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

• From: steve weir <weirsi@xxxxxxxxxx>
• To: si-list@xxxxxxxxxxxxx
• Date: Fri, 01 Jul 2011 01:20:54 -0700

```Use a field solver.

Steve.
On 7/1/2011 1:04 AM, Jayasuryan KG wrote:
> 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?
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> Best Regards,
>>
>> - Jayasuryan
>>
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>

--
Steve Weir
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