[SI-LIST] : Distinguishing Differential Models

Abe Riazi ([email protected])
Sat, 25 Sep 1999 09:17:38 -0700

Dear All:

The archives of this forum contains a significant amount of
information regarding differential signals. Numerous authors have
discussed various key characteristics of differential transmission lines
such as impedance of differential pairs and single-ended lines as a
function of odd and even mode impedance, broadside coupling vs. edge
coupling, equal trace length vs. constant trace separation, return path
current distribution, termination, common-mode (CMM) noise,
measurements, etc. However, very little has been written about
differential models. Actually these models possess some unique features
well worthy of consideration.

Let us explore differential models by means of a simple example.
Presented below is a part (which includes only the differential pairs)
belonging to the [Pin] and [Diff Pin ] sections of IBIS model of a
differential clock generator:

[Pin] signal name model_name
R_pin L_pin C_pin
. . .
. . .
. . .
. . .
. . .
. . .
18 CLKB outbufb
20.0 m 1.60 nH 0.3 pF
20 CLK outbuffb
20.0 m 1.60 nH 0.3 pF


[Diff Pin] inv_pin vdiff
tdelay_typ tdelay_min tdelay_max

20 18 0.00 V
0.00 S -55.0 pS +55 pS

The [Diff Pin] section above is the complete section of the model.
However, the [Pin] section includes only those pins associated with the
differential pair, and the remaining pins have been omitted.

The IBIS keyword [Diff Pin] can be utilized to relate differential
pins, differential threshold voltages and timing delays. The first
column (i.e. [Diff Pin] ) represents the non-inverting pin name. The
second column (i.e. inv_pin ) corresponds to the inverting pin name
for the I/O buffer. It is necessary for each pin name to match the pin
names listed in the [Pin] section of the model. The third column (i.e.
vdiff ) presents the output and differential threshold voltage between
pins (provided they are Input or I/O pin types). The typical, Minimum
and Maximum launch delays (of non-inverting pins relative to inverting
pins) are given by the fourth, fifth and sixth columns respectively.

Before utilizing an IBIS model in a simulation, it is frequently
required to translate the IBIS file into the native model format of the
simulator. Usually, the translated models also identify the
differential pins by means of polarity indicators. For example, the
differential pins of XTK models of above example include POS and NEG as
illustrated below:

PIN 18 TYPE outbuff 20 NEG L=1.6 C=0.3
PIN 20 TYPE outbuff 18 POS L=1.6 C=0.3

All of the differential IBIS models that I have worked with included
the [Diff Pin] section. However, it is possible for a differential
model to be missing this section. This is especially true because based
on IBIS specifications the [Diff Pin] section is not a requirement. I
have investigated this possibility by removing the [Diff Pin] section,
translating the model to Quad, then testing it in a simulation. The
converted models now lacked polarity indicators (as expected), that is:

PIN 18 TYPE outbuff 20 L=1.6 C=0.3
PIN 20 TYPE outbuff 18 L=1.6 C=0.3

and caused erroneous simulation.

In summary, a differential IBIS model (driver or receiver) can be
distinguished by presence of a [Diff Pin] section which follows the pin
mapping part. According to IBIS specs, [Diff Pin] keyword is not a
requirement, but its absence (in a differential IBIS model) can lead to
errors during the translation and simulation stages.

Your comments are much appreciated, and I feel obligated to thank
Walt Otto, Dale Terrien and John Yuratovac for having expertly
described to me many interesting concepts related to differential pairs
and signal integrity.

Best Regards,

Abe Riazi
SI Engineer
Anigma, Inc.

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