Owing to the low complexity and high level of system integration, the quadrature direct-conversion architecture is widely used in Doppler radar for noncontact detection of slow periodic motions such as mechanical vibrations and physiological motions of respiration and heartbeat. However, precise detection of the complete motion pattern has been challenging due to the high-pass characteristics of the ac-coupled baseband circuitry. A few techniques have been proposed to preserve the actual motion pattern in radar sensing based on hardware modifications that add system complexity and cost. In this paper, a digital post-distortion (DPoD) technique is proposed to compensate for the signal distortions in the digital baseband domain. Without any cumbersome hardware modification, the complete pattern of slow periodic motions can be detected using a simple quadrature direct-conversion architecture with ac-coupled baseband. Experimental results show that the proposed Doppler radar with the DPoD technique is robust to compensate signal distortions and can be used for precise detection of slow Doppler motions (near dc) where ac coupling typically attenuates the signal.