On the equivalence of z-domain and s-domain models in system identification

The step-invariant or ZOH-transformation of discrete-time transfer function models to continuous time is discussed. It is shown that negative real poles which are commonly considered as not transformable, can be transformed to poles on the aliasing margin. Thus a generalized equivalent can be determined, whose ZOH-transform is the original system. Numerical algorithms are also discussed, and examples are shown.