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

  • From: Jory McKinley <jory_mckinley@xxxxxxxxx>
  • To: Alvaro alkschbirs <alvaro.alkschbirs@xxxxxxxxx>, steve weir <weirsi@xxxxxxxxxx>
  • Date: Tue, 9 Nov 2010 12:55:24 -0800 (PST)

Hello Alvaro,
Benefits are to name a few reduced strain on the board routing with no need to 
maintain differential impedance through more dense areas keeping in mind that 
what is most important is the electrical skew of both paths should be held to a 
reasonable design margin value.  Common mode noise should be minimized through 
loosely coupled lanes but if any common mode makes it to the RX then you have 
termination set.
Regards,
-Jory




________________________________
From: Alvaro alkschbirs <alvaro.alkschbirs@xxxxxxxxx>
To: steve weir <weirsi@xxxxxxxxxx>
Cc: si-list@xxxxxxxxxxxxx
Sent: Tue, November 9, 2010 3:41:24 PM
Subject: [SI-LIST] Re: Differential pairs commom mode impedance

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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>
>
> --
> Steve Weir
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