An overview of decimator structures for efficient sigma-delta converters: Trends, design issues and practical solutions

Classical decimation structures usually use comb filters at first stage due to the simplicity of comb filters. However, comb filters cannot satisfy high performance demands of state-of-the-art Sigma-Delta (ΣΔ) analog-digital converters (ADCs). Some possible solutions are comb based structures which are power and area efficient and posses an improved magnitude characteristic. The principal issues in the comb-based filter design are: power and area efficiency, high alias rejection and approximately flat passband characteristic, considering also high values of the decimation factors. In this paper we first review the new trends in ΣΔ ADCs and demands for the decimation block design. Next we review power and area efficient structures. The methods to improve the alias rejection of comb filters, as well as the methods for the compensation for the comb passband droop, and those strategies which simultaneously improve the alias rejection and the passband droop are reviewed in the following. Finally we review the multirate decimation structures.