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Subject: Differential Impedance
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YSShin
Posts: 8
Online: User is Offline
3/16/2006 9:00 PM  
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 = 10.5mil
Then 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
Kalevi2
Moderator
Posts: 69
Online: User is Offline
3/23/2006 9:30 AM  
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.
Kai Keskinen
dallas
Posts: 0
Online: User is Offline
3/23/2006 11:15 AM  
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.
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