[sdiy] What's all this decoupling cap business??

[Magnus Danielson]

From: "Paul Schreiber" <synth1 at airmail.net>
Subject: Re: [sdiy] What's all this decoupling cap business??
Date: Wed, 26 Nov 2003 14:58:39 -0600
Message-ID: <000a01c3b460$1913e460$0201a8c0 at speakeasy.net>

Paul,

> I'm *not* disagreeing that 0.01uf caps are best *above 100Mhz* (ie the EDN article written 17
> years after the arrival of the monolythic cap). In 1982, 14Mhz was considered "really fast".

Indeed. Now I was a bit nit-picking, but I was it for the purpose of adding to
what you said and possibly have a few others see some more light, when we had
the topic of capacitors open for discussion.

Oh, I have an audio-effect box with over 100 MHz in it from 1978, so fast that
it contains both ECL and 74S logic. Blitzing fast for its time! ;O)

> Another 'fly in the oinment': what avout SMT caps? 1206 vs 0805 vs 0603? Now, the cap's lead
> inductance is zip, and you have other things into play.

1206 you can just plain forget. We're talking 0805 as large, 0603 as normal and
0402 as small these days, and I'm not joking, soon 0603 will be large and 0402
will be normal.

> My point was: many things in EE that are "given" evolved from several
> sources.

That is VERY true. Your point is much better than your example, regardless of
how enligthening that was as such.

One of the things about those "rule of thumbs" is that people run according to
these rules without all the context which helped to create them. So, we have
one set of engineers running according to these when they really should be
doing something else, and we have another set of engineers feeling the rules of
thumbs is just garbage and is not using them, when they infact is quite sane.
Oh, then we have a set of engineers who is just unaware of "rule of thumbs" and
they have a feel of "big mystery" in designs and those that pull it of must be
able to do "black magic". The only cure is to learn more, much more.

Cheers,
Magnus