A low noise current-mode readout circuit for CMOS image sensing applications

A major obstacle in current-mode CMOS image sensors is the transconductance gain mismatch across pixels, which translates to Fixed Pattern Noise (FPN), degrading the image quality. In this paper, we propose a highly linear and compact current-mode readout circuit that eliminates this mismatch by performing the voltage to current conversion within a unity gain feedback loop. It is further proposed that by using the Active Column Sensor (ACS) architecture, only two transistors from the readout circuit are required to be present at the pixel site, while the rest of the circuitry resides common to an entire column. This setup saves a significant amount of real estate. The circuit is designed in TSMC 0.25 μm process, and simulation results are presented.