Low-noise readout using active reset for CMOS APS

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Abstract
Pixel reset noise sets the fundamental detection limit on photodiode based CMOS image sensors. Reset noise in standard active pixel sensor (APS) is well understood and is of order kT/C. In this paper we present a new technique for resetting photodiodes, called active reset, which reduces reset noise without adding lag. Active reset can be applied to standard APS. Active reset uses bandlimiting and capacitive feedback to reduce reset noise. This paper discusses the operation of an active reset pixel, and presents an analysis of lag and noise. Measured results from a 6 transistor per pixel 0.35 micrometers CMOS implementation are presented. Measured results show that reset noise can be reduced to less than kT/18C using active reset. We find that theory simulation and measured results all match closely.

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