[SI-LIST] Re: Diff.Pairs
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: bmgman@xxxxxxxxxx
- Date: Sun, 12 Oct 2003 09:57:17 -0700
Mike
That is my point. This is an example of routing a differential pair
with matched skew, but on different layers. I do not recommend this
routing at all. It is, however an example of what Lee has advocated in
his post, which I quote here:
"It is time to stop representing differential signals as needing to be
tightly coupled to each other in order to operate properly. It is simply
not so. I have routed thousands of differential signal where each member
of the pair is on a different layer. If this were not possible, 1 mm pitch
BGAs with differential signals would be un routable. There are tens of
thousands of such parts being shipped every month on PCBs where they are
routed apart from each other. "
This is an extremely bad practice and bad advice and should be shunned.
regards,
scott
Mike Brown wrote:
>Scott, the stackup shown has image current flowing in all 4 Pwr/Gnd
>planes for any signal transition, differential or common mode. The only
>place where they cancel is at the driver's power/ground pins. Noise on
>any of the planes will cause current flow in that plane pair; so to the
>extent that current in the plane can be considered common mode, you are
>correct. I just do not see this stackup as being in any sense
>differential.
>
>That it can be used to connect a differential driver to a differential
>receiver is true. That it is in any way electrically optimal is false.
>That it may be the only route available may or may not be true. The
>schedule would have to be extremely pressing for me to accept a board
>routed in this manner.
>
>Mike
>
>Mike
>
>Scott McMorrow wrote:
>
>
>
>>Mike
>>
>>see below
>>
>>
>>
>>>Scott,
>>>how did the difference between the noise voltages get to be common
>>>mode? Given identical noise voltages on both traces, the difference
>>>is zero - and that is differential noise. The CM noise is the
>>>average of the two noise voltages (sum/2)
>>>
>>>
>>
>>I am talking about a case where Lee has suggested that the
>>differential pair can be split between two different routing layers.
>>The stackup might be as follows:
>>
>>Ground
>>
>>diff layer 1
>>
>>Power 1
>>
>>Ground
>>
>>diff Layer 2
>>
>>Power 2
>>
>>
>>The noise on each of these layers is totally different. Thus at any
>>point along the differential pair there is a common mode voltage
>>differential that occurs, causing a net current flow in one direction
>>or the other. You are correct for signals that are referenced to the
>>same layer.
>>
>>Oh, and your point is well taken. -1.5 dB of loss would be 1.5 dB of
>>gain. Oops!
>>
>>
>>regards,
>>
>>scott
>>
>>
>>
>
>
>
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--
Scott McMorrow
Teraspeed Consulting Group LLC
2926 SE Yamhill St.
Portland, OR 97214
(503) 239-5536
http://www.teraspeed.com
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