Zeroes of Smapled Data Transfer Functions in the Presence of Continuous Time Unmodeled Dynamics

Sampled systems zeroes are sensitive to the presence of unmodeled dynamics in the continuous time system. In this report an expression which approximates the numerator of sampled systems in this situation is given. In particular it is shown that the numerator in the z-domain can be computed on the basis of the sampled data transfer function of the unperturbed system (exact modeling) and the knowledge of two constants relative to the unmodeled poles and zeroes.