RE: [SI-LIST] : deCoupling caps and there placement

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From: Larry Miller (ldmiller@nortelnetworks.com)
Date: Tue Nov 14 2000 - 07:20:49 PST


From an EMI standpoint, if you can consider the cap to be an ac short then
putting it closer to ground will mean less inductance between the capacitor
and the ground plane. This means you will have less area radiating on the
capacitor body, pads, etc..

This same theory holds for if you have an AC termination on a logic line.
You have a capacitor and a resistor in series. If you put the capacitor "on
the bottom", i.e., attached to ground, then the junction of the resistor and
capacitor is also at RF ground and you have less radiating area.

So I vote for putting the decoupling caps closer to the ground plane if you
have to make a choice.

Larry Miller

> -----Original Message-----
> From: Kowal, Keith [SMTP:kowal@dnpg.com]
> Sent: Friday, November 10, 2000 3:11 PM
> To: si-list@silab.eng.sun.com
> Subject: [SI-LIST] : deCoupling caps and there placement
>
> I just heard a school of thought that says place decoupling caps
> physically as close as possible to GND pins....I have always keep the
> lead length as short as possible to the VCC pin which decouples the
> power pins. I understand that one should keep the ground lead as short
> as possible also ... so if one has a trade off to make , which lead
> should be longer and why...??? any thoughts
>
> keith
> DIGITAL NETWORKS
>
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