ok....ive seen where some designers will use a LC filter, while others use a
pi filter when feeding different sections of a board.  Generally, its used
when feeding a noise sensitive circuit such as a PLL, but I've seen it also
on just to different parts of a board when large ASIC's are used.

So, a LC is used when the impedance is different, while a pi used when the
impedances are more matched, granted that a pi is a LC with the additional
cap in front.

So, the couple of questions running around in my noggin are......does this
filter both ways...in that if you have a 3.3 rail, and feed 3 chips...each
off its own filter, do you actually gain anything by doing all three, or
would two suffice...meaning that will the filter also prevent noise from
coming back from the chips onto the rails.  Second is.....it seems there is
no rhyme or reason to some of the designs in that one might use the LC while
another a pi.  The source voltage is a small POL (point of load) module, no
specs on the output impedance but most suggest a pi filter on the output.

So, if I want to try and do the best job on keeping the power clean on the
pins of the chips...I'm leaning toward a LC  consisting of a ferrite bead, a
.1uF cap and a 1uF cap.

Comments?

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