A CMOS 0.8-μm transistor-only 1.63-MHz switched-current bandpass ΣΔ modulator for AM signal A/D conversion

This paper presents a CMOS 0.8-mu m switched-current (SI) fourth-order bandpass Sigma Delta modulator (BP-Sigma Delta M) IC capable of handling signals up to 1.63 MHz with 10.5-bit resolution and 60-mW power consumption from a 5-V supply voltage. This modulator is intended for direct A/D conversion of narrowband signals within the commercial AM band, from 540 kHz to 1.6 MHz. Its architecture is obtained by applying a lowpass-to-bandpass transformation (z(-1) --> -z(-2)) to a I-bit second-order low-pass Sigma Delta modulator (LP-Sigma Delta M). The design of basic building blocks is based upon a detailed analysis of the influence of SI errors on the modulator performance, followed by design optimization. Memory-cell errors have been identified as the dominant ones. In order to attenuate these errors, fully differential regulated-folded cascode memory cells are employed. Measurements show a best SNR peak of 65 dB for signals of 10-kHz bandwidth and an intermediate frequency (IF) of 1.63 MHz. A correct noise-shaping filtering is achieved with a sampling frequency of up to 16 MHz.