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

  • From: Alvaro alkschbirs <alvaro.alkschbirs@xxxxxxxxx>
  • To: steve weir <weirsi@xxxxxxxxxx>
  • Date: Tue, 9 Nov 2010 18:41:24 -0200

Thank you Scott and Steve,
But, What is the benefict to use loosely coupled pairs?
When I need to use it? when I have common mode terminations?


Rgds,



On Tue, Nov 9, 2010 at 6:02 PM, steve weir <weirsi@xxxxxxxxxx> wrote:

> 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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>
>
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