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Diffpair impedance calculation

myil17

Hi Everyone,
I am a new hardware engineer. I am little bit confused while I compare the results from cadence tool and saturn pcb design tool
for diffpair impedance matching. why do these two results differ?

  • excellon1

    What version of the PCB Editor are you using & What is the diffpair impedance you are wanting ?

  • myil17
    in reply to excellon1

    I am using 17.4-2019 S019 [7/14/2021] Windows SPB 64-bit Edition. I would like to get 100 ohms for ethernet.

  • excellon1
    in reply to myil17

    So, in order to design the diff-pair one has to really know what the intended stackup of the board is and the material that is used. A good place to start is to look at a couple of board manufacturers and figure out for your design what would be the best type of board. At a minimum you would probably need a 4Lr board so as to accommodate the correct impedance of the diff pair. Each manufacturer has their own recipe for making boards so you would need to know the details of the stackup they normally use.

    The Saturn Tools are good and can be used to calculate the DIFF-Pair when you know what the intended stackup you wish to use. Most tools will give a fairly close approximation of the correct size traces and spaces to use. The built in stackup editor in Allegro is also pretty good.

    There can be a difference in the calculated results between different tools, it basically comes down to the formulas used. Some tools are better in that they can consider things such as etch-factor etc.

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