Octagonal CMOS Image Sensor for Endoscopic Applications

Abstract

An octagonal shaped CMOS image sensor (CIS) has been developed for endoscopic applications. The octagonal shape of the outlined die is also applied to the pixel matrix which has suppressed corners, resulting in an octagonal pixel matrix of 422'640 effective pixels. The sensor die fits on a circumference of 3.2 mm diameter being this the best die cut ratio between the cut corners and the pixel placement. The missing pixels on the corners are internally compensated by the readout electronics to maintain a rectangular image output format of 680x680 pixels. This paper presents the solutions found to overcome the layout challenges of fitting both row and column logic on the same side of the diagonally cut corners. Furthermore, the sensor features an area efficient column parallel ADC converter, having on the column readout side, only 402 μm overhead total space between the utmost pixel row until the edge of the silicon die. This space is used to place CDS, ADC, column addressing, readout, additional electronics and power routing. The sensor also has a fully autonomous operation, requiring only an external clock supply to drive the internal state machine. The interface requires just 6 connections: 2 for power supply, 2 for bidirectional data and 2 for main clock supply. The frame rate and the exposure are configurable by register. The maximum frame rate is 50 frames per second (FPS) and data is delivered over LVDS serial data link. The integration time can be configured from 147 μs to 20.1 ms. The power supply is 3.3 V with internal Power-On-Reset (POR) and has a power consumption of 91 mW manufactured in 180 nm CIS process.