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

About this list Date view Thread view Subject view Author view

From: Scott McMorrow (scott@vasthorizons.com)
Date: Wed Mar 01 2000 - 16:15:24 PST


Chris,

For pll inputs, where there is a small current flow and small
or negligible instantaneous current, inductors can be placed
in the power and ground circuits to totally isolate the PLL
power from high frequency noise components. The assumption
here is that the PLL circuit is running in a linear mode
of operation with little or no transient current.

One could argue that isolating only the Vcc pin for an analog
device, without also isolating the ground pin through ferrites
only isolates the device from half of the possible noise. Since
ground still wiggles, there will be noise on the PLL Vcc pin
relative to ground. Isolation using two ferrites would provide
higher noise isolation. However, one must be sure to provide
adequate isolation from other parasitics where noise
could be coupled into the circuit, such as nearby signal lines,
coupled pins in the package, large planes or fill areas where
there could be additional capacitive coupling parasitics ... etc.

regards,

scott

--
Scott McMorrow
Principal Engineer
SiQual, Signal Quality Engineering
18735 SW Boones Ferry Road
Tualatin, OR  97062-3090
(503) 885-1231
http://www.siqual.com

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 > > **** To unsubscribe from si-list or si-list-digest: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP. > si-list archives are accessible at http://www.qsl.net/wb6tpu > ****

**** To unsubscribe from si-list or si-list-digest: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu ****


About this list Date view Thread view Subject view Author view

This archive was generated by hypermail 2b29 : Thu Apr 20 2000 - 11:35:15 PDT