RE: [SI-LIST] : Differential pair - How to compensate for differe nt length?

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From: Alokby, Ahmed (ahmed.alokby@intel.com)
Date: Mon Jan 29 2001 - 12:37:36 PST


I just ran into this trying to match differential clocks on a new design. I
twirled one of the lines near the receiver and the design came back fine.
Obviously you compromise differential coupling to get better matching. You
do need to match the lines pad to pad especially if you are concerned about
35 mils (approximately 5 ps of flight time).
Ahmed
Intel Corps.

-----Original Message-----
From: Michael Nudelman [mailto:mnudelman@tellium.com]
Sent: Monday, January 29, 2001 6:43 AM
To: 'Shahar Eytan'; 'si-list@silab.eng.sun.com'
Subject: RE: [SI-LIST] : Differential pair - How to compensate for
differe nt length?

We usually run such a pair as "differential" (parallel) as possible right to
the very end of line, where the shorter wire will go additional distanse.
However, depending on you frequency rate, 35 mils may not even call for
compensation; just try to see what part of UI it is; it may be unnecessary.

Mike.

-----Original Message-----
From: Shahar Eytan [mailto:shahar@mellanox.co.il]
Sent: 28 января 2001 г. 08:21
To: Shahar Eytan; 'si-list@silab.eng.sun.com'
Subject: [SI-LIST] : Differential pair - How to compensate for different
length?

        Hi All,

        I need to connect high speed lines from a Fine BGA to a connector
using differential lines. The positive and negative outputs from the FBGA
resides in row 1 and 2 of the FBGA, that means ~35 mils of difference
between the lines.
        What is the right way to compensate for this difference?
        I was thinking that the best way is to go strait out with from the
internal ball, while the second one (from the external row) will "pay" for
his shorter way by few turns as close as possible to the source. This way
the differential pair will be coupled together all the way to the connector.
Same for the receiver.
        What do you think?

                Shahar Eytan
                Mellanox Technologies Ltd.
                <mailto:shahar@mellanox.co.il>
                Tel: 972-4-9593244 ext 264
                Fax: 972-4-9593245

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