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This paper presents a 4bit SAR ADC for ultra-low energy radios. It is not obvious to maintain good power-efficiency for low resolution, low data rate ADCs given fixed overhead and scaling limitations. Nevertheless, an excellent FOM of 25fJ/conversion-step is achieved by using a dedicated capacitor implementation, asynchronous dynamic logic, an optimized layout and a reduced power supply. The prototype in a 90nm CMOS technology achieves an ENOB of 3.9bit while operating at 1.024MS/s. The power consumption is only 0.37μW from a 0.7V supply, which is an absolute minimum for 1MS/s ADCs.