A High Dynamic Range CMOS Image Sensor using Programmable Linear-logarithmic Counter for Low Light Imaging Applications

A high dynamic range image sensor which uses a linear-logarithmic counting scheme to provide user programmable sensitivity variation for low light imaging application is presented. In a 3T pixel, if the photo-signal information is converted to time domain instead of voltage, the dynamic range for low light signals is no longer limited by sensor noise floor. However, it is still limited by the readout circuitry which is required to obtain digital bits for image acquisition. The proposed algorithm prevents the saturation of the readout circuitry by modifying the frequency of chip level counter so that it counts in a linear-logarithmic manner. The linear-logarithmic counter is integrated with the column circuitry and pixel in such a way that it enables low light photo-signal capture without losing the high light signal information. A chip with the proposed technique has been designed in 0.35 mu m AMS OPTO process. The test chip has a 128 x 128, 3T pixel array with 9 mu m pixel pitch. Simulation results show that maximum dynamic range obtained is 95 dB. The imager provides a user-programmable sensitivity curve depending upon the application. The extra hardware is added only at the chip level and no ramp generator is used, making the technique area and power efficient efficient.