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.

Measuring AC Impedance

Complex impedance of a node to ground, also between two nodes.

5Spice

Add a voltage TestPoint to the schematic and connect it to the node whose impedance you want to measure.

Add a current Signal Source and connect it from the node to ground. Set it's magnitude to 1.0, phase = 0.

Be sure no other Signal Sources in the schematic are active.

Run AC analysis and Plot the TestPoint voltage.

Z = V/I.   I = 1.0 due to using the current source.

Impedance between two nodes: use a differential voltage TestPoint connected between the nodes and connect the current Signal Source between the nodes.

Traditional Spice

Add a current source connected from the node to ground. Set its DC value to zero and its AC magnitude to 1.0. Phase will default to 0
example:  Iz 13 0  0  AC 1.0

Be sure no other voltage/current sources have "AC" specified.

Run AC simulation and plot the voltage at the node.

Z = V/I.   I = 1.0 due to using the current source.

Impedance between two nodes: connect the current source between the nodes and plot the voltage between the nodes.

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