A 108dB DR Hybrid-CTDT Direct-Digitalization ΔΣ-ΣM Front-End with 720mVpp Input Range and >300mV Offset Removal for Wearable Bio-Signal Recording

Abstract

This paper presents a direct-digitalization front-end for wearable bio-signal recording. The proposed front-end is built with a 2nd order hybrid-CTDT $\Delta \Sigma - \Sigma$ modulator, taking the benefits of oversampling and noise shaping. The $\Delta \Sigma - \Sigma$ topology removes electrode DC offset and shapes signals as well as motion artifacts at the input by adding $a\Sigma -$stage in the feedback loop, while the $\Sigma -$stage recovers the bio-signals by quantizing the difference of the consecutive samples. To meet the requirements of noise, input impedance of a bio-potential interface, a capacitively-coupled chopper amplifier serves as an input stage and also an active adder. An asynchronous 5-bit differential-difference SAR quantizer combines the functionalities of a coarse ADC and a passive adder in a traditional $\Delta \Sigma$ loop, leading to a compact output stage. The prototype IC is fabricated in a standard TSMC $0.18 \mu m$ CMOS process and achieves the peak SNR of 105.6dB and DR of 108.3dB with the maximum linear input range of 720mVpp. Its input referred noise is $0.98 \mu $ Vrms over 100Hz bandwidth. ECG and EEG measurements verify the bio-potential signals acquisition capability of this IC.