[SI-LIST] : RE: Charge moving from decoupling capacitors

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From: George_Tang@Dell.com
Date: Wed May 17 2000 - 21:16:58 PDT


Barry,

This is true on most boards. But that is because the caps are tied to the
planes through vias and lead traces, which are inductive. Also, the caps
are further away from the IC power pins than the planes. This does not mean
that the cap by itself cannot work at 1 GHz. If you have a 50 mil trace
above a gnd plane terminated to a cap without using vias or other lead
traces and measure VSWR and reflections from the cap, you will see that the
cap works well at 1 GHz and beyond. You must solder the cap to the gnd
plane and to the 50 mil trace directly.

Regards,

George Tang

-----Original Message-----
From: Barry Ma [mailto:barry_ma@altavista.com]
Sent: Wednesday, May 17, 2000 9:16 AM
To: si-list@silab.eng.sun.com; EMC-PSTC
Cc: weirsp@atdial.net
Subject: Re: Charge moving from decoupling capacitors

Steve,

Thanks a lot for the very nice hierarchy description below.

If there's a 10 mil or less spacing between pwr and gnd planes, the plane
cap is available. The plane cap and the decaps are complementary in whole
frequency range. Plane cap takes care of high end, and decaps cover low
portion. Then the location of decaps are not critical. And then decaps can
be shared by other chips, according to the excellent research conducted by
EMC lab at UMR.

Please allow me to modify a bit your description as follows.

The capacitance inside the device supports the chip first, but usually not
enough.
Charge from the planes also supports the chip and replenishes the device
capacitance,
Decaps replenish the plane on low frequency portion, while plane cap
responds itself on HF end,
Bulk capacitors replenish decaps and plane cap,
The voltage regulator replenishes the bulk capacitors.

Please correct me. Thanks.

Regards,
Barry
bma@ANRITSU.com
-----------------
From: "sweir" <weirsp@atdial.net>, on 5/11/00 9:28 PM:
....

The capacitance inside the device supports the chip,
Charge from the planes replenishes the device capacitance,
HF capacitors on the board replenish the planes,
Bulk capacitors replenish the HF capacitors,
The voltage regulator replenishes the bulk capacitors.

....[edited by bm]

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