Original Message-----
> From: Neil Johnson [mailto:nej22 at hermes.cam.ac.uk]
> Sent: Donnerstag, 8. Mai 2003 23:24
> To: Paul Maddox
> Cc: synth-diy
> Subject: Re: [sdiy] power supply decoupling capacitors
>
> Paul Maddox wrote:
> > As a rule, anything IC has one 100nF cap for each supply for things I do..
>
> A good rule, but not always necessary....as in anything "it depends".
> Some logic boards you can get away with 10nF per chip.  If you have a
> flooded ground plane you can afford to be even more light on decoupling
> due to the much lower ground path inductance.
>
> > > I also wonder why they use two caps go from + to gnd, and gnd to -.
> > > Wouldn't one cap between + - be sufficent?
> > noooooo....
> > the cap passes HF noise to ground, if you connect one across supplies,
> > you allow HF on the +ve rail to goto the -ve rail and vicaversa,
> > allowing for the noise on +ve and -ve to be present on BOTH supplies...
>
> True, but sometimes that's better than letting it get near the ground.
> Remember, a typical op-amp will have a PSRR of around 100dB (for a TL071),
> whereas putting that onto the ground could be worse.
>
> But, as always, it depends!
>
> > > Some people say that connecting caps to gnd will introduce unwanted
> > > noise in your gnd, which is a bad thing if gnd and signal gnd is the
> > > same thing.
> >
> > yes...
> > So you shoud design your grounds carefully...
>
> Amen to that!!
>
> > This PCB I mentin has TWO grounds, one analogue and one digital.
>
> *swoon* Only _two_??  Paul, you're not trying hard enough!
>
> One board I did a few years ago (blimey, was it really that long ago?) had
> four grounds---one for the digital section, one for the RF generator, one
> for the low-noise amplifier, and one for the receiver amplifier.  We even
> put the LNA in its own shieled metal box on the PCB, using its own ground
> plane as the bottom plate.  Fun times indeed.
>
> There's a lot goes into designing PCBs.  In many, many cases the layout
> can follow standard rules, but when you want low noise, high speed, or any
> combination, that's when the "fun" kicks in.  And that's before I mention
> RF....
>
> Neil
> (currently fighting Catalan numbers and exploding algorithms...)
>
> --
> Neil Johnson :: Computer Laboratory :: University of Cambridge ::
> http://www.njohnson.co.uk          http://www.cl.cam.ac.uk/~nej22
> ----  IEE Cambridge Branch: http://www.iee-cambridge.org.uk  ----