[sdiy] filter cap needed?

[David G. Dixon]

In my (somewhat limited) experience with building electronic circuits, I
have yet to detect any effect of adding or not adding 0.1uF bypass caps.  I
have settled on the habit of adding one or two pairs of such caps to any
circuit.  I generally try to line up my IC's so that they straddle a single
pair of parallel power rail traces.  I typically put one pair of caps on
either end of the string of IC's, very close to the rails.  I don't like
cluttering up the areas in front of IC power pins with them, as this would
complicate my PCB layout strategy.  I may add another pair between digital
chips, but not usually between opamps.

However, something has been troubling me about these caps: If several of
them are connected between a rail and ground at various locations, wouldn't
they end up fulfilling the same function as one cap, several times as large
(given that they are capacitances in parallel)?

> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-
> bounces at dropmix.xs4all.nl] On Behalf Of Colin Hinz
> Sent: Thursday, March 26, 2009 9:31 PM
> To: Synth-Diy
> Subject: Re: [sdiy] filter cap needed?
> 
> On Thu, 26 Mar 2009, Simon Brouwer wrote:
> 
> > Colin Hinz schreef:
> >> On Wed, 25 Mar 2009, db wrote:
> >>
> >>> I guess I've now been convinced to do it. :)
> >>>
> >>> Am I correct in that it just needs a .1uf  from 5v to gnd?
> >>
> >> That should do it. Close to the regulator pins too, naturally enough.
> >
> > Actually it doesn't matter if it is far from the regulator.
> 
> OH YES IT DOES!
> 
> Otherwise, it wouldn't be necessary to specifiy it at all, in most
> cases, because IC decoupling would take care of it.
> 
> > The capacitor on the *input* of the 78xx must be close to it, though.
> 
> Bypass caps in general must be close to whatever they're stabilising,
> because otherwise you're just building itty bitty L-C tank circuits.
> 
> And if your output L-C is resonant with load transients, you can get the
> 4V p-p "regulator" oscillator I mentioned earlier.
> 
> - Colin
> 
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