This paper presents an ultra low power and low voltage successive- approximation-register (SAR) analog-to-digital converter (ADC) that uses an adaptive timing optimized asynchronous clock generator. Compared to asynchronous converters that use the conventional clock generator, the frequency range is expanded by 50% at 0.4V analog and 0.7V digital power supply voltage. By calibrating the delay time of the clock generator, the DAC settling time is optimized to counter the device mismatch. Test chip has been fabricated in 40nm standard CMOS process and achieved figure of merit (FoM) of 8.75-fJ/conversion- step with 2.048MS/s at 0.6V analog and 0.7V digital power supply voltage. The ADC operates from 50S/s to 8MS/s performing over 7.5-ENOB.