Electrical – How to use a subcircuit in capture / spice

The answer below is how to properly import a PSpice model (usually generated in a third-party software) into Orcad Capture and PSpice so that both the schematic editor and the simulator work without errors.

Summary
Firstly, the model must be imported into Orcad Capture (so that a new component and a new library .olb is created. Secondly, the model must be properly "fed" to PSpice.

Step 1. Creation of Orcad's component library (.olb file)

• Open Orcad Capture -> File -> Import -> PSpice -> OK

Notes:
a) Field Open: path to the PSpice model
b) Field Save as: path to an Orcad library to be created with a new component. Extension must be .opj
c) Field Schematic Configuration File: leave it as suggested

Step 2. Setting up PSpice simulator

• Copy and paste your model file (e.g. Model.mod).
• Rename the file from Model.mod to Model.lib. This is done, because PSpice works only with .lib extension.
• Open Orcad Capture, PSpice -> New Simulation Profile -> Enter any name for the profile (e.g. tran1) -> In an opened window go to "Configuration files" tab -> Library category

• Enter the path to Model.lib in the Filename field -> Press Add to Design button
• Make sure that the file {Orcad installation directory}\tools\pspice\library\nom.lib is also added to the Configured Files as a Global
• Make sure that in the Library Path there is a correct path to the PSpice main library (see the picture above as an example of a proper path).

Final comments
a) Wrt the error you mentioned "ERROR(ORPSIM-15108): Subcircuit LM741/NS used by X_U5 is undefined", this is caused by improper setting of PSpice simulation profile (either improper path to the Model.lib or main Library Path).
b) Overall, the trick is that import of a PSpice model is done twice: once in Orcad Capture and once in PSpice (would have been better if done automatically by the program).
c) Of course, to add the component in Capture, one needs to add the library (Model.olb) in a Place Part subwindow.
d) Not sure, but likely names of the files Model.lib and Model.olb must be the same, so that PSpice knows what Spice model to use.

That's a good step using current sources over voltage ones. You can use transfer functions under the form of Laplace expressions, looking like this:

Laplace=(s + 1)/(s^2 + 2);

This, as seen, would be entered as the value of a G source, for example. LTspice will know to transform s into the complex exponential. It can also work in a behavioural source.

BUT while the Laplace expressions will work flawlessly in frequency domain, they can result in garbage in time domain, something mentioned in the manual.

This is why, unless you're dealing with exotic transfer functions having sqrt(s), or similar, non-multiple of s, it's better to derive your transfer functions using the basic RLC elements. For example:

These can be combined into full-sized transfer function expressions, but they should be proper transfer functions; for improper ones, you'll have to juggle with the basic building blocks above, somehow. For stability issues, it's also a good idea to split the longer expressions into 2nd order ones. Here is an example:

You could also use S-param files, but, IIRC, they're based on Laplace. These are just examples on how to do it and, remember, it's just to avoid the awful performance in time domain of Laplace expressions. Ultimately, the choice is in your hands.

The generic transfer function for the 2nd order block is this:

$$H(s)=\frac{a_{2}s^2+a_{1}s+a_0}{s^2+b_{1}s+b_0}$$

where \$a_n\$ and \$b_n\$ are the ones in the image, while \$s\$ is expressed as \$\frac{1}{2\pi f}\$ (since, as seen in the 1st image, G+C means \$1/s\$).

As a reminder, the transfer function should be proper: the order of the numerator is less than or equal to the denominator's.