Spice is a powerful tool for circuit analysis. But it often surprises users with
. holes in its abilities and strange definitions
. lack of industry-wide standards
. the need for the user to model the physics of their circuit, not just draw the schematic

I hope this blog will educate users and promote discussion in these areas.

Connecting Subcircuits to a Schematic Symbol - clearing up misconceptions

This will be very brief, just to clear up common confusion. The actual details vary from program to program.

Spice subcircuits (.SUBCKT) usually do not have the same connectivity order as the pins of the physical device package. Often the subcircuit does not even have the same number of terminals as the physical device has pins.

So if you use a subcircuit with a schematic capture that supports both PCB layout and Spice, you either need to create separate PCB and Spice schematic symbols, or be able to enter both sets of connectivity information into one schematic symbol.

Spice programs that do not do PCB layout, like 5Spice, only need the Spice connectivity information entered in the schematic symbol. This is obtained manually from the subcircuit listing or may be included in a database by the Spice program vendor.

Spice itself knows nothing about physical packages and schematic symbols.
A Spice subcircuit does not contain this information.

Connectivity in Spice is based on circuit nodes. Each circuit is described by a "nodelist". The main circuit's nodelist is generated from the schematic by the Spice vendor's proprietary algorithm. Spice then embeds the nodelist of the subcircuit into the nodelist of the main circuit. Homework - what is a nodelist?

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