Design optimization for low light CMOS image sensors readout chain

For a CIS readout chain based on 4T pixel, column amplification and CDS, we confirm that thermal noise can be reduced to be neglected compared to 1/f noise using parameters independent of the pixel design, namely column level gain, bandwidth control or correlated multiple sampling (CMS). Based on analytic noise calculation and simulation results using 180nm process, we show that CMS has no advantage over CDS for thermal noise reduction but offers slightly more 1/f noise reduction (about 20% less 1/f noise if high number of samples is used). 1/f and RTS noise originating from the in-pixel source follower transistor are reported to be the dominant noise sources in CIS readout chain. Based on analytic noise calculation, we demonstrate that, for a given CMOS process, the input referred 1/f noise is minimal for a unique pair of gate dimensions of the in-pixel source follower and we give its expression as a function of technological parameters.