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

  • From: Scott McMorrow <scott@xxxxxxxxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Tue, 09 Nov 2010 14:56:24 -0500

Alvaro

the spec also requires differential and common mode termination.  This 
is in an apparent attempt to control common mode emissions.  If you 
follow the strict specification, all traces should be designed to be 50 
ohm single-ended impedance with large spacing to achieve low coupling, 
and as close as possible to 100 differential / 25 ohm common mode impedance

This will cause you to increase your trace width, rather than reduce 
it.  You will need to route the traces as single-ended in the BGA 
fan-out region.

Scott


Scott McMorrow
Teraspeed Consulting Group LLC
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On 11/9/2010 2:27 PM, 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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