A switched current sigma delta modulator using a low distortion feedfoward topology

Abstract

Implementation of feedforward paths for low-distortion topologies on switched capacitor sigma-delta (SC-SD) modulators was successfully reported elsewhere [13]. In this paper we propose the use of a similar low distortion topology on an implementation of a switched current sigma-delta (SI-SD) modulator. The objective is a reduction of harmonic distortion observed in implementations with simple integrator cells. In this approach, one of the main SI cell drawbacks, the harmonic distortion due to conductance variation, can be reduced by alleviating the signal in the integrators path. In this approach, the integrators ideally process only noise. Relaxing requirements of integrators allows simpler and faster switched-current integrator circuits. The feedforward SI-SD modulator was designed in XFAB CMOS 0.6µm technology. Simulated results points to a reduction of harmonic distortion from 2% in a classical 2nd order feedback topology to 0.2% in the feed forward design. It also provides a reduction of 30% in the silicon area, and higher sampling frequency.