Criteria are presented to ensure that all zeros of the equivalent discrete transfer function of a continuous-time plant with a sampler and a zero-order hold are stable. An algorithm is proposed which is more efficient than methods based on the Nyquist curve, and works with transfer function poles of any multiplicity. A method has been developed to modify the location of the zeros of the discretized system which is based on the addition of an analogue first-order lag-lead filter between the computer DA converter and the plant.