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About the influence of a ground plane on the differential pair impedance

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  • About the influence of a ground plane on the differential pair impedance

    Hi everyone,

    I am looking for articles about this topic.
    Thanks for help.
    Attached Files

  • #2
    Something like this: https://www.signalintegrityjournal.c...o-return-plane


    • #3
      qdrivesI don’t think that it is what I was waiting for.
      I didn’t explain very well.
      I meant the gnd plane being on the same layer than the differential pair. Please refer to screenshot.


      • #4
        I didn't find any article about the topic.
        Now the idea could be to set up a new rule for polygon clearance around differential pairs in order to enlarge it, something between 1 and 2mm instead 0.2mm.
        What do you think?


        • #5
          And the result with a clearance of 1mm is as shown on the screenshot.


          • #6
            Seeing I got the first part wrong, let me ask what it is you are thinking of.
            1) Do you have a Gnd return (plane/polygon) below/above the tracks?
            2) I do not think you would design this on a single layer board, what is the distance from this signal layer to the next?
            ​3) Are you asking because of the other discussion on co-planar waveguides? The one where I mentioned that the 'Gnd' on the same layer still had an influence > 2mm according to most calculators.
            4) Are you thinking here as to have the two signals couple into each other and not a return plane? Thus making them more true differential pairs.

            If it is point 3, then I found it very interesting and frightening at the same time. However, I think most calculators are wrong as otherwise you could never calculate impedance correctly in a 'normal' situation.


            • #7
              qdrives 1/ Yes, we have
              2/ This is a 16 layers stackup
              3/ No

              So, for you, the best option is to remove all gnd polygon pours on all signal layers?
              Is there any calculator for differential pairs guarded by gnd polygon pour?


              • #8
                So, for you, the best option is to remove all gnd polygon pours on all signal layers?
                NO. Definitely not.

                iCD Coplanar Waveguide (CPW) Planner - Model microstrip Coplanar Waveguides to reduce radiation loss, of high-speed serial links, significantly improving product performance. The iCD Design Integrity software incorporates the iCD Stackup Planner, iCD PDN Planner, iCD CPW Planner and SmartSolver software plus a myriad of functionality specifically developed for high-speed PCB design. The iCD Stackup Planner offers unprecedented simulation speed, ease of use and industry leading impedance accuracy at an affordable price. In-Circuit Design is also a PCB Design Service Bureau specializing in High Speed Multilayer PCB Design and Board Level Simulation.


                How can I compute the differential impedance of an Edge-Coupled Coplanar Waveguide With Ground? I couldn't find any free calculator online, so I wrote a small program which computes the the impeda...

                Last edited by qdrives; 07-19-2023, 03:54 PM.


                • #9
                  qdrivesThank you so much for all this information.
                  I checked everything with different calculators, there is a slight difference of impedance between with or without coplanar waveguide for a same width and gap.
                  The tolerance about the impedance is generally 10% and 20% for LAN.
                  The calculation shows we are always within this range if we consider "differential" in AD instead "differential coplanar".