**From:** Tom Dagostino (*tom_dagostino@mentorg.com*)

**Date:** Mon Nov 15 1999 - 14:43:38 PST

**Next message:**D. C. Sessions: "[SI-LIST] : Transient impedance"**Previous message:**Mark Randol: "[SI-LIST] : Commercialism solution?"**In reply to:**Mike Degerstrom: "Re: [SI-LIST] : Differential Pair Theory"**Next in thread:**Phares, Charles C: "RE: [SI-LIST] : Differential Pair Theory"

There are many different "single ended" trace values that will give you a

100 Ohm differential line, all dependent on the coupling between the two

traces. Do not assume unless you know for a fact that the trace impedance

is half of the differential impedance. That assumption only holds true for

totally uncoupled lines. Twisted pairs tend to have a lot of coupling and

thus the self impedance is a lot different from 100 Ohms divided by 2. If

the system is 100 Ohms differential then differentially terminate into 100

Ohms.

Tom Dagostino

-----Original Message-----

From: owner-si-list@silab.eng.sun.com

[mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Mike Degerstrom

Sent: Monday, November 15, 1999 2:18 PM

To: si-list@silab.eng.sun.com

Subject: Re: [SI-LIST] : Differential Pair Theory

On Nov 15, 1:27pm, Chris Bobek wrote:

*> Subject: [SI-LIST] : Differential Pair Theory
*

*> 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
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*> 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?
*

You want to match your 100 ohm twp to your board impedance. When excited

differentially, the characteristic impedance of your 100 ohm twp behaves

as 50 ohms to ground. I believe it is called 100 ohm cable instead

of 50 ohm cable because the impedance is related between the true

and complement and not to ground. So two 50 ohm Zo's in series is

100 ohms. One thing to keep in mind is that if you route your

t/c pair closely together on your pcb then you must design for

the odd mode impedance which is typically several ohms lower

than your single-ended Zo.

*> 2) How did I know to terminate the pair by placing a 100ohm resistor
*

*> between the two traces at the receiver?
*

You don't unless the manufacturer of the IC that drives your

differential interface specified this termination. As opposing

examples take LVDS. You should terminate with 100 ohms between

t/c at the load end. But if it was ECL you need to terminate

both t/c with 50 ohms to -2.0. In the former case you cannot

terminate to ground or you will unbias the LVDS output buffer.

In the latter case if you did not terminate the output buffer

to -2 volts then you don't bias the ECL output buffer correctly.

It can be difficult to visualize why 100 ohms 'works' for

a 50 ohm environment on your pcb. However, simply visualize

the 100 ohm resistor as two 50 ohm resistors tied to a 'virtual

supply'. This virtual supply is whatever your output buffers

set the voltage to be which is (1/2)*(VHIGH+VLOW). Then again

you can visualize your pcb Zo to be the impedance between true

and complement which is 2*50 ohms.

*> 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?)
*

Only if you were using SPICE with ideal voltage sources. Otherwise

I answered this above for real world scenarios.

*> 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?
*

*>From your driver you can think of 'I' flowing from a power and/or
*

ground supply from the true leg and '-I' flowing from a power and/or

ground supply from the complement leg. The net result that there

is no AC switching return current. Over a pcb with controlled Zo

stripline or microstrip you can visualize most of the return current

traveling in the reference plane above and/or below the conductors.

When you launch onto the twp, the pcb return currents for true and

complement sides can be visualized as simply canceling themselves

out to yield no current. Over the twp, you can visualize the

return currents as flowing from true to complement and vice-versa.

The return pcb is the same as the launching pcb. For termination

between the t/c pair, the return current flows through the resistor.

For t/c pair termination to a supply then you dump I into the

power supply from the true side and -I into the supply from the

complement side - again the switching current is zero.

Mike

*>
*

*> Thank you very much,
*

*>
*

*> Chris
*

*>
*

*>
*

*> **** To unsubscribe from si-list: send e-mail to
*

majordomo@silab.eng.sun.com.

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*>
*

*>-- End of excerpt from Chris Bobek
*

-- _______________________________________________________________ Mike Degerstrom Email: degerstrom.michael@mayo.edu Mayo Clinic 200 1st Street SW Gugg. Bldg. RM 1042A Phone: (507) 284-3292 Rochester, MN 55905 FAX: (507) 284-9171 WWW: http://www.mayo.edu/sppdg/sppdg_home_page.html _______________________________________________________________**** 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 ****

**** 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:**D. C. Sessions: "[SI-LIST] : Transient impedance"**Previous message:**Mark Randol: "[SI-LIST] : Commercialism solution?"**In reply to:**Mike Degerstrom: "Re: [SI-LIST] : Differential Pair Theory"**Next in thread:**Phares, Charles C: "RE: [SI-LIST] : Differential Pair Theory"

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