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

  • From: Scott McMorrow <scott@xxxxxxxxxxxxx>
  • To: Alvaro alkschbirs <alvaro.alkschbirs@xxxxxxxxx>
  • Date: Tue, 09 Nov 2010 15:55:20 -0500

Pros
wider traces
better control of differential and common mode impedance
lower loss

Cons
more space required

Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
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On 11/9/2010 3:41 PM, Alvaro alkschbirs wrote:
> 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
>> IPBLOX, LLC 150 N. Center St. #211
>> Reno, NV  89501 www.ipblox.com
>>
>> (775) 299-4236 Business
>> (866) 675-4630 Toll-free
>> (707) 780-1951 Fax
>>
>>
>>
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