An ultra low power 8-bit current-mode successive approximation (SAR) analog-to-digital (ADC) converter for Wireless Sensor Network (WSN) applications is presented. The proposed ADC contains a new asynchronous clock generator, which works only during data processing and is turned off afterwards, thus enabling a power saving mode. The clock generator is controlled by a bias current, and can be adjusted for a wide range of sampling frequencies. The proposed ADC with the clock generator occupy a chip area of 3500 μm2 and dissipate 400nW power from a 0.6V supply voltage in a CMOS 0.18-μm technology, enabling input data processing with a frequency of 100 kHz.