[sdiy] Nice info on decoupling caps

[Ian Fritz]

At 12:54 PM 1/19/2012, David G Dixon wrote:
> > A nice little intro on decoupling caps for the uninitiated.
> > Some good links for more info at the bottom of the article too:
> >
> > http://www.analog.com/en/all-operational-amplifiers-op-amps/op
> > erational-amplifiers-op-amps/products/RAQ_JBryant_In_and_Out_I
> > ssue76/resources/faq.html?display=popup
>
>I found this article to be frustratingly terse.  "Many ICs have internal
>circuitry which generates HF noise on the power rails" doesn't really tell
>me anything!  Am I to believe that an opamp generates HF, just because?
>Why?  If so, then wouldn't every transistor also require decoupling?  Where
>does it end?
>
>I tend to use only one pair of decoupling ceramics on each module; two pairs
>if it is a big module, or if it has digital stuff on it (which is fairly
>rare).  I've never had any problem which could be solved with more
>decoupling.  Maybe I've just been lucky, but I think that, for most analog
>synth circuits, this advice of putting caps on every IC is simply needless
>overkill.

Absolutely correct.  I've lost the reference, but a long time ago one of 
the semiconductor manufacturers did an actual experiment on a board with a 
large analog circuit.  Their conclusion was that you need one pair of 
bypass caps for every "few" ICs.  I've never seen a problem that could be 
fixed with more bypass caps, either, as long as sensible bypassing as 
specifically recommended by the manufacturer for certain chips (555 timer, 
311 comparator) is used.  In fact some spec sheets even say explicitely 
that bypassing next to the chip is Not required.  Most x-talk problems I 
have had were due to radiation or improperly laid out supply lines.

Guess I'm just "uninitiated". :-)

Ian