The design of high-order bandpass sigma-delta modulators using low-spread single-stage structure

Abstract

A structure for single-stage high-order bandpass sigma-delta modulators (BPSDMs) is presented. The proposed structure introduces an additional internal path in each resonator, thus, adding one degree of freedom in coefficient determination. Coefficient spread can therefore be reduced, resulting in significantly reduced capacitance area in switched-capacitor BPSDM circuits. High-order BPSDMs with different quality factors (Q) are demonstrated. It shows that coefficient spread is significantly reduced, especially for high-Q applications. For comparable eighth-order 3-bit BPSDMs, the maximum coefficient spread are respectively 15369 and 7693 for conventional cascade-of-resonator-with-feedback (CRFB) and cascade-of-resonator-with-feedforward (CRFF) designs, and 114 for the proposed structure. For an eighth-order 1-bit example, these respective values are 8994, 2638, and 74. With coefficient mismatch, peak signal-to-noise ratio (PSNR) degradation of the proposed structure is less than those of the CRFB and CRFF structures, demonstrating reduced sensitivity to component mismatch. Hence, the proposed structure can reduce chip area and ease circuit implementation of BPSDMs.