Pi filter vs bypass cap

Discussion in 'General Electronics Chat' started by MisterNo, Apr 25, 2011.

  1. MisterNo

    Thread Starter Member

    Jun 28, 2010
    26
    0
    I came across this data sheet http://www.syfer.com/category_docs/filtsmSBSP.pdf and they offer pi filters in a 1206 package. Now all we know that 0.1uF are commonly used as bypass caps, how it will be if these Pi chip filters are used as bypass caps? I have a situation that there is a lot of swithcing noise from ICs induced back on power supply lines and I was thinking that adding extra inductance on PCB traces with these Pi filter will keep close spaced ICs more decoupled in terms of noise. So one can think of these Pi chip filters are ferrite chip bead + bypass cap.
    Any thoughts?
     
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    That's exactly what they are, an inductor with a capacitor at each end.
     
  3. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    Pi-filters provide a RF isolation between the powersupply and the powered part.
    In the PDF there is a table with the "insertion loss" for a range of frequencies.

    Bertus
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Bypass caps help the board with all the components. It prevents the adjacent chips and other devices from interfering with each other. The pi filter is to isolate the board from other devices. It is global, the bypass caps are local.
     
  5. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    You cannot use it "as a bypass cap" It needs to be placed in series. It has a current limit. It is harder to "stick in a circuit": you can almost always retrofit a cap, by jumping the power supply pins on a chip with a cap. All that said it may be a useful device.
     
  6. MisterNo

    Thread Starter Member

    Jun 28, 2010
    26
    0
    Thanks for all your replies.
    Current limit is 1A so that is more than enough for individual chips. If I pick a pi chip filter with 100nF capacitance then that is the same as using standard bypass cap, but here I have extra 0.25uH inductance in the filter.
    My reasoning was that with bypass caps decoupling is based on the fact that bypass cap forms LP filter with PCB power traces impedance and that picking pi filter with known inductance I can better control and know attenuation in the RF range.
    Now that 0.25uH is really not much extra inductance, I though that there may be some situation where these Pi filter will do the better job than bypass cap since if power traces between ICs are short then ESR and ESL of bypass cap might make if worthless above some frequency.
     
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