The optimization of noise and dynamic range with variable-gain column amplifier in CMOS image sensors

A low noise and relatively high dynamic range CMOS active pixel sensor (APS) using a variable-gain column amplifier is presented and analyzed. On this signal path there are a pixel source follower, a switched-capacitor, noise-cancelling, variable-gain amplifier, and a correlated double sample (CDS) circuit in each column. The using of high gain for the column amplifier reduces input-referred random noise, but it may reduce the dynamic range of this device at meanwhile. In this paper, we present a detail analysis for the noise and the dynamic range with the variable gain of the column amplifier. It is revealed that the total random read noise can be analyzed in three parts: the first part is from the pixel circuit, including the pixel-related fixed-pattern noise, reset noise and pixel source follower amplifier noise; the second part is from the column circuit, including the column-related fixed-pattern noise and the column amplifier noise; and the third part is from the output amplifier in the chip-level circuit. The analysis suggests that the noise components from the pixel and column can be significantly cancelled by the double-stage column noise canceller, and the noise components from the output amplifier in the chip-level circuit, are the major noise source and can be greatly reduced if the signal is amplified before this noise is added. Both the analysis and measured result indicate that we can achieve a low input-referred noise and keep a relatively high dynamic gain by choosing a proper column amplifier gain.