A 10 bit 16-to-26 MS/s flexible window SAR ADC for digitally controlled DC–DC converters in 28 nm CMOS

Abstract

This paper presents a 10 bit charge redistribution successive approximation analog-to-digital converter (ADC) for integrated digitally controlled DC---DC converters. A timing efficient implementation of a window function is proposed, where only a reduced input range is converted. A redundant search is applied to overcome the speed limitation of the analog components. The window mode enables further speed enhancement without an increase of clock frequency and power consumption. Position of the set-point and size of the window is digitally adjustable every conversion. Fabricated in 28 nm low-power CMOS technology the ADC occupies only 110 × 85 µm2. In full range mode a conversion rate of 16 MS/s is achieved and in window mode 26.7 MS/s, respectively. With a measured total power consumption of 710 µW and 9.1 bit ENOB a FOM of 81 fJ/conv-step is reached. A large input range with constant resolution, highly linear characteristic, and high robustness to PVT variations together make this ADC an advantageous alternative to delay line or ring oscillator based window ADCs. The highly digital nature of the proposed architecture allows implementation in modern sub-100 nm CMOS technologies.