This paper presents a 0.35 V 100 kS/s 10-bit successive approximation register (SAR) ADC with adaptive window (AW) in 90 nm CMOS. The SAR ADC uses the transient information of the latch comparator to create redundancy ranges. Furthermore, the proposed technique also uses the transient information to produce AW for each bit which can significantly reduce the power consumption of the comparator, the DAC settling time and also digital control logic. Last but not least, the timing control window can also avoid ADC from encountering meta-stability. The measurement result achieves an SNDR of 57.18 dB, an ENOB of 9.2 bits, a power consumption of 74 nW, and a resulting FoM of 1.25 fJ/conv.-step.