Noise in photodiode-type CMOS active pixel sensors (APS) is primarily due to the reset (kTC) noise at the sense node , since it is difficult to implement in-pixel correlated double sampling for a 2-D array. Figure 1 shows the pixel schematic of a photodiode APS.Signal integrated on the photodiode sense node (SENSE) is calculated by measuring difference between the voltage on the column bus (COL) – before and after the reset (RST) is pulsed. Lower than kTC noise can be achieved with photodiode-type pixels by employing “soft-reset” technique. Soft-reset refers to resetting with both drain and gate of the n-channel reset transistor kept at the same potential, causing the sense node to be reset using sub-threshold MOSFET current. However, lowering of noise is achieved only at the expense higher image lag and low-light-level non-linearity. In this paper, we present an analysis to explain the noise behavior, show evidence of degraded performance under low-light levels, and describe new pixels that eliminate non-linearity and lag without compromising noise.
June 01, 2001