[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: Thu, 29 Sep 2011 14:21:00 -0700
De nada!
--------------------------------------------------
From: "Grasso, Charles" <Charles.Grasso@xxxxxxxxxxxx>
Sent: Tuesday, September 27, 2011 3:10 PM
To: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>; "steve weir"
<weirsi@xxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
Subject: [SI-LIST] Re: Routing differential lines as single ended traces ?
> Thank you Lee!
>
> 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: Lee Ritchey [mailto:leeritchey@xxxxxxxxxxxxx]
> Sent: Monday, September 26, 2011 4:50 PM
> To: Grasso, Charles; steve weir; si-list@xxxxxxxxxxxxx
> Subject: Re: [SI-LIST] Re: Routing differential lines as single ended
> traces ?
>
> 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
>>> (866) 675-4630 Toll-free
>>> (707) 780-1951 Fax
>>>
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