A mixed analog–digital fast hamming-weight filtering circuit using switched-capacitor arrays

Abstract

Many circuit design applications rely on an intermediate sequence to carry a decision to the next circuit stage. The decision may be carried by a weighted pattern of N bits, with the weights being selected in a way that optimizes the circuit implementation or some aspect of performance. For example, when the weights are consecutive powers of 2 beginning with 1 = 2 0 , we have the standard binary representation. As another example, when all the weights are 1, we have the unary representation that encodes a value k by k asserted bits and N-k unasserted bits (a weightk bit vector of length N). In this paper, we present the design of a circuit that screens a unary representation to verify that the represented value falls between preset lower and upper limits l and u, passing through any string that represents a value in the interval [l, u] and outputting the all-0 s bit pattern otherwise. Our mixed analog-digital circuit implementation, based on switched-capacitor arrays, provides a decision output within a clock cycle of 4 ns for 16-bit unary representation, when realized with 0.15 lm TSMC technology. The latter results were obtained with normal, per-bit capacitance of 200 fF and single-clock-cycle operation. As an added benefit, our filtering circuit can form the basis for designing a cost-effective Hamming decoder circuit.