A 99.95% Linearity Readout Circuit With 72 Db Dynamic Range For Active Pixel Sensors

One of the main sources of nonlinearity and sensitivity degradation in conventional active pixel sensor (APS) architectures is the source follower buffer amplifier that isolates the photodiode node from the column bus. To extend the linear region of the output voltage, correct the sensitivity degradation, and improve the imager dynamic range, an output regulated voltage follower is proposed. All these enhancements are achieved without decreasing the pixel fill factor nor increasing the number of transistors and the addressing circuits complexity of a conventional 4T CMOS APS. The performance of the circuit is assessed using a 0.35-mu m CMOS technology. The obtained results show a 14-dB dynamic range improvement for highly linear applications over the conventional APS readout circuit.