This paper presents an extremely low-voltage operation and power efficient successive-approximation-register (SAR) analog-to-digital converter (ADC). Tri-level comparator is proposed to relax the speed requirement of the comparator and decrease the resolution of internal Digital-to-Analog Converter (DAC) by 1-bit. The internal DAC employs unit capacitance of 0.5fF and ADC operates at nearly thermal noise limitation. To deal with the problem of capacitor mismatch, reconfigurable capacitor array and calibration procedure were developed. The prototype ADC fabricated using 40nm CMOS process achieves 46.8dB SNDR with 1.1MS/sec at 0.5V power supply. The FoM is 6.3-fJ/conversion step.