A novel bit-serial design of comb filters for oversampling A/D converters

The non-recursive algorithm of comb filters suitable for bit-serial implementation is described in this paper. It performs decimation on the oversampling delta-sigma modulator output in several stages by utilizing the commutative rule. Each stage performs very simple FIR filtering and decimates the output by a factor of 2. A novel bit-serial design based on this algorithm is then presented. It features regularity and area and power-consumption efficiency. It also provides the possibility of higher frequency operation than existing designs. As an example, a comb filter of the response sinc³(f) with a decimation ratio 32 has been designed in AMS´ 1 μm double-metal CMOS process. It contains only 4500 transistors even by using standard static gates and occupies about 1.1 mm² active chip area