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PSpice User Guide Mixed analog/digital simulation October 2019 645 Product Version 17.4-2019 © 1999-2019 All Rights Reserved. Specifying digital power supplies Digital power supplies are used to power interface subcircuits that are automatically created by PSpice A/D when simulating analog/digital interfaces. They are specified as follows: ■ PSpice A/D can instantiate them automatically. ■ You can create your own digital power supplies and place them in your design. When using parts from the standard libraries in your design, you can usually have PSpice A/D automatically create the necessary digital power supply. If you use custom digital parts created in technologies other than those provided in the standard model library, you may need to create your own digital power supplies. Because digital power supplies are used only by analog/digital interface subcircuits, digital power supplies are not needed for digital-only designs. We recommend avoiding placing a power supply to a digital-only design because it may increase simulation time and memory usage. Default power supply selection by PSpice A/D When PSpice A/D encounters an analog/digital interface, it creates the appropriate interface subcircuit and power supply according to the I/O model referenced by the digital part. The I/O model is specific to the digital part's logic family. The power supply provides reference or drive voltage for the analog side of the interface. By default, PSpice A/D inserts one power supply subcircuit for every logic family in which a digital primitive is involved with an analog/digital interface. These power supply subcircuits create the digital power and ground nodes that are the defaults for all parts in that family. If multiple digital primitives from the same logic family are involved with analog/digital interfaces, one instance of the power supply subcircuit is created with all primitives connected to the power supply nodes. Table 15-2 summarizes the default node names and values. For instance, TTL power supplies have a default value of 5.0 volts at analog/digital interfaces.