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

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From: Ritchey Lee (leeritchey@earthlink.net)
Date: Fri Mar 03 2000 - 16:50:44 PST


Perhaps a better question to ask is for the science that supports the plane cutting approach. The science one might offer is the fact that copper planes that are not cut, have very low impedances from
place to place along their surfaces. As a result, currents travelling in them, produce very small voltage drops (noise). You can do this math for yourself by getting the sheet resistance of the foil
copper used to build PCBs.

You can also make measurements on real PCBs that you have around you taht are operating. That's the best way as it is a very direct method and you don't have to take anyone's word for it.

Lee

Martin thompson wrote:

> Hi Lee,
> Can I ask for some science to back this sort of comment up, otherwise I
> have to choose between one comment saying its good and another saying
> its bad. I suppose I could use some metric like 'number of typos in
> message' to decide :-)
>
> Thanks,
> Martin
>
> Martin Thompson.
>
> TRW Automotive Advanced Product Development,
> Stratford Road, Tel: +44 (0)121-627-3569
> Solihull, Fax: +44 (0)121-627-3584
> B90 4GW. UK mailto:martin.j.thompson@trw.com
>
> -----Original Message-----
> From: Ritchey Lee [SMTP:leeritchey@earthlink.net]
> Sent: Friday, March 03, 2000 1:48 AM
> To: si-list@silab.eng.sun.com%INTERNET
> 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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