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

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From: Hansen_Chris@dpt.com
Date: Fri Mar 03 2000 - 08:58:55 PST


Along these lines, does anyone have any experience with the new low inductance
caps giving better performance in the power system decoupling network? In
particular, AVX advertises several types of low inductance caps. They have
their reverse aspect ratio 0508 and 0612 type that are supposed to has 1/2 the
inductance of their 0805 and 1206 counterparts. They have their IDC family of
parts. These parts give a designer 8 connection points on the chip so ideally 4
short traces instead of 1 could run to VCC and ground, thereby reducing the
inductance contribution of the connection of the device to the power system and
therefore the whole inductance of the system. On the super low inductance side,
AVX offer their LICA parts. These are actually little BGAs that contain an
array of capacitors and are supposed to reduce the inductance down into the 100
pH range. Before committing to the expense of totally switching over to any of
these devices, it would be nice to know if anyone has had experience to show a
justifiable improvement in system performance in going from the traditional 0805
or 1206 package style ceramic chip caps that we currently use, to one of these
other low inductance varieties.

Thanks,
Chris Hansen
DPT/Adaptec

"Mayer, Mike" <mikem@cp.artesyn.com> on 03/03/2000 08:48:38 AM

Please respond to si-list@silab.eng.sun.com

To: "'si-list@silab.eng.sun.com'" <si-list@silab.eng.sun.com>
cc: (bcc: Chris Hansen/DPT)

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

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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