基于自适应相关多采样技术的CMOS图像传感器列级单斜ADC设计

为了降低CMOS图像传感器的噪声水平,本文提出了一种基于自适应相关多采样技术的单斜ADC,可以根据光强自动选择合适的斜坡信号完成A/D转换.通过在低照度下使用小范围、高增益的斜坡信号进行多采样操作,可以充分降低读出噪声.多斜坡发生器由温度计码电容型DAC实现,此外还分析并校准了斜坡信号的失调电压和增益误差.提出的单斜ADC是在标准的110 nm 1P4M工艺下设计仿真的.仿真结果表明:基于自适应相关多采样技术的单斜ADC不会增加总A/D转换时间,同时在8倍的前置放大器增益下实现了最低59μVrms的读出噪声.此外当前置放大器为2倍增益时,读出噪声从强光下的211μVrms降低到弱光下的92μVrms,噪声的降低对应于7.21 dB信噪比的提高.另外,本文的品质因数为3.77 nVrms·s.

A Single-Slope ADC with Adaptive Correlated Multiple Sampling Technique for CMOS Image Sensors

This paper presents a single-slope ADC with adaptive correlated multiple sampling technique for low-noise imaging. Four ramps of different gains and durations are adaptively switched according to the light intensity. Therefore ,the readout noise is reduced by using a small-range and high-gain ramp multiple times at low-light illumi-nation. The multiple-ramp generator is based on the thermometer coded switch-capacitor array. Moreover,the offset voltage and gain error of the ramps are analyzed and calibrated. The single-slope ADC is implemented in a standard 110 nm 1-poly 4-metal process. The simulation results show that the ADC with adaptive correlated multiple sampling technique achieves 59 μVrms readout noise at 8 × pre-amplifier gain without increasing the total A/D conversion time. Additionally, when the pre-amplifier gain is equal to 2, the readout noise is reduced from 211 μVrms to 92 μVrms,which corresponds to 7.21 dB signal-to-noise ratio improvement. Furthermore,the figure of merit of this paper is 3.77 nVrms·s.