How to measure input impedence of an opamp?

[analogpr]

Hello! I have download a pspice model of a commercial opamp. I would like to know how to calculate the input impedence of the opamp using pspice. My model has 5 ports. 2 inputs, 2 voltage supply ports and 1 output.
Thanks

 

[FvM]

You have OP common mode and differential input impedance and the input impedance of a specific amplifier circuit. What are you looking for?

 

[goldsmith]

How to measure input impedence of an opamp?

Hello! I have download a pspice model of a commercial opamp. I would like to know how to calculate the input impedence of the opamp using pspice. My model has 5 ports. 2 inputs, 2 voltage supply ports and 1 output.
Thanks

Hi analogpr

As FvM Mentioned above , there are some modes for an Op amp . input impedance of an op amp depends on which circuit you've used it , in . for instance if you are running an amplifier within that , and it has negative feedback input impedance will be affected and must be calculated with considering that issue . it also depends of frequency of operation too . with variations in frequency of operation of your circuit , input impedance will be affected somehow .
Best Wishes
Goldsmith

 

[crutschow]

One way to measure the impedance with Pspice is to simply measure the input current change with input voltage of the circuit for whatever circuit configuration you are interested in. The input impedance is then ΔV / ΔI.

 

[godfreyl]

I wouldn't trust any measurement of input impedance from a simulator because:
a) Most SPICE models are inaccurate.
b) Input imedance can vary over a wide range for different opamps of the same type.

Why not just look in the datasheet?

 

[LvW]

I wouldn't trust any measurement of input impedance from a simulator because:
a) Most SPICE models are inaccurate.
b) Input imedance can vary over a wide range for different opamps of the same type.
Why not just look in the datasheet?

Perhaps the background of the question is to investigate/proof HOW GOOD the model properties reflect the data sheet values? In this case, it makes much sense to perform simulations.

 

[FvM]

Simulated "measurement" results aren't necessarily less accurate than datasheet specifications but hardly better. Everything depends on the model quality.

Simulated measurements may be nevertheless meaningful to determine the effective input and output impedances of different amplifier circuits.