RE: [SI-LIST] : Selection of Optimum Termination

Bob Davis ([email protected])
Tue, 20 Oct 1998 01:28:31 -0700


Signal Integrity is about energy control, from the source of energy to the
one or more destinations.

Series termination schemes absorb the excess energy early, while parallel
termination schemes absorb it later. The topology directs the energy to
where it is needed, when it is needed. Excessive energy ends up going where
it is not wanted and coming back to bite you later. A 100ma 1ns driver with
a 3 volt edge from a CMOS device pushes much more energy (~6-9 times) into
the topology than an ECL/PECL driver does. Almost all modern technology
receivers require input currents ranging from 1 nanoamp to 1 microamp to
cover the leakage of the input protection diodes, which is 5 to 8 orders of
magnitude below what the driver is capable of driving. The remaining
energy, with high spectral content, is either absorbed in the termination,
the lossy board material, protection diodes, nearby lines or is radiated for
the FCC to look for.

As a circuit must work with both strong case and weak case drivers, the
starting energy varies from a low ratio of 2:1 to as much as 5:1 with some
loosely controlled processes and still must make a reasonably clean, timely,
transition of the receiver threshold.

Think spectral energy control. Remember the energy is in the fields between
the signal and the return path.

I hope this provide a simpler way of thinking about SI.

Bob Davis
Summit Computer Systems, Inc
Signal Integrity Specialists - High Speed, Critical PCB Design
[email protected] ,

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Abe Riazi
Sent: Monday, October 19, 1998 8:59 PM
To: '[email protected]'
Subject: [SI-LIST] : Selection of Optimum Termination

Hi Everyone:

Termination constitutes an important topic of signal integrity. I
have been interested in reviewing articles which describe the required
steps for determination of optimum termination for a high speed digital

Many papers written on this subject, include an excellent description
of various types of termination (i.e. Series, Parallel, Thevenin, Diode,
etc.). Some of the articles illustrate that to select the correct
termination for a high speed digital design, it is necessary to take
into consideration the technology (i.e. CMOS, TTL, ECL, etc.) of the
components. This is due to the fact that several characteristics of the
drivers and receivers (for example rise and fall times, input and output
impedances) play a critical role in determination of the optimum
termination method and component values. I was impressed by a more
recent publication which discussed another significant element, namely
the routing scheme (i.e. daisy chain, star, point to point, etc.) or
topology, which should be also carefully evaluated when choosing a
termination technique. For instance, parallel termination is well
suited for daisy chain routing, and series termination is frequently a
superior choice for the star topology.

Topology, Termination, and Technology are at times referred to as
the three "T"s of signal integrity. A careful appraisal of each of the
three is often a necessity when determining the most effective
termination type.

I would appreciate your comments and suggestions related to required
considerations, computations, or steps towards selection of optimum

Best Regards,

Abe Riazi
SI Engineer
Anigma, Inc.

email: [email protected]

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