A Sigma-Delta Modulator with Single-Pole Double-Throw Analog Switch

Abstract

In this paper, a high precision and high energy efficiency discrete-time switching capacitive 3-order Sigma-Delta modulator (SDM) is proposed. The SDM is applied to portable electroencephalograms (EEG) because of its little energy and good performance. The energy efficiency levels of the current mainstream modulator systems are analyzed and compared, and the CIFF modulator architecture which is most conducive to realizing high energy efficiency is selected. The single-loop CIFF structure is selected to give consideration to the accuracy and stability of the circuit. The circuit is implemented in a hierarchical structure, and a single-pole double-throw (SPDT) analog switch is adopted to overcome the difficulty of opening conventional CMOS analog switches at low supply voltages and the increase in threshold voltage Vth of NMOS devices due to the base bias effect. The proposed discrete-time Sigma delta ADC modulator is designed with SMIC 0.18um CMOS technology and achieves an SNDR of 101.6dB under a 1.8V power supply voltage and a signal bandwidth of 2kHz. The power consumption is 520uW and the significant bit (ENOB) is 16.58 bits.