An oscillator collapse-based comparator with application in a 74.1dB SNDR, 20KS/s 15b SAR ADC

Abstract

This paper presents a new energy-efficient ring oscillator collapse-based comparator, which is demonstrated in a 15-bit SAR ADC. The comparator automatically adjusts comparison energy according to its input difference without any control, eliminating unnecessary energy spent on coarse comparisons. The employed SAR ADC supplements a 10-bit differential main CDAC with a 5-bit common-mode CDAC. This offers an additional 5 bits of resolution with common mode to differential gain tuning that improves linearity by reducing the effect of switch parasitic capacitance. A test chip fabricated in 40nm CMOS shows 74.12 dB SNDR and 173.4 dB FOMs. The comparator consumes 104 nW with the full ADC consuming 1.17 μW.