[SI-LIST] Re: Differential Impedance Mismatch Rule of Thumb

In a message dated 12/19/2002 2:30:49 PM Pacific Standard Time, 
Jacob.Foor@xxxxxx writes:

> I've heard that the propagation delay of the split
> should be no more than 1/5 of the rise time of the signal in order for
> the mismatch to not affect the signal. 

This is a reasonably valid guideline, but like all things, it is a 
compromise. Martyn's comments "... increase the line width appropriately for 
the duration of the split whilst you route around the obstacle, at least this 
way you could keep the odd mode impedance matched" is a superior approach. 

When using 100-Ohm differential pairs with 10-15% coupling, separating the 
traces will yield a 10-15% increase in the even-mode (single-ended or SE) 
impedance of each trace and about a 20-30% increase in the 
odd-mode/differential impedance.
Widening the traces to lower the SE impedance to 50 Ohms will maintain the 
differential impedance near the 100-Ohm target. The net result is that the 
even-mode and odd-mode impedances are little changed. Yes, Martyn, I have 
successfully used this technique, particularly in coursing through AMP HS-3 
connector pin fields. The differential TDR results verify negligible 
differential impedance disturbance. Simply separating the traces and 
maintaining the same/original trace widths indeed yields 20-30% impedance 
discontinuities (also verified by TDR measurements). Of course in pin fields, 
the parasitic loading of nearby pin vias must be accounted for, and the 
coupling to other signal pins needs to be balanced to minimize common-mode to 
differential-mode conversion.

Enjoy the holidays,

Mike

Michael L. Conn
Owner/Principal Consultant
Mikon Consulting

                   *** Serving Your Needs with Technical Excellence ***


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