[sdiy] power supply decoupling capacitors

[Paul Maddox]

Martin,

> Some people do not use any, others use caps a lot.

As a rule, anything IC has one 100nF cap for each supply for things I do..

> 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...

> Wouldn't the situation be better if the pcb traces for + and -
> would enclose as little area as possible (inductance)?

yes, but thats not always possible..
I'm working on a board that is 300mm by 250mm, so its not going to happen..

> 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...
This PCB I mentin has TWO grounds, one analogue and one digital. They
connect only at the GND pin bringing power to the PCB along with 4 caps, 2
10uF (one for each rail) and 2 100nF (one for each rail), other than that
the two grounds are isolated...

Paul
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