[SI-LIST] Re: Routing differential lines as single ended traces ?

  • From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
  • To: "steve weir" <weirsi@xxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
  • Date: Mon, 26 Sep 2011 09:10:53 -0700

I always route differential pairs to a "not closer than" rule.  This 
guarantees that there will be no unwanted interaction such as one line 
driving the impedance of the other down.  I have seen no cases where this 
has resulted in a lack of board space for routing.  Perhaps there are some 
somewhere.


I arrive at the "not closer than" rule by using a field solver to determine 
how close the two lines can be without adversely affecting the impedance of 
either one.

When you use the "not closer than" rule all traces are single ended and 
usually 50 ohm.  That means you don't need to add complexity to bare board 
test by insisting on a 100 ohm diff pair  measurement.  Makes life much 
easier.  The drivers always wanted to see 50 ohm lines any way.

Lee

--------------------------------------------------
From: "steve weir" <weirsi@xxxxxxxxxx>
Sent: Sunday, September 25, 2011 7:22 PM
To: <si-list@xxxxxxxxxxxxx>
Subject: [SI-LIST] Re: Routing differential lines as single ended traces ?

> On 9/25/2011 6:57 PM, Low Jerry wrote:
>> Hi,
>> I recently came across a validation platform where the LVDS differential
>> lines are routed as single ended traces traces instead of tightly coupled
>> differential pairs. The motivation I found was so that they could use 
>> each
>> of the pairs as single ended traces when needed as well. When I probed
>> deeper it seems like the person who proposed this scheme has left. So I
>> would like to seek help here on some clarification
>>
>>     - Will a scheme like this impact the performance of the differential 
>> pair
> The scheme itself will not.
>>     ? Since I understand that differential routing is more immune to 
>> noise.
> That is largely a myth.  There are only a few very special circumstances
> where tightly coupled pairs exhibit better noise rejection to PCB trace
> noise aggressors than reasonably routed, loosely coupled pairs.
>>     - What are the considerations/feasiblity studies that need to be done
>>     before implementing a scheme like this ?
> They are the same as with any signal integrity requirements.  The scheme
> doesn't impose anything extra.  It does remove some headaches.
>>     - Since this is a validation platform what measures can be taken 
>> ensure
>>     that the performance seen are similar to a production platform if the
>>     production platform is routed in differential.
> Homework gets done or it doesn't.
>
> Steve
>>
>> Thanks in advance for the feedbacks. Have a great day.
>>
>>
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>
>
> -- 
> Steve Weir
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