4^th-Order Switched-Current Multistage-Noise-Shaping Delta-Sigma Modulator With a Simplified Digital Noise-Cancellation Circuit

Abstract

This paper proposes a fourth-order (2-2) switched-current (SI) multistage-noise-shaping (MASH) delta-sigma modulator (DSM) with a simplified digital noise-cancellation circuit (DNCC) by using a Taiwan Semiconductor Manufacturing Company (TSMC) 0.18 μm 1P6M CMOS process. In view of area efficiency, we propose a small-area current-mode sample-and-hold circuit (S/H) with a modified feedback memory cell (FMC) and cross-connected bias circuit. As a result of modifications to the feedback impedance, the input impedance of the current-mode differential FMC was decreased by [2 + (g'm3/gm1 - 1) x A] times relative to a traditional FMC. Any input current can be processed faster than usual given low input impedance. The MASH architecture inherited a superior signal-to-noise ratio (SNR) due to a simplified DNCC, consisting of six unit-delay circuits using a master-slave D flip-flop (DFF) and a logic circuit using a Karnaugh map. Post-layout simulations reveal that the simulated SNR was 87.1 dB and the effective number of bits (ENOB) was 14.18 bits. Measurements indicated that the SNR was 64.5 dB and the ENOB was 10.42 bits-at a sampling frequency of 10.24 MHz, an oversampling ratio of 256, a signal bandwidth of 20 kHz, and a supply voltage of 1.8 V. The designed chip was measured to have a power consumption of 18.19 mW, a chip area of 0.13 mm2, and a measured figure of merit (FoM) of 331.9 (pJ/conv-step). The advantages of our modulator are its small chip area and high processing speed at all input currents.

Keywords

• Delta–sigma modulator
• switched-current
• multistage-noise-shaping
• feedback memory cell
• digital noise-cancellation circuit
• master–slave DFF