**From:** Mike Jenkins (*jenkins@lsil.com*)

**Date:** Mon Nov 15 1999 - 14:52:22 PST

**Next message:**Tom Dagostino: "RE: [SI-LIST] : What speed scope should I consider?"**Previous message:**D. C. Sessions: "[SI-LIST] : Transient impedance"**In reply to:**Chris Bobek: "[SI-LIST] : Differential Pair Theory"**Next in thread:**Josh Nickel: "Re: [SI-LIST] : Differential Pair Theory"**Reply:**Josh Nickel: "Re: [SI-LIST] : Differential Pair Theory"

Chris,

I'm sure you will get multiple responses to your question. Here's

a theoretical view. (Apologies to the math-phobics out there.)

Looking onto a single-ended line is a one-port network. Differential

turns this into a two-port. The dif'l voltage is defined typically

as V(port1) - V(port2) and the common mode as [V(port1) + V(port2)]/2.

Some simplifying assumptions (which may or may not be true and should

be checked for each application) are:

1) The dif'l pair is symmetric (i.e., port1 and port2 can be

interchanged and the 2-port looks the same).

2) The signal is dif'l only (i.e., V(port1) = - V(port2)). Your

example would probably fail this assumption badly, as ground

loops would induce common mode signals.

Back to the 2-port thing....The input impedance is now a 2x2 matrix

rather than a single number:

| V(port1) | | Z11 Z12 | | I(port1) |

| | = | | * | |

| V(port2) | | Z21 Z22 | | I(port2) |

Z12 = Z21 for passive networks like this dif'l line. The dif'l

impedance is Z11+Z22-2*Z12. If the pair is symmetric, Z11 = Z22,

so the dif'l impedance is 2(Z11-Z12). On your board, if the two

traces are separate, Z12 is neglible, so Z11=50 ohms (100 ohms dif'l).

On your shield-less cable, Z11 and Z12 are large, but Z11-Z12=50 ohms.

These two structures (PCB and twisted pair) cannot be matched both

for dif'l and common mode signals.

Hope that gives you a framework to start with.

Regards,

Mike

Chris Bobek wrote:

*>
*

*> Hi,
*

*>
*

*> I am trying to understand how differential pairs behave and how you
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*> terminate them. I've read app notes and books that tell you how to
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*> terminate them, but I don't know how they derived the values and
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*> methodology. I would just like a general understanding of what's going
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*> on. I understand how and why termination works on normal, single-ended
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*> lines.
*

*>
*

*> As an example, suppose you had a 100ohm, unshielded, twisted pair cable
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*> that ran for 5000 feet between two boards. Once the pair entered the
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*> board, it was routed over several inches to a differential receiver.
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*> The two boards do not share a ground (suppose they are battery powered).
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*>
*

*> 1) How did I know to route the traces on the board as two 50 ohm
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*> traces?
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*> 2) How did I know to terminate the pair by placing a 100ohm resistor
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*> between the two traces at the receiver?
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*> 3) Could I terminate each line separately as if they were two,
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*> single-ended lines (say, using a 50ohm resistor to ground on each line
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*> at the receiver?)
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*> 4) Where does the return current flow as the signal is launched down
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*> the line from the driver, across the twisted pair, across the diff pair
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*> on the board, and to the receiver?
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*>
*

*> Thank you very much,
*

*>
*

*> Chris
*

-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Mike Jenkins Phone: 408.433.7901 _____ LSI Logic Corp, ms/G715 Fax: 408.433.7461 LSI|LOGIC| (R) 1525 McCarthy Blvd. mailto:Jenkins@LSIL.com | | Milpitas, CA 95035 http://www.lsilogic.com |_____| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~**** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****

**Next message:**Tom Dagostino: "RE: [SI-LIST] : What speed scope should I consider?"**Previous message:**D. C. Sessions: "[SI-LIST] : Transient impedance"**In reply to:**Chris Bobek: "[SI-LIST] : Differential Pair Theory"**Next in thread:**Josh Nickel: "Re: [SI-LIST] : Differential Pair Theory"**Reply:**Josh Nickel: "Re: [SI-LIST] : Differential Pair Theory"

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