Ferrite Bead Question

Apr 15, 2006 5 Replies
R

We're making a pcb that's goal is to take in frequencies and filter out anything not in the 20-40Khz range. We will have a dc/dc converter, specifically this one:

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and we are using decoupling capacitors on all filter opamps Vin and Vouts; and decoupling caps on other components, including the



+/-Outputs of the dc/dc to get rid of noise.

so my question is: someone suggested we use a ferrite bead to make sure we eliminate any other noise on the dc/dc.. so do we need a ferrite bead?



All of the Ferrite beads i'm seeing are for Mhz range.. and I thought we didn't need one that high, but then the data sheet has a section on pg 3 that says "Ripple & Noise (20Mhz)". What does that mean? Would the ferrite bead solve that problem? If so, what are the specs of a ferrite bead that I should be looking for?


-Thanks-



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M

100mV noise 20Meg BW....... Thats a lot, I dont think a ferrite bead will be a lot of help. Check out the Jim Williams amazing app note, I forget the number, on the LT1533 at Linear. Also think about a Common Mode filter, from Murata etc. A CM filter might help a lot. It might be easier to try for a lower noise converter

martin

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J

Hello Martin,

Or roll your own.

Mine never have that much noise.

Regards, Joerg

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J

Sure wouldn't hurt. Some dc/dc converters kick out horrendous multi-MHz spikes or ringing things, which could easily get into low-level front-ends, or possibly heterodyne with a signal component.

Go for a high-impedance bead, 600 ohms or so (beads are usually rated for impedance measured at 100 MHz.) Put one in series with the dc/dc input, and another in series with the output, with ceramic bypasses on both sides of the beads. If the input and output grounds aren't the same, put one in the output return lead, too.

HF spikes can play hell with sensitive front-ends.

John

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G

also EDN article by Jim Williams:

and a ferrite EMI suppression presentation from Steward Ferrite can be downloaded from

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