Re: [SI-LIST] : App note assumptions

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From: Michael Schmitt (mschmitt@baumerident.com)
Date: Mon Apr 17 2000 - 00:07:38 PDT


A several days seminar about EMC of PCB showed me exactly this what
cypress says.

We use a software to calculate the capacitor bulk the get rid of these
resonace peaks. This tools also takes care about the physical pcb
dimensions of the power planes LxWxH and Er and tells me what caps to
use and what the effective radius of the bulk caps will be.

Dipl.-Ing. (FH) Michael Schmitt
Baumer Ident GmbH
Entwicklung / Development Department
Hertzstr. 10
D-69469 Weinheim
Deutschland / Germany
Tel. +49 (0) 6201 9957 - 30
Fax. +49 (0) 6201 9957 - 99
E-Mail : mschmitt@baumerident.com
Web: http://www.baumerident.com

Mark Geddes schrieb:
>
> Has anyone else noticed in this "app note" (see below) that Cypress has a
> very good
> scientific explanation about why using several values of decoupling
> capacitors can cause problems, and then they go on to recommend a bunch of
> general routing rules that they haven't substantiated?
>
> This is how false information is spread.
>
> 1+1=2, because if I have one cow in a field, & then put another cow in that
> field, I will have two cows in the field. Therefore use one cap per power
> pin, 1+1=2, place caps close to the IC & on the same side, and keep leads
> below 6mm. Can anyone pick out the assumptions I made after giving an
> example why 1+1=2?
>
> Thanks Lee Ritchey, for making me skeptical of app notes!
>
> Mark Geddes
>
> >
> > What worries me about using bulk capacitors with different capacitance
> > in parallel as in the picture you sent is well described in
> > the following
> > article:
> > http://www.cypress.com/pub/appnotes/decouple.pdf
> > (look especially picture 5 on page 3)
> >
>
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