Step Invariant Approximation

The idea here is that $C(s)$ and $D[z]$ have the same response at the sample times to a unit step
Solve for D[z] from
$L^{-1}\left(C(s)\frac{1}{s}\right){|}_{t=kT}=z^{-1}(D[z]\frac{z}{z-1})$

$D[z]=\frac{z-1}{z}z\left(L^{-1}\left(C(s)\frac{1}{s}\right){|}_{t=kT}\right)$