[SI-LIST] : Differential Pair Theory

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From: Chris Bobek ([email protected])
Date: Mon Nov 15 1999 - 13:27:58 PST


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

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
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,


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