Input impedance analysis

This is how we at Röcklinger set up LTspice for input impedance analysis

Use a current source with the fixed magnitude of 1 Ampere:

1. In parallel with the input.
2. In a point in the circuit where analysis is required.

A shut resistor is not needed if Rinput is in parallel with with the current source. In other cases use a 100G resistor in parallel with the current source.
From Ohms law we find that U = I * Z and as the current source is set to 1 Ampere then in this case the magnitude of the Voltage is equal to the Impedance.
Do an AC analysis and plot voltage of interest divided by the current source. Set the scale of Y-axis to linear and voila impedance is ploted.