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

  • From: "Havermann, Gert" <Gert.Havermann@xxxxxxxxxxx>
  • To: "Jayasuryan KG" <suryan_tvm@xxxxxxxxxxx>
  • Date: Fri, 1 Jul 2011 13:12:54 +0200

Jayasuryan,

the coupling between individual traces of a differential pair is a function of 
distance. The wider you put the traces apart, the lower the coupling. In your 
case with 45Ohm individual and 90 Ohm differential impedance, you will go for 
zero coupling, thus the traces need to be far away.
For stripline, as a rule of thumb, 1.5 times the Signal Plane to GND Plane 
distance (prepreg thicknes) would be a good starting point for zero coupling.

Just couriosity, why do you need 45 Ohm single ended impedance? In many 
differential applications a tighter coupling is beneficial.

BR
Gert


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-----Ursprüngliche Nachricht-----

Von: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] Im 
Auftrag von Jayasuryan KG
Gesendet: Freitag, 1. Juli 2011 10:05
An: Rohit MISHRA
Cc: si-list@xxxxxxxxxxxxx
Betreff: [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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Other related posts:

  • » [SI-LIST] AW: Re: Differential Pair - Impedance Question - Havermann, Gert