Analysis of pixel gain and linearity of CMOS image sensor using floating capacitor load readout operation

In this paper, we demonstrate that the floating capacitor load readout operation has higher readout gain and wider linearity range than conventional pixel readout operation, and report the reason. The pixel signal readout gain is determined by the transconductance, the backgate transconductance and the output resistance of the in-pixel driver transistor and the load resistance. In floating capacitor load readout operation, since there is no current source and the load is the sample/hold capacitor only, the load resistance approaches infinity. Therefore readout gain is larger than that of conventional readout operation. And in floating capacitor load readout operation, there is no current source and the amount of voltage drop is smaller than that of conventional readout operation. Therefore the linearity range is enlarged for both high and low voltage limits in comparison to the conventional readout operation. The effect of linearity range enlargement becomes more advantageous when decreasing the power supply voltage for the lower power consumption. To confirm these effects, we fabricated a prototype chip using 0.18um 1-Poly 3-Metal CMOS process technology with pinned PD. As a result, we confirmed that floating capacitor load readout operation increases both readout gain and linearity range.

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