The integration of 2D ultrasonic transducer arrays and pitch-matched ASICs has enabled the realization of various 3D ultrasound imaging devices in recent years -. As applications such as 3D intravascular ultrasonography, intra-cardiac echocardiography, and trans-fontanelle ultrasonography call for miniaturization and improved spatial resolution, higher-frequency transducers (>5MHz) with a correspondingly smaller array pitch (<150m) are needed. Such devices generally employ a large number of transducer elements, calling for channel-count reduction in the ASIC while meeting stringent restrictions on per-element power consumption and die area. Micro-beamforming (BF) is an effective way of reducing channel count by performing a delay-and-sum operation on the echo signals received within a sub-array . However, prior BF implementations employ per-element capacitive memory to realize the delay , , making it increasingly difficult to apply BF in smaller-pitch arrays.