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From [log in to unmask] Sat Apr 6 10: |
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I agree that this matrice will work in most instances and generally the
engineer will make notation, if it is to be otherwise.
Henry R. Linneweh
PCB Designer
On Fri, 5 Apr 1996, Waddell, Pete wrote:
>
> I've heard it said both ways. But what is the deal? Well since decoupling caps
> are intended to minimize switching noise, which shows up at the pwr pin, what
> we're trying to do is dump the noise to gnd. This means minimizing noise on
> traces, so vcc trace(s) should be as short as possible. I've seen the Moto book
> too and it confuses the hell out of a lot of people. Does the Moto fact book say
> why the caps should be closer to gnd??
>
> Theoretically the multi layer construction would seem to mean that since there
> are no traces, (a surface mount cap does require a trace/via, however short, to
> feed to the plane) the caps are filtering noise all across the board. Does this
> make sense?
>
> Pete Waddell
>
> ______________________________ Reply Separator _________________________________
> Subject: Re: Placement of bypass caps
> Author: [log in to unmask] at Internet
> Date: 4/5/96 4:18 AM
>
>
> Mr Brooks
> The old rule of tumb was to set by-passes next to VCC so you did
> not have a pwr trace running on top of your board the length of your
> IC.
> How ever today most boards have planes for pwr, gnd So a even
> matrics of bypasses across the board will do the trick.
>
> All you are doing is decoupling vcc,gnd, if done correctly the board
> and parts will not make noise.
>
>
>
> Hope this helps
> DM PAVONE
> Micros
>
>
> ______________________________ Reply Separator _________________________________
> Subject: Placement of bypass caps
> Author: [log in to unmask] at SMTPLINK-Micros
> Date: 4/4/96 5:45 PM
>
>
> I have several sources (including the Motorola FACT book) that say bypass
> caps should be placed as close as possible to the GROUND pin of the IC (not
> the power pin.) But I have seen few really specific discussions of why this
> is so.
>
> Some of my customers insist on having them placed as close as possible to
> the power pin.
>
> Can anyone give me a definitive explanation of where they should be placed,
> and why? Thanks
>
> Doug Brooks
>
>
>
>
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