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Impulse invariance method for analog-to-digital filter conversion

`impinvar`

performs the impulse-invariant method of analog-to-digital
transfer function conversion discussed in reference [2]:

It finds the partial fraction expansion of the system represented by

`b`

and`a`

.It replaces the poles

`p`

by the poles`exp(p/fs)`

.It finds the transfer function coefficients of the system from the residues from step 1 and the poles from step 2.

[1] Antoniou, Andreas.
*Digital Filters*. New York: McGraw-Hill, Inc., 1993.

[2] Parks, Thomas W., and C. Sidney Burrus. *Digital
Filter Design*. New York: John Wiley & Sons, 1987.