This paper presents a flexible SAR ADC in 90nm CMOS for wireless sensor nodes. By supporting resolutions from 7 to 10bit and sample rates from DC to 2MS/s, this design can be used for a variety of applications such as sensor interfacing and receiver frontends. Flexibility is achieved by a reconfigurable comparator and a reconfigurable DAC. Compared to prior art, this work substantially improves power-efficiency and enables low-voltage operation by employing a pseudo-differential DAC switching scheme, offset compensation and simplified asynchronous logic control. The measured chip achieves power-efficiencies of 2.8-6.6fJ/conversion-step at 2MS/s and 0.7V supply. The FOM is maintained down to kS/s-range as the leakage is only 2nW.