RE: [SI-LIST] : Power filtering?

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From: Patel, Bhavesh (bpatel@ciena.com)
Date: Wed May 09 2001 - 11:37:50 PDT


Hi! In my case the PLL power pins are somewhere in the middle for a BGA and
I cannot get a 20mil trace to the pin so using a plane but where I am
getting confused is that by using isolated plane approach I can take care of
the noise rejection but the filter components are connected to gnd so
wouldn't I pick up noise from there(gnd). I am not advocating cutting up the
gnd plane but I am trying to justify using this isolated plane approach for
a single PLL power pin.
Thanks for all the inputs so far, its a real help
Bhavesh

-----Original Message-----
From: Michael Nudelman [mailto:mnudelman@tellium.com]
Sent: Wednesday, May 09, 2001 5:09 AM
To: Patel, Bhavesh; SI_LIST (E-mail)
Subject: RE: [SI-LIST] : Power filtering?

Bhavesh:

What we did (successfully) - we did not cut out planes; for PLL power we
just used two-pole LC filter (actually Ferrite-C and LC filter), and fat
trace around the chip. Right now we will do the experiment on the RC filter.

Mike.

-----Original Message-----
From: Patel, Bhavesh [mailto:bpatel@ciena.com]
Sent: Tuesday, May 08, 2001 9:24 PM
To: SI_LIST (E-mail)
Subject: [SI-LIST] : Power filtering?

Hi! Gurus, I had a question regarding the amount of noise rejection that can
be achieved 1)by using a filter 2) using separate power plane.
If the IC manufacturer said that the PLL power to be supplied has to be
'clean' then he is mentioning about the noise rejection that he would like
to have on these power pins that feed power to his internal PLL's.
The manufacturer will typically recommend an RC filter and I believe the IC
mfgr has verified on his test board that this the filter values that will
achieve the noise rejection.
Now, some designers want to cut up the power plane i.e. say from the main
3.3V plane cut up a small portion use the Resistor as a bridge to feed the
power to the island.
I believe we have achieved two levels of rejection 1)filter 2) cut up plane.

Can I just use the filter and not cut up the power plane and if I have to
how do I calculate the noise rejection number by using the filter & using
the island approach.
Thanks in advance
Bhavesh Patel

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