[SI-LIST] Re: Differential pairs commom mode impedance

  • From: steve weir <weirsi@xxxxxxxxxx>
  • To: Alvaro alkschbirs <alvaro.alkschbirs@xxxxxxxxx>
  • Date: Tue, 09 Nov 2010 12:02:09 -0800

It's basically telling you to use loosely coupled pairs where each line 
has a single ended impedance of 50 Ohms.  For zero line to line 
coupling: Zcm = Zse /2, and Zdiff = Zse*2.

Steve.


Alvaro alkschbirs wrote:
> 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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