OTA-free 1-1 MASH ADC using Fully Passive Noise Shaping SAR & VCO ADC

Abstract

We present an OTA-free 1-1 MASH ADC utilizing a fully passive noise shaping (FPNS) SAR as first-stage and open-loop VCO ADC as second stage. The key contribution of this work is to address the challenge of driving large sampling capacitor for high resolution NS-SAR. The proposed architecture reduces resolution of SAR stage and leverages residue attenuation due to passive charge sharing in the FPNS SAR to linearize the VCO. Combining an FPNS SAR with a VCO ADC shapes in-band thermal noise of VCO and SAR comparator at ADC output. Additionally, we demonstrate a computationally inexpensive foreground inter-stage gain calibration algorithm for the proposed ADC architecture. The prototype ADC consumes 0.16mW while achieving an SNDR/DR of 71.5/75.8dB over a 1.1MHz bandwidth and walden FoM of 23.3fJ/step which is the lowest in 65nm technology.