[SI-LIST] Re: Differential Impedance Mismatch Rule of Thumb
- From: MikonCons@xxxxxxx
- To: si-list@xxxxxxxxxxxxx
- Date: Fri, 20 Dec 2002 14:03:52 EST
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 ***
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
