## 1 Introduction  
The high-resolution three-dimensional (3-D) time-of-flight (TOF) laser detection and ranging (LADAR) sensors have been developed in various fields such as remote sensing, 3-D imaging, geographical mapping, and range-finding [1; 2; 3; 4; 5]. A LADAR sensor has difficulty processing all reflected TOF signals for the region of interest (ROI) in every direction when obtaining 3-D imaging information in real-time. To resolve this, many solutions such as the rotational motor (RM)-based method [6], focal-plane-array (FPA) of the photodetectors-based method [7], and the static unitary detector (STUD)-based method [8; 9] have been proposed. As both RM-based and FPA-based methods have a structural limitation to increase 3-D resolution, the STUD-based method has been developed to obtain high spatial resolution 3-D images with many advantages. It has one signal processing chain and does not need micro-lenses to enhance the signal-to-noise ratio (SNR), resulting in cost effectiveness. In a STUD-based LADAR sensor, as shown in Figure 1, two high-speed optical scanners (in vertical and horizontal direction) and a large-sized photodetector are conjugated instead of a rotational motor or the FPA of a photodetector. The transmitter illuminates a collimated pulse laser beam in a specific direction over the entire ROI using two high-speed optical scanners with a fixed scanning step size, and the receiver detects returned pulse laser signals using a large-area photodetector. In order to increase 3-D resolution in a STUD-based LADAR sensor, the photosensitive area of a photodetector has to be increased to enlarge the ROI. However, as the increase in the photodetector area decreases its bandwidth due to an increased parasitic capacitance (\(C_{PD}\)), this causes a large timing error (i.e., walk error [10; 11]). In order to effectively increase the photodetector area with no harm to its bandwidth, the partitioning photosensitive cell (PPC) method has been proposed in earlier studies [8; 9], as shown in Figure 2. In order to independently amplify the received signal, each partitioned cell has its own transimpedance amplifier (TIA), and a signal combiner sums the outputs from all TIAs. It has theoretically no limitation in increasing the number of cells, and a high-resolution 3-D image acquisition is possible on a large ROI even with a short laser pulse. However, previous works [9] focused only on implementing the proposed PPC method embedded in a STUD-based LADAR sensor with a fully integrated four-input-combining receiver, which did not consider the high spatial resolution of 3-D imaging. Although a bandwidth limitation caused by a large CPD could be solved by applying the PPC method, the unexpected performance degradation could be caused by the pattern of multiple partitioning and signal combining as the number of partitioning photosensitive cells increases. In this study, we propose a prototype STUD-based LADAR sensor with a large-area photosensitive cell for high spatial resolution 3-D imaging commercial applications. With the fixed scanning step size of the optical scanner, a large ROI with a photosensitive area of approximately 500 \(\upmu\)m \(\times\) 2240 \(\upmu\)m corresponding to the CPD of approximately 32 pF is required in a STUD-based LADAR sensor for high spatial resolution commercial applications. Based on the analysis of the PPC method, the effective number of partitioning photosensitive cells and signal-combining stages is selected to read out a large photosensitive cell with optimum performances. A 16-to-1 transimpedance-combining amplifier (TICA) is proposed and fully implemented on a single chip as a front-end readout circuit. The rest of this paper is organized as follows. The analysis of the PPC method is discussed in Section 2. The circuit implementation details of the proposed STUD-based LADAR receiver are  
Figure 1: Block diagram of the STUD-based LADAR sensor. Figure 2: Partitioning photosensitive cell method. described in Section 3.