[sdiy] filter cap needed?

[Roy J. Tellason]

On Friday 27 March 2009 01:46:17 pm Scott Nordlund wrote:
> You're thinking about it the wrong way.  High frequency signals (hopefully)
> aren't emanating from the power supply to the rest of the circuit.
>
> Switching in a CMOS circuit will cause a transient spike in the current
> supplied to the chip.  

Not nearly as much as switching  in a TTL chip will.

> Parasitic inductance or long leads in the power supply lines will limit how
> quickly the power supply can respond to this,  leading to a local drop in
> supply voltage.  This enters nearby circuits via the power rails and can
> cause noise, glitches, etc..  A decoupling capacitor on the power leads of
> each chip will stabilize the power during these transient conditions and
> limit their affect on other parts of the circuit.
>
> Or you could think about it this way: decoupling caps provide a low
> impedance path to ground for high frequency signals (noise).

Another aspect to consider is the self-resonant frequency of the high-value 
capacitors in power supplies,  and for board decoupling (as opposed to chip 
decoupling).  I've been known to use a 1000uF electrolytic with a 1uF 
tantalum across it and a 10nF ceramic disc across _that_ just in the power 
supply of stuff I've built.

And lots of decoupling caps all over the place.

If they weren't really all that necessary the bean counters would have phased 
them out of much commercial gear by now,  and I still see them all over the 
place.


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