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Ferrite Bead Question

Discussion in 'Electronic Design' started by [email protected], Apr 15, 2006.

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  1. Guest

    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: http://www.calex.com/pdf/3wdsmt.pdf
    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-
     
  2. On 15 Apr 2006 09:37:35 -0700, in sci.electronics.design
    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
     
  3. martin griffith wrote...
    an70.
     
  4. Joerg

    Joerg Guest

    Hello Martin,
    Or roll your own.

    <brag_mode>
    Mine never have that much noise.
    </brag_mode>

    Regards, Joerg
     
  5. John  Larkin

    John Larkin Guest

    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
     
  6. Glen Walpert

    Glen Walpert Guest

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