[SI-LIST] Re: Differential Pair - Impedance Question

Dear Rohit,

Thanks for the detailed reply. Is there any tool to make sure that the 
impedance of the individual trace is 45 Ohm and it is 90 Ohm (no mutual 
coupling) when it will be differential?
How can we make sure it is meeting the our requirement in manufacturing too. 
Impedance coupons are the only way or do we have any other solution(s)?

Best Regards,
Jayasuryan

-----Original Message----- 
From: Rohit MISHRA
Sent: Thursday, June 30, 2011 7:06 PM
To: si-list@xxxxxxxxxxxxx
Cc: suryan_tvm@xxxxxxxxxxx
Subject: RE: [SI-LIST] Re: Differential Pair - Impedance Question

Jayasuryan,

When you talk about "45 Ohm impedance for individual", you are talking about 
characteristic impedance(Z0) of individual or single ended line which is 
intrinsic to that line only and can't be changed by changing differential 
impedance actually it is other way around, once you have set the 
characteristic impedance of your single ended line then differential 
impedance between two lines depends on characteristic impedance(Z0) of lines 
and mutual coupling between them.

So if characteristic impedance of single ended line (D+ & D-) is Z0 and 
coupling coefficient between them is k then

Zdiff = 2 * Z0 ( 1- k) or
      = 2 (Z11 - Z12)

-> k = 0, Zdiff = 2 * Z0

Hope it should be clear now.

Rgds,
Rohit



-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of steve weir
Sent: Thursday, June 30, 2011 1:21 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Differential Pair - Impedance Question

The single-ended impedance can be half the differential impedance when
the lines have no mutual coupling.

Steve
On 6/30/2011 12:02 AM, Jayasuryan KG wrote:
> Dear  Experts,
>
>
> I have a question regarding differential pair.
>
> In my UBS differential pair, the impedance is 90 Ohm.
>
>
>
> Is it possible to achieve 45 Ohm impedance for individual (D+ and D+) 
> traces(45 Ohm+45 Ohm? Ohm) in the differential traces?
>
>
>
> Kindly share your feedbacks.
>
>
>
> Thanks in advance.
>
>
>
> Best Regards,
>
> - Jayasuryan
>
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-- 
Steve Weir
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