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

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From: Vinu Arumugham (vinu@cisco.com)
Date: Wed Mar 01 2000 - 15:35:14 PST


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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