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

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From: Mayer, Mike (mikem@cp.artesyn.com)
Date: Fri Mar 03 2000 - 05:48:38 PST


Someone tried to tell me that connecting the VCC pins of clock drivers to
the plane through inductors would "keep noise off of the planes". But the
whole point of a properly designed power distribution system is provide a
low impedance over a large frequency range. If your power distribution
system has a high impedance at a frequency that your clock buffer is drawing
current, then the problem is the power distribution system and not the clock
driver's connection to it. And do clock drivers draw more high frequency
current than modern processors? I doubt it.

============================================================================
=
Mike Mayer Artesyn Communication Products, Inc
Senior Hardware Design Engineer Madison, WI
Mike.Mayer@cp.artesyn.com http://www.artesyn.com/cp
============================================================================
=

> -----Original Message-----
> From: Ritchey Lee [mailto:leeritchey@earthlink.net]
> Sent: Thursday, March 02, 2000 7:47 PM
> To: si-list@silab.eng.sun.com
> Subject: Re: [SI-LIST] : Adding inductors to ground?
>
>
> It's a common practice that is a bad idea. Can't say how it
> got started, but it's a sure way to degrade the performance of parts.
>
> Lee
>
> 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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