Sigma-delta modulators with multibit quantising elements and single-bit feedback

The performance of multibit sigma-delta coders is analysed and compared with that available from single-bit designs. It is demonstrated that the basic difficulty with multibit coders is the need for a high-accuracy digital-analogue convertor (DAC) in the feedback loop. An expression for the reduction of in-band noise power as a function of coder order, quantiser resolution, and oversampling ratio is derived, together with a subsidiary condition imposed by the use of multibit DACs. New coder topologies are presented which use multibit quantisation with single-bit feedback, avoiding the need for high-resolution DACs. Analysis and simulations are described which show that these have a similar performance to multibit designs with multibit feedback