A 20-MHz bandwidth, 75-dB dynamic range, continuous-time delta-sigma modulator with reduced nonidealities

Abstract

This letter presents a 4-bit continuous-time delta-sigma modulator (CT-DSM) fabricated using a 65-nm CMOS process. The circuit is designed for wide-bandwidth applications, such as those related to wireless communications. This CT-DSM has an oversampling ratio of 16 with a 640-MHz sampling frequency. To reduce the clock jitter sensitivity and excess loop delay effect, the first DAC pulse is a nonreturn-to-zero (NRZ)-type pulse, whereas the second DAC pulse is a return-to-zero (RZ)-type pulse; this is accomplished using a current-steering DAC. In order to reduce mismatch without using a data-weighted averaging circuit, the size and layout of the unit current source in the current-steering DAC are considered carefully. The CT-DSM achieves a signal-to-noise ratio (SNR) of 67.3 dB, a signal-to-noise and distortion ratio (SNDR) of 63.4 dB, and a dynamic range of 75 dB for a 20-MHz signal bandwidth.