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

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From: Michael Nudelman (mnudelman@tellium.com)
Date: Fri Nov 10 2000 - 17:01:30 PST


Keith:

Basically, you have to remember a mantra:"The fast current flowing into my
GND pin flows into my VCC pin. There is no free current". Repeat it 100
times before going to sleep.

Let's take sourced current.
If you have a driver, than the current you source first flows into your VCC,
than into your load (TRX line, resistor, solder short - choose one).
The return current (if you treat it nicely) flows under your TRX line in
your GND plane back to your GND pin and/or in your VCC plane through decoup.
cap to your GND pin.
Same process when sinking.

In books of Hi-Speed design, pictures often times mention return current
through the GND plane and the GND pin inductance. For some reason they forgo
mentioning the other side.

Power supply is symmetrical, and if you have two power planes, than there is
no basic difference between Sinking/Sourcing - just choose your reference
plane to be the other one and your sourcing become sinking.

Lots of traces are referenced to VCC planes, as well as to GND.

Some of my hi-speed routing layers consist of pairs GND-sig-sig-VCC.

To summ up - you have to decide which pin/plane you like to bounce more -
VCC or GND.

Personally, I am an equal opportunity bouncer.

"Kowal, Keith" wrote:

> 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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