An ultra-low power fully dynamic capacitance-to-digital converter (CDC) that exploits a novel charge reuse technique is proposed. The CDC includes a capacitive bridge as the sensing frontend and an asynchronous SAR ADC for signal digitization. Passive charge sharing between the frontend and ADC is used to enable a fully dynamic operation. Instead of resetting the capacitive bridge (with large sensing and reference capacitors) for each measurement, the charge is maintained and reused over many measurements to save energy. A power gating technique is employed to reduce the stand-by power. As a result, a FoM as low as 4.3fJ/conv-step is achieved for the CDC, which is >3x better than the state-of-the-art. Furthermore, it supports an inherent scaling of power vs. speed with a minimum power of only 44pW and a compact chip area of 6440μm2.