[SI-LIST] : Power filtering?

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From: Patel, Bhavesh (bpatel@ciena.com)
Date: Tue May 08 2001 - 18:24:05 PDT


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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