Hello, I want to know the Formula for differential impedance that is using Allegro PCB SI.Acctually, if I set up layers in Layout Cross Section, Then I can see the Impedance of target trace's width under the condition. But after layout, in SigXp, the impedence of the trace is not same with it.In short, Layout Cross Section window shows, Single Line Width = 6.5mil Differential Spaceing = 10.5milThen I can get the target impedance - 100ohm.But even I used those design rule, in SigXp, the tracemode what has those stackup informations shows different impedance - 92ohm.Is there any one who have seen this kind of problem?Thanks for your concerning.YS Shin
Do you have the same dielectric constant for the two cases? Are the reference planes marked as conductor or plane? If they are conductor, you will see the right impedance in the board but the extraction to sigxp will give you the wrong reference planes.
Did you remember to fill in the dielectric constant for the conductor
layers in the Allegro stackup? The dielectric value specified on
the conductor layers represents the dielectric beside the traces.
It should be the same as the prepreg material.