In this paper, we propose a systematic design method for continuous-time sigma-delta modulators. The modified-Z-transform technique is used to take into account any variation occurring between two sampling instants in the continuous-time DAC feedback signal. The proposed discrete-time to continuous-time transformation method is general and well-suited to design automation. A fifth-order lowpass as well as a sixth order bandpass modulators with RZ and NRZ feedback signals are given as design examples.