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

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From: sweir (weirsp@a.crl.com)
Date: Wed Mar 01 2000 - 17:21:52 PST


The usual good intention of this is to keep noise from the planes out of
the PLL, not the other way around. The fear is that the VCO will get
thrown off by PS noise. It can be more dangerous to use an inductor,
unless the filter is properly damped, than to use either a resistor or
direct connection. In either case, high quality bypass from the PLL VCC to
the PLL ground is very important. This is also one of the places where
judicious moating can be very useful.

Regards,

Steve.
At 03:58 PM 3/1/00 -0800, you wrote:

>Yes, but then what is the point of putting an inductor if it is only
>supplying the on-chip PLL?
>
>My (limited) understanding of why the inductor is needed is that it prevents
>the switching noise on the PLL clock driver from coupling back to the power
>planes reducing EMI and such related problems.
>
>Giri
>
>
> > -----Original Message-----
> > From: Vinu Arumugham [SMTP:vinu@cisco.com]
> > Sent: Wednesday, March 01, 2000 3:35 PM
> > To: si-list@silab.eng.sun.com
> > Subject: Re: [SI-LIST] : Adding inductors to ground?
> >
> > If it is a PLL GND pin that is only supplying the on-chip PLL section
> > which has no I/Os, there may be no signal return currents through this
> > pin. An inductor in this case will not have a detrimental
> > effect.
> >
> > Vinu
> >
> > "Zabinski, Patrick J." wrote:
> >
> > > Chris,
> > >
> > > I never heard of this either, but I thought of one possible
> > > scenario where it might be of use.
> > >
> > > In traditional ECL/TTL/etc. logic, Ground (0 V) is used
> > > as the primary reference for transmission lines and return
> > > paths. In these cases, it's best to have a low-inductance
> > > return path for the signals, and it is sometimes better
> > > (debatable) to insert an inductor/choke/ferrite bead in
> > > the power path to increase isolation between circuits and
> > > reduce EMI.
> > >
> > > However, PECL uses it's positive power supply for signal
> > > reference, and it should be used as the return path,
> > > not 0V ground. In a PECL system, I can envision where
> > > the supply should have low-inductance and the ground could
> > > have intentional inductance inserted for isolation.
> > >
> > > Can't say I've ever seen it, but depending upon the
> > > technology you're using (i.e., return-path dependent),
> > > I can see where it makes sense. Comments?
> > >
> > > Pat
> > >
> > > > 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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