Re: Parallel caps in schematics

On Aug 2, 9:13 am, John Larkin
<jjlar...@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
On Thu, 02 Aug 2007 11:00:22 -0500, Vladimir Vassilevsky





<antispam_bo...@xxxxxxxxxxx> wrote:

John Larkin wrote:

Capacitors have series inductance, so for good high frequency
bypassing it helps to put them in parallel. Also the multiple caps can
be put in different places, bypassing multiple power pins and such.

There's less justification for using different values; none in my
opinion.

Several identical caps connected in the parallel are making for the
worst of the possible combinations of resonances and antiresonances. If
a very solid wideband decoupling is required, the paralleled caps should
have the values different from each other by 10 times or so.

No. Putting different caps in parallel just moves the resonances
around; Spice it. But a good power-ground plane structure, with
large-value bypass caps scattered about, is a very stiff, low-Q
structure, and Spice is a poor-to-worthless model of that reality.

I occasionally put SMA footprints on my power/ground planes, and TDR
the planes without and with bypass caps, and later measure the actual
HF noise on operating boards. Try it, it's educational. Most of the
literature on bypassing is absurd.

Hmmm...

You suggest Spicing, then point out that Spice is a poor tool for
analyzing real-worl layout situations.

.

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