A high dynamic-range CMOS image sensor with locally adjusting charge supply mechanism

Abstract

This work proposes a charge supply mechanism to increase the dynamic range of the CMOS image sensor. This charge supply mechanism consists of two NMOS transistors in a serial connection and a feedback circuit. One of two NMOS transistors is connected to the N node of a photodiode for providing supplied charges and is controlled by the feedback circuit. The other NMOS transistor is addressed by the row-select control signal to reduce sensor's power consumption. The feedback circuit senses the pixel's output and converts it to a control signal for the operation of charge supply. A testing chip with a 128 /spl times/ 128 pixel array was fabricated by using the TSMC 0.35 /spl mu/m 2P4M CMOS technology. The measurement results demonstrate that the responses are around 25mV, 260mV and 150mV per decade of light intensity during high, middle and low luminance, respectively. The overall dynamic range could attain 120dB. With capability of using a single reset signal to read pixels, the proposed sensor is easily integrated with the correlated double sampling (CDS) circuit to effectively minimize fixed pattern noise (FPN). As compared to the conventional image sensors, the proposed image sensor would have high sensitivity under low light intensity. Moreover, the feedback circuit would automatically control the amount of supply charges during high light intensity to delay photodiode's saturation without any external control signal. Particularly, the pixel array can be partitioned into multiple sub-arrays that are addressed by different feedback circuits. The proposed image sensor can effectively increase its dynamic range and clearly sense the high contrast picture.