Low power 9-bit 500 kS/s 2-stage cyclic ADC using OTA variable bias current

Abstract

This paper presents a 9-bit, 2-stage cyclic analog to digital converter (ADC) with a variable bias current control circuitry to reduce its power dissipation. Each stage outputs a three-bit digital word and the circuit requires four subcycles to perform a whole conversion. Since the accuracy required is higher in the first stage and first subcycle and decreases in subsequent cycles, the bias current of each operational transconductance amplifier is regulated depending on the subcycle of the conversion process. The resolution and sampling frequency of the converter make it suitable to be integrated with 8-bit CMOS imagers with column-parallel ADC architectures. The ADC has been designed using a 1.2 V 110 nm CMOS technology and the circuit consumes 27.9 µW at a sampling rate of 500 kS/s. At this sampling rate and at a 32 kHz input frequency, the circuit achieves 56 dB of SNDR and 9 bit ENOB. The Figure of Merit is 109 fJ/step.