Re: [SI-LIST] : Differential Pair Theory

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From: sweir (weirsp@a.crl.com)
Date: Mon Nov 15 1999 - 16:57:39 PST


Chris,

I reasonable tutorial is included in Lucent's quad driver / receiver
literature. That also has the advantage of explaining some of the common
mode range issues, as the Lucent parts are PECL.

Regards,

Steve.
At 01:27 PM 11/15/99 -0800, you wrote:
>Hi,
>
>I am trying to understand how differential pairs behave and how you
>terminate them. I've read app notes and books that tell you how to
>terminate them, but I don't know how they derived the values and
>methodology. I would just like a general understanding of what's going
>on. I understand how and why termination works on normal, single-ended
>lines.
>
>As an example, suppose you had a 100ohm, unshielded, twisted pair cable
>that ran for 5000 feet between two boards. Once the pair entered the
>board, it was routed over several inches to a differential receiver.
>The two boards do not share a ground (suppose they are battery powered).
>
>1) How did I know to route the traces on the board as two 50 ohm
>traces?
>2) How did I know to terminate the pair by placing a 100ohm resistor
>between the two traces at the receiver?
>3) Could I terminate each line separately as if they were two,
>single-ended lines (say, using a 50ohm resistor to ground on each line
>at the receiver?)
>4) Where does the return current flow as the signal is launched down
>the line from the driver, across the twisted pair, across the diff pair
>on the board, and to the receiver?
>
>Thank you very much,
>
>Chris
>
>
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