An Autofocus Sensor With Global Shutter Using Offset-Free Frame Memory

An autofocus (AF) sensor with global shutters using offset-free frame memory is presented. A cross-shaped AF sensor array has left and right horizontal arrays and top and bottom vertical arrays. AF is achieved by calculating the phase difference from the digital code of the light illumination difference between the left and right (or top and bottom) arrays. The global shutter was implemented by using offset-free frame memories for accuracy enhancement. The proposed offset-free frame memories consist of flip-around sample-and-hold amplifiers with correlated double sampling. An active pixel sensor array, offset-free frame memories, a programmable gain amplifier, a 10-bit pipelined analog-to-digital converter, and digital control circuits are fully integrated on the chip fabricated on the 0.18-μm CMOS image sensor technology. It occupies 7 × 8 mm² with bonding pads. Each active pixel size is 15 × 100 μm². The total power consumption of the AF sensor is 660 mW at 3.3-V supply voltage.