[SI-LIST] Re: Routing differential lines as single ended traces ?
- From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
- To: "Grasso, Charles" <Charles.Grasso@xxxxxxxxxxxx>, "steve weir" <weirsi@xxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Mon, 26 Sep 2011 15:49:32 -0700
Charles,
I knew someone would ask that question! I usually design stackups with 4
mils of laminate between the stripline layer and its plane. When I do that,
a 10 mil edge to edge separation results in one trace diminishing the
impedance of its neighbor by about 1%. That's not a big drop and 10 mil
separation has not turned out to be a problem for routing the board.
As you can see, there is a judgment call to be made here.
5% would be half the entire impedance error budget we allow and 10% would be
all of it.
Hope this helps.
lee
--------------------------------------------------
From: "Grasso, Charles" <Charles.Grasso@xxxxxxxxxxxx>
Sent: Monday, September 26, 2011 1:04 PM
To: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>; "steve weir"
<weirsi@xxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
Subject: [SI-LIST] Re: Routing differential lines as single ended traces ?
> Hello Lee - How much interaction is too much? 1% 5% - 10%??
>
>
> Best Regards
> Charles Grasso
> Compliance Engineer
> Echostar Communications
> (w) 303-706-5467
> (c) 303-204-2974
> (t) 3032042974@xxxxxxxxx
> (e) charles.grasso@xxxxxxxxxxxx
> (e2) chasgrasso@xxxxxxxxx
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Lee Ritchey
> Sent: Monday, September 26, 2011 10:11 AM
> To: steve weir; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Routing differential lines as single ended traces ?
>
> 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
>> IPBLOX, LLC
>> 150 N. Center St. #211
>> Reno, NV 89501
>> www.ipblox.com
>>
>> (775) 299-4236 Business
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