User`s guide

Interface Subcircuit Selection by PSpice A/D 15
-3
Interface Subcircuit
Selection b
y
PSpice A/D
AtoD and DtoA interface subcircuits handle the translation
between analog voltages/impedances and digital states, or vice-
versa. The main component of an interface subcircuit is either a
PSpice A/D N device (digital input: digital-to-analog) or a
PSpice A/D O device (digital output: analog-to-digital).
PSpice A/D “N” and “O” devices are neatly packaged into
interface subcircuits in the model library . The standard model
library shipped with your MicroSim software installation
includes interface subcircuits for each of the supported logic
families: TTL, CD4000 series CMOS and high-speed CMOS
(HC/HCT), ECL 10K, and ECL 100K. This frees you from ever
having to define them yourself when using parts in the standard
library.
Every digital primitive comprising the subcircuit description of
a digital part has an I/O model describing its loading and driving
characteristics. The name of the interface subcircuit actually
inserted by PSpice A/D is specified by the I/O model of the
digital primitive at the interface. The I/O model has parameters
for up to four analog-to-digital (AtoD) and four digital-to-
analog (DtoA) subcircuit names.
The reason for having four possible levels of interfaces is that in
some instances you may need more accurate simulations of the
input/output stages of a digital part. In other instances, a simple,
smaller model will suffice. Therefore, the four interface levels
allow you to choose among different models depending on the
accuracy required from the simulation.
Currently, digital parts provided in the standard libraries only
use interface levels 1 and 2. With the exception of the HC/HCT
series (described below), levels 3 and 4 reference the same
subcircuits as levels 1 and 2, though future releases may make
use of these. Table 15-1
summarizes the four interface levels.
The difference between levels 1 and 2 only occurs in the
AtoD interfaces, described below. In all cases, the level 1 DtoA
interface is the same as the level 2 DtoA interface.
That’s the letter O, not the
numeral zero.
If you are creatin
g
custom di
g
ital
parts in technolo
g
ies other than
those provided in the standard
model library, you may have to
create your own interface
subcircuits.