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

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From: Zabinski, Patrick J. (zabinski.patrick@mayo.edu)
Date: Wed Mar 01 2000 - 15:15:02 PST


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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