[SI-LIST] Re: Placement of decoupling capacitors
- From: Ray Anderson <Raymond.Anderson@xxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 31 Jul 2001 14:53:02 -0700 (PDT)
Khalid Ansari wrote:
>
>I just have enough space to place this one close to the pin, the rest have
>to go quite further away due to space constraint. Placing them too far
>away I have to worry about the loop inductance.
>
The loop inductance you need to be concerned with is
that of the loop formed by the decap, the mounting pads,
vias and power planes. The distance between the decap
and the IC doesn't determine the loop inductance.
That being said, the distance IS an issue when you consider
the time of flight between the decap and the IC consuming
the current. A commonly used rule-of-thumb is that
the decap should be within 1/10 wavelength (at the frequency
of interest) of the chip it is supplying in order to be effective.
If you are too far away, the chip requiring the current may not
get it in time from the decap sourcing the current.
-Ray Anderson
Sun Microsystems Inc.
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