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Differential trace characteristic impedance question

Discussion in 'Electronic Basics' started by billcalley, Apr 9, 2008.

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  1. billcalley

    billcalley Guest

    Guys,

    Since I have to deal with differential circuits now, and I am only
    use to single-ended, I was wondering if the characteristic impedance
    of a differential trace matters. To clarify: If I design
    differential traces to terminate into say, 100-ohms differential, I
    could use traces that are 90 mils wide with a trace-to-trace spacing
    of 100 mils to accomplish this, or I could use traces that were 30
    mils wide with a spacing of 5 mils. Is either OK, even though the
    characteristic impedance of each individual trace is significantly
    different in the two examples? Would it matter to the circuit at all,
    as it does with the characteristic impedance of a microstrip trace in
    a single-ended circuit -- or is it only the differential impedance
    that matters in such applications?

    Thanks!

    -Bill
     
  2. Phil Allison

    Phil Allison Guest

    "billcalley = Shit For Brains Cunthead "


    ** What sort of " differential circuits" are you on about ?

    Kindly supply a link to an example.


    BTW

    Please be more vague in future.

    ****.



    ....... Phil






    To clarify: If I design
     
  3. Phil Allison

    Phil Allison Guest

    "Bob Wanker Congenital Anencephalic "



    ** Puke, puke puke......
     
  4. Phil Allison

    Phil Allison Guest

    "Terry Given ASD Fucked Pile of Kiwi Scum "



    ** Same as all the others.




    ....... Phil
     
  5. It ultimately depends upon the receiver topology used, but differential
    impedance usually does not matter. It's single ended that matters.
    See here:
    http://www.speedingedge.com/PDF-Files/diffsig.pdf

    However, it is possible (but sometimes tricky) to design your tracks to meet
    both 100ohms differential and 50ohm SE if you are that way inclined.

    If you look at some app notes for the high speed diff interfaces on various
    top end FPGA's (like Xilinx), you'll find similar guidelines. 50ohm SE is
    what is needed.

    Dave.
     
  6. Fred Kruger

    Fred Kruger Guest

    Do the world and yourself a favor - walk into your local Walmart, lay a
    franklin on the counter for a sturdy double-barrelled shotgun, load it,
    then walk into the bathroom and decorate the ceiling with your skullcap.
     
  7. Phil Allison

    Phil Allison Guest

    ** My god, the vilest scumbags on the planet and all their pathetic
    sock-puppets are coming out of the cracks now.

    Phewwweeee - what a stench !!



    ....... Phil
     
  8. billcalley

    billcalley Guest


    Thanks guys for all those very clarifying answers! I feel I
    understand much better how to use balanced microstrip now. So
    basically the single-ended impedance doesn't really matter, just so
    long as the differential impedance of the traces meets our specs
    (because, as stated above, to place a 100-ohm balanced microstrip with
    50-ohm single-ended characteristic impedance on, let's say, a 62mil
    PCB, would take up a lot of space!).

    Cheers,

    -Bill
     
  9. Fred Kruger

    Fred Kruger Guest

    Go **** yourself with an 18V cordless drill fitted with a 1/2" hole saw
    bit, Phil. You should be brutally flogged using the cured, platted skin of
    your own dead mother, be rolled in salt, sand & dog shit and then dipped
    in bleach before being battered to death with a two foot black rubber
    dildo with a nail through it.
    Gee Phil, that's just your breath blowing back in your face. One would
    have thought you'd be used to that by now.
     
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