Datasheet

7–221
Introduction
The data in this application report demonstrates the skew between the outputs of a sample of Texas Instruments Advanced
BiCMOS (ABT) devices. This report explains which output skew is being examined, where the data comes from, and how the
data is analyzed. Some of the errors that may be present in the data are discussed.
Skews
Skew is a term that defines the difference in time between two signal edges. Several different types of skew being used are
defined in JEDEC 99 clause 2.3.5.
Output Skew [t
sk(o)
] – The difference between two concurrent propagation delay times that originate at either a
single input or two inputs switching simultaneously and terminating at different outputs.
Input Skew [t
sk(i)
] – The difference between two propagation delay times that originate at different inputs and
terminate at a single output.
Pulse Skew [t
sk(p)
] – The difference between the propagation delay times t
PLH
and t
PHL
when a single switching
input causes one or more outputs to switch.
Process Skew [t
sk(pr)
] – The difference between identically specified propagation delay times on any two samples
of an IC at identical operating conditions.
Limit Skew [t
sk(l)
] – The difference between: 1) The greater of the maximum specified values of t
PLH
and t
PHL
and
2) The lesser of the minimum specified values of t
PLH
and t
PHL
.
The skew discussed here is the skew of propagation delays across the outputs of a device. More specifically, it is the difference
between the largest value obtained for a propagation delay and the smallest value across all of the outputs. For example, if
output 3 has the largest propagation delay t
PLH
and output 14 has the smallest, the output skew for this device would be the
difference between the propagation delays for output 3 and output 14 (see Figure 1).
The majority of the curves presented in this paper consist of data taken on devices that have one output switching at a time.
This produces a skew that should not be confused with the defined data-sheet skew t
sk(o)
. The data-sheet value for t
sk(o)
is found
by switching all of the outputs simultaneously. Two of the devices examined in this paper (ABT16240 and ABT16500A)
include curves that present t
sk(o)
data.
Source of Data
The data used to produce the curves presented in this paper was extracted from the characterization data bases used to prepare
the data sheets for the devices presented. The sample size of the data base is approximately 30 devices for each characterization
lot (wafer) used.
The data was sorted so that the maximum skew for each device at a particular V
CC
and temperature combination could be
determined. Next, the maximum skew values were averaged to produce a data point for each transition. Further statistical
analysis of this data was performed to calculate a standard deviation of the maximum skew across the devices. This value was
then used to produce a three-standard-deviation data point for each V
CC
and temperature combination. The data is presented
as a family of curves across V
CC
, with each member of the family being an output skew versus temperature curve. The curves
for each device are broken out by output transition (i.e., t
PLH
, t
PHL
). Each transition is further separated into a set of curves
depicting the average skew across the devices and a set representing the average skew, plus three standard deviations.