One of the important features in CMOS image sensors regarding high sensitivity is that the random readout noise can be better than that of the CCD, if the property of narrow noise bandwidth in CMOS active pixel sensors is effectively used. This is especially important for mega-pixel video-rate image sensors. To meet the requirement, the use of high-gain amplifier at the column of the CMOS imager is effective, because the noise due to wideband amplifier at the output of the image sensor can be relatively reduced. However, it has not been clarified how much the column amplifier can contribute to the noise reduction effect.
In this paper, we present a noise calculation model of a switched-capacitor type column amplifier. The total noise consists of a noise component due to the noise charge sampled and held at the charge summation node of the amplifier and transferred to the output, and a noise component directly fluctuates the S/H stage at the output of the column amplifier. The analytically calculated noise has well agreement with that of the simulation results using a circuit simulator.
In this paper, we propose a method of low-noise signal detection technique using frame oversampling and a CMOS image sensor with non-destructive high-speed readout mode. The technique enables the use of the high-gain column amplifier and the digital integration of signals without noise accumulation. The column amplifier is effective to reduce the noise due to the wideband amplifier and the quantization noises. The least square estimation of the noise using the intermediate non-destructive outputs further reduces the noise level. Simulation results show that the input referred noise can be reduced to a few electrons.