Re: [SI-LIST] : Differential Pair Theory

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From: David Instone (david_instone@uk.xyratex.com)
Date: Tue Nov 16 1999 - 07:08:28 PST


The best description/tutorial of terminating differential lines etc I
have ever seen was in 'Printed Circuit Design March 1999' Pages 25-28
article title is 'Terminating Differential Signals On PCBs'

It was written By Steve Kaufer and Kellee Crisafulli, both of Hyperlynx

Dave Instone
__________________
Chris Bobek 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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