*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 ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: http://www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: http://www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu

**Follow-Ups**:**[SI-LIST] Re: Differential pairs commom mode impedance***From:*Scott McMorrow

**[SI-LIST] Re: Differential pairs commom mode impedance***From:*steve weir

- » [SI-LIST] Differential pairs commom mode impedance - Alvaro alkschbirs
- » [SI-LIST] Re: Differential pairs commom mode impedance - Scott McMorrow
- » [SI-LIST] Re: Differential pairs commom mode impedance - steve weir
- » [SI-LIST] Re: Differential pairs commom mode impedance - Alvaro alkschbirs
- » [SI-LIST] Re: Differential pairs commom mode impedance - steve weir
- » [SI-LIST] Re: Differential pairs commom mode impedance - Jory McKinley
- » [SI-LIST] Re: Differential pairs commom mode impedance - Scott McMorrow
- » [SI-LIST] Re: Differential pairs commom mode impedance - Vinu Arumugham
- » [SI-LIST] Re: Differential pairs commom mode impedance - steve weir
- » [SI-LIST] Re: Differential pairs commom mode impedance - Vinu Arumugham
- » [SI-LIST] Re: Differential pairs commom mode impedance - steve weir
- » [SI-LIST] Re: Differential pairs commom mode impedance - Vinu Arumugham