This thesis presents an ultra-low-power 8-bit asynchronous current-modesuccessive approximation (SAR) ADC for single channel neuron spike recording.The novel design exploits current mode operating in weak inversion forhigh power efficiency and is designed to operate at a 1.8V supply. The ADC isrunning at a 16kHz sampling frequency using under 1uW of power, thoughis adjustable using the featured calibration registers. A finished layout ispresented, occupying less than 0.078mm2. Linear operation through mismatchand process variations is obtained using a current calibration circuitconnected to both the current mode DAC and all the biases. This ensuresINL < 0.5 and DNL < 1, yielding no missing codes and a 3sigma productionyield. Calibration is needed because of the relatively large mismatch causedby sub-threshold operation of the current mirrors. The design also offers anewly developed current comparator with high resolution and fast settlingrelative to the current level and is completable with other state-of-the-art solutions,though still feature some voltage scaling issues left for future work.