An ultra low power and low voltage successive-approximation-register (SAR) analog-to-digital converter (ADC) with timing optimized asynchronous clock generator is presented. By calibrating the delay amount of the clock generator, the DAC settling waiting time is adaptively optimized to counter the device mismatch. This technique improved the maximum sampling frequency by 40% keeping ENOB around 7-bit at 0.4V analog and 0.7V digital power supply voltage. The delay time dependency on power supply has small effect to the accuracy of conversion. Decreasing of supply voltage by 9% degrades ENOB only by 0.1-bit, and the proposed calibration can give delay margins for high voltage swing. The prototype ADC fabricated in 40nm CMOS process 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 can operates from 50S/s to 8MS/s keeping ENOB over 7.5-bit.