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Symmetrical stackup = asymmetrical impedance?

AndiLoop

Hello all together,

I'm trying to design my very first flex PCB with impedance 50 Ohm single ended and 100 Ohm differencial signals.

I have some problems to understand the calculation from Cadence in the stackup.

Its a three layer stackup and reference plane is GND in the middle.

From GND to TOP and BUTTON its symmetrical so I'm expecting equal impedance on both outer layers.
But Cadence seems to have another calculation.

What do I miss in this situation?

And as far as I understand Cadence only know single ended impedance.
So assuming a VERY large gap between two differential pairs each 50 Ohm impedance add together to 100 Ohm differencial.
So there is no way to get a correct impedance for parallel routed differencial pairs directly in Cadence from the contraint manager, right?

Thank you very much!

Parents
  • redwire

    Hmmmm....on a test board I made with 3 layers I get identical impedance on TOP and BOTTOM.  So maybe what JC Tessier said that it's related to the interconn.iml file?  You can always exit Allegro and delete that file and re-check.

    Also, Cadence fully understands differential pairs and calculates the impedance correctly.  The diff impedance should be available if you click on the enable diff impedance check box.

  • jc teyssier
    in reply to redwire

    When impedance calculation retrun "strange" results; in 99% it's a stackup definiton problem: checking and correct it will correct the result. In smoe rare case (say 1%), result is still inscorrect. In such case, closing allergro, remove tinterconn.iml and reopen allegro (it crete the file if not exist) correct the problem.

    I have seen 1 brd in past where this was not enought; it was a16.6 brd.

    16.6 does not show the trapezoidal angle in stackup window.

    Opening the brd in 17.2, i saw that one layer have a different value for this parameter.

    Jean-Charles

Reply
  • jc teyssier
    in reply to redwire

    When impedance calculation retrun "strange" results; in 99% it's a stackup definiton problem: checking and correct it will correct the result. In smoe rare case (say 1%), result is still inscorrect. In such case, closing allergro, remove tinterconn.iml and reopen allegro (it crete the file if not exist) correct the problem.

    I have seen 1 brd in past where this was not enought; it was a16.6 brd.

    16.6 does not show the trapezoidal angle in stackup window.

    Opening the brd in 17.2, i saw that one layer have a different value for this parameter.

    Jean-Charles

Children
  • redwire
    in reply to jc teyssier

    Actually 16.6 does show the trapezoidal angle.  It depends on the license you open the brd file with.  A basic license will not show it but an SI license, for example, will.  

  • jc teyssier
    in reply to redwire

    You are right. Thanks for the precision

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