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This paper proposes a new pulse-frequency-modulation (PFM) digital pixel sensor (DPS) with an in-pixel variable reference voltage generator to increase the resolution of the pixel, especially when the level of illumination is low. The proposed PFM DPS offers the attractive characteristics of a reduced integration time when the level of illumination is low, reduced complexity of routing and silicon cost, and comparable fill factor as that of conventional PFM DPS, however, with a significantly improved dynamic range. The proposed digital image sensor is implemented in TSMC-0.18 mum 1.8 V CMOS technology and validated using Spectre with BSIM3V3 device models. Simulation results demonstrate that the PFM digital pixel has a dynamic range of 150 dB when integration time is set to 60 mus, approximately 70 dB more than the conventional PFM digital pixel sensors.