access icon free High-loop-delay sixth-order bandpass continuous-time sigma–delta modulators

© The Institution of Engineering and Technology
Received 29/10/2011, Accepted 19/10/2012, Revised 02/10/2012, Published

This study focuses on the design of high-loop-delay modulators for parallel sigma–delta conversion. Parallel converters, allowing a global low oversampling ratio, consist of several bandpass modulators with adjacent central frequencies. To ensure the global performance, the noise transfer function (NTF) of each modulator must be adjusted regarding its central frequency. In this thematic a new topology of sixth-order modulators based on weighted-feedforward techniques is developed. This topology offers an adequate control of the NTF at each central frequency by simple means. Additive signal paths are moreover proposed to obtain an auto-filtering signal transfer function. An optimisation method is also developed to calculate the optimised coefficients of the modulators at different central frequencies. The main concerns are improving the stability and reducing the sensitivity of the continuous-time circuit to analogue imperfections. This is essential for parallel conversion since, in each channel, the modulator works at a central frequency which differs from the fourth of the sampling frequency. The performance of the optimised modulator is compared with its discrete-time counterpart with good argument.

Inspec keywords: sigma-delta modulation; filtering theory

Other keywords: sixth-order bandpass continuous-time sigma-delta modulator; auto-filtering signal transfer function; optimisation method; continuous-time circuit; high-loop-delay modulator; bandpass modulator; parallel converter; noise transfer function; parallel sigma-delta conversion; additive signal path; parallel conversion; weighted-feedforward technique

Subjects: A/D and D/A convertors; A/D and D/A convertors; Filtering methods in signal processing

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