In this paper, a Vcm-based thermometer switching technique, which can greatly improve the static linearity of SAR ADCs with coarse and poorly matched unit capacitors, is proposed. In conventional prior arts, the total capacitance of CDAC has been governed mostly by the mismatch and the static linearity, rather than the thermal noise, or by the capacitor weight calibration algorithms which further complicates the ADC operations with much hardware overhead. Through the use of proposed Vcm-based thermometer switching algorithm, the total capacitance of CDAC could be effectively reduced to a near thermal noise limited value with trivial hardware overhead. A flash-assisted SAR ADC structure with time-interleaved SAR sub-ADCs is implemented to maximize power efficiency in enhancing static linearity and conversion speed.
본 논문에서는 mismatch 특성이 좋지 않은 작은 capacitor 로도 높은 linearity 를 얻을 수 있는 Vcm-based thermometer switching 기법을 제안하였다. 기존 linearity가 낮은 switching 방식으로 인해 capacitance를 높게 설정하거나 복잡한 capacitor weight calibration을 사용하였던 것 대비, switching 방법 만을 바꿈으로써 CDAC의 capacitance 를 kT/C limit 에 근접하도록 획기적으로 줄일 수 있었다. 또한 flash-assisted SAR ADC 구조와 더불어 time-interleaved 구조를 사용함으로써 linearity와 conversion speed 향상에 따른 전력 효율성을 극대화 하였다.
