A multiple operation mode CMOS digital pixel sensor dedicated to a visual cortical implant

A CMOS image sensor with pixel level analog to digital conversion is presented. The circuit of each pixel contains a photodiode, with a fill factor of 21%, a shutter/follower, a comparator and a storing unit, for a total of 44 transistors. The comparator uses the maximum voltage swing available and is insensitive to fabrication process variations. A digital to analog converter is used to deliver a voltage reference in order to compare it with the pixel voltage for the analog to digital conversion. The digital value of the pixel voltage is stored in an 8-bit DRAM. The circuit can operate in one of the three following modes: logarithmic, linear integration or differential. The first two modes are used for image capture and the third mode is used for an active range finder system. All these modes are required by a visual cortical implant. The circuit layout of a pixel is being fabricated in CMOS 0.18 μm technology, and a matrix of 45×90 pixels was submitted for fabrication.