A logarithmic response CMOS image sensor with on-chip calibration

CMOS image sensors with logarithmic response are attractive devices for applications where a high dynamic range is required. Their strong point is the high dynamic range, Their weak point is the sensitivity to pixel parameter variations introduced during fabrication. This gives rise to a considerable fixed pattern noise (FPN) that deteriorates the image quality unless pixel calibration is used. In the present work a technique to remove the FPN by employing on-chip calibration is introduced, where the effect of threshold voltage variations in pixels is cancelled, An image sensor based on an active pixel structure with five transistors has been designed, fabricated, and tested. The sensor consists of 525 x 525 pixels measuring 7.5 mu m x 10 mu m, and is fabricated in a 0.5-mu m CMOS process, The measured dynamic range is 120 dB while the FPN is 2.5% of the output signal range.