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

• From: Rohit MISHRA <rohit.mishra@xxxxxx>
• To: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
• Date: Thu, 30 Jun 2011 21:36:40 +0800

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