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

• From: "Jayasuryan KG" <suryan_tvm@xxxxxxxxxxx>
• To: "Rohit MISHRA" <rohit.mishra@xxxxxx>
• Date: Fri, 1 Jul 2011 13:34:34 +0530

```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
IPBLOX, LLC
150 N. Center St. #211
Reno, NV  89501
www.ipblox.com

(866) 675-4630 Toll-free
(707) 780-1951 Fax

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```