A 144 x 144 current-mode image sensor with self-adapting mismatch reduction

We present a novel CMOS continuous-current imager that uses nonvolatile floating-gate charge storage in each pixel for automatic cancellation of fixed-pattern noise (FPN) and vignetting artifacts. We demonstrate the ability to reduce image artifacts over a wide range of incident light intensities. Adaptation occurs for each pixel in parallel, using a unique pixel circuit that employs hot-electron injection in stable feedback to accurately match a reference value. The adaptation mechanism stores a reference image, which may be uniform for FPN cancellation or nonuniform for various imaging applications. The design has been fabricated in a commercially available 3-metal, 2-poly 0.5-[micro]m standard CMOS technology. Experimental results confirm the ability to reduce the FPN variance by a factor of 178x at the intensity at which adaptation was performed, and by a factor of 34x over five orders of magnitude of intensity. Adaptation takes as little as 4 s and the 144 x 144 image can be acquired at 0.9 frames/s. During normal operation, the chip consumes 140 [micro]W under standard office lighting conditions at room temperature. Index Terms--Analog memories, automatic programming, calibration, hot carriers, image sensors, insulated gate field-effect transistors (FETs), error compensation.