Power supply LC filter placement

[philwinder]

lc filter power supply

Hi,
Where is the best place to put a LC power supply filter?

Since when I place it before any voltage regulators {i.e. directly on the power input to the board (DC from a S.M. power supply) } I still get a lot of noise on some high gain amplifiers even when using low noise regulators (LP3878 - National). I would have thought that the regulators PSRR would have improved the noise, not made it worse.

However, placing the LC after the regulators, before the op amp power terminals, I get minuscule amounts of noise! Although, I dont want to put a big LC pair on every amp power input!

Cheers,
Phil

 

[dipnirvana]

power supply filter

what is the L &C value ...i hope u don't have large inductor.

 

[philwinder]

power supply lc filter

For testing im using 560uH and 2200uH = 143Hz, but when I build I will use 1mH and 1mF = 159Hz.

How would this matter? Apart from the obvious damping problems?

Cheers,
Phil

Added after 1 hours 22 minutes:

Just a bit more info, the noise is due to the vcc/2 biasing supply which must be then getting amplified by 60dB. Would it be better to swap it for a voltage reference, although I am driving a shield from it. I dont think a voltage reference would have a low enough impedance.