Digital filter and method for a mash delta-sigma modulator

Abstract

A digital filter receives signals from each stage of a MASH delta-sigma modulator and filters noise components from the signals prior to combination as a single sequence of values decimation. Each stage of the MASH delta-sigma modulator provides an output sequence of one-bit, binary values, which are then filtered to remove high-order, out of band quantization noise. After filtering, the output sequences are then combined through a cascade-combiner, which may be similar to the pre-processing stage of a MASH delta-sigma modulator architecture. The digital filter processes signals of each stage separately. Consequently, the digital filter does not perform multiplication of two, multi-bit values. Multiplication of two values, the first of which is a one-bit, binary value, may be implemented with a multiplexer selecting either the second value or a zero value based on the first one-bit, binary value (i.e., logic 1 or 0, respectively). Duplicate FIR filters, or a single FIR filter with bit-interleaving by a multiplexer, master-slave delay chain and demultiplexer controlled by system clock transitions, may be used to process the sequences on a single bit basis, replacing multi-bit multipliers of the digital filter with multiplexers. Alternatively, the FIR filter may be implemented with a multiplexer, master-slave delay chain, ROM look-up table and demultiplexer if bit interleaving is employed.