[SI-LIST] Differential pairs commom mode impedance

  • From: Alvaro alkschbirs <alvaro.alkschbirs@xxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Tue, 9 Nov 2010 17:27:54 -0200

Hi gurus,
I have a question about differential pairs routing.

The XFP MSA Specifications says:

"XFI differential reference impedance for driver, *transmission line*, and
receivers are defined as 100 Ohms. XFI common mode reference impedance for
driver, *transmission lines*, and receivers are specified as 25 Ohms."

I think that it says that my signal need to have both 25ohms commom mode
impedance and 100ohms differential impedace. In pratical terms, I need to
make this traces thinner and more spaced. Thinner traces are more critical
and add  too.

Is this requirement true for any type of 100ohms differential impedance
pair?

If yes, this requeriment will add some problems to make BGA fanouts without
route the pairs separately by other pins fanout vias.

Why I need control common mode impedance?
How critical is this requirement?
Is 30 ohms commom mode impedance acceptable?

I will appreciate your help!
Rgds,
Álvaro

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