Our
last meeting was..
Differential
Impedance Finally Made Easy
Presentation
Download Instructions
PICTURES
Presenter:
Dr Eric Bogatin, Bogatin Enterprises
Date :
October 20 2000
Times :
1:30 pm - 4:30pm
What we
saw: An increasing number of designs
are levering the performance features of
differential pairs. Yet, their behavior and
design principles are confusing to most
engineers. Dr Bogatin took time from his busy
schedule to visit us at the RMCEMC Society to
help us understand the somewhat confusing world
of differential impedance. 32 clearly confused
engineers turned up to finally understand
Differential Impedance and we were not to be
disappointed. Some special mentions: The RMCEMC
officers were especially glad to see students
from Colorado University at the meeting and a
special mention to one intrepid fellow who flew
in from Salt Lake City, Utah to participate. I
think that that is another first for the RMCEMC -
having a out-of-state- guest at one of our
monthly shindigs. Dr Bogatin started the meeting
with the assertion that once the concept
impedance was understood, then differential
impedance will become clear. The first order
equivalent circuit models were reviewed and
discarded because although correct, they do not
help in the understanding of differential
impedance. In order to better understand
impedance, Eric led us into a Zen like spiritual
exercise - in short we had to "...be the
signal"!!. Using a lot of pacing along the
podium, Eric then described another way to look
at impedance using the velocity of propoagation
and the capacitance of the line.We learned that a
good intuition of the impedance of a geometry can
be obtained by looking at the capacitance first.
After reviewing the current flow in a
transmission line, Dr Bogatin then got into the
meat of the presentation - the Differential Pair.
We reviewed some of the impedance characteristics
of the differential pair and then looked at how
to drive and terminate the signal. We were
introduced to the Odd and Even mode
characteristics of the differential pair and how
they relate to the impedances. After the first
hour we understood that a differential pair is
any two transmission lines with the special case
of two symmetrical lines. In the second hour, Dr
Bogatin discussed first how to measure a
differential pair. He then displayed some
measurements taken on a board that drove home the
concepts presented in the first hour. Dr Bogatin
then added the bane of every engineers existance
- the split ground plane. Impedance and current
plots were shown that emphasized the superior
performance of differential drive over a split.
After the second hour we understood that the
impedance of one line in a differential pair
depends on how the other is driven. We reviewed
how to measure differential pairs and verified
again - do NOT cross a split. Questions were
peppered at Dr Bogatin during the meeting and a
better understanding of differential signalling
was obtained.
Biography:
Eric is the President of Bogatin
Enterprises, specializing in training for Signal
Integrity and Interconnect Design. Topics focus
on the electrical properties of interconnects,
including on-chip, IC packages, MCMs, boards and
cables, and how these structures can be designed
for optimized high speed performance. In addition
to private and public short courses from
fundamental to advanced levels, Bogatin
Enterprises publishes many papers on Signal
Integrity topics.
Contact Information:
Eric Bogatin
BOGATIN ENTERPRISES
Training for Signal Integrity and Interconnect
Design
v: 913-393-1305
f: 913-393-1306
e: eric@bogent.com
web: <http://www.gigatest.com>
ftp: ftp://ftp.BogatinEnterprises.com
Presentation
Download:
Bogatin Enterprises was aquired by GigaTest in
2001. You can get a pdf copy of the presentation here.
Pictures
from our October Meeting
Click on
image to enlarge
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