Re: [SI-LIST] : Adding inductors to ground?

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From: Chris Bobek (cbobek@cadence.com)
Date: Fri Mar 03 2000 - 08:49:54 PST


Hi,

Thanks for all of your responses! I asked a similar question of Howard Johnson when I was at his seminar. From what I remember, his technique was as follows (I apologize if I mess it up!)

1) Create an island for AVcc. It has no electrical connection to anything on the PWB.
2) Create a moat for Gnd that has a small "drawbridge" of cu to Gnd on one side.
3) Add an inductor or ferrite from Vcc to the AVcc island.
4) Add several decoupling caps of various size (including some large values in the several uF range). These caps go from AVcc directly to the semi-isolated Gnd island underneath.
5) NOTHING crosses the moat, except the inductor!
6) Tie any grounds from ICs within the island directly to the semi-isolated Gnd.
7) All signals leaving the isolated circuitry must pass over the cu drawbridge.

This may not work for every application, but I'm sure it does for a lot of them. I should add that Howard said to only use this method as a last resort. Placing sensitive circuitry in the corner of the
board may be a better alternative.

Thanks,

Chris

Chris Bobek wrote:

> Hi,
>
> I came across a schematic that shows the PLL ground of an IC connected
> to ground through an inductor. The Vcc pin of the device is connected
> to a bunch of caps and an inductor to Vcc.
>
> I understand and have used an inductor (or a resistor) with a bunch of
> decoupling caps on Vcc for applications like this. But, I've always
> tied the Gnd pin(s) directly to ground. It doesn't make any sense to me
> why you would want to add inductance in the path of any ground. It
> seems you would just add switching noise to your device.
>
> Can somebody explain whether this is common practice, or whether it was
> a poor design (sorry I don't have more information on the particular
> device).
>
> Thanks,
>
> Chris
>
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