[sdiy] power supply decoupling capacitors

[Neil Johnson]

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
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