US 6,271,785 B1Granted

CMOS imager with an A/D per pixel convertor

AssigneeTexas Instruments Incorporated

InventorsDavid Andrew Martin, Krishnaswamy Nagaraj

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Abstract

An image sensor (10) which can be fabricated using conventional CMOS processes uses a comparator circuit (18) at each pixel (14) having a first input coupled to a photodetector (16) and a second input coupled to a ramp signal generator (30, 32). The ramp signal generator (30, 32) is comprised of a counter (32) and a D/A conversion circuit (30) with the analog output of the D/A conversion circuit (30) forming an analog ramp input to the comparator circuit (18). A counter circuit (32) can be used to drive the digital side of the D/A conversion circuit (30) and configured to count from 0 to 2n−1 to 0, N being the resolution of the photodetector (16). The output of the D/A conversion circuit (30) causes comparator circuit (18) to flip when the ramp signal is equal to the value of the output from the photodetector (16). The comparator circuit (18), in turn, drives a load signal to a register (38) which stores the counter values 32 from pixel (14) at the instant the comparator 18 flips. In this way an A/D conversion of the image data takes place.

Claims1

Dependent claims

An image sensor comprising:

a ramp signal generator;
a counter circuit counting from an initial count to a final count in synchronism with said ramp signal generator;
a plurality of pixel cells disposed in a two-dimensional array of M rows and N columns, each of the cells containing a photodetector and a comparator circuit, each comparator circuit having first and second input terminals, each photodetector having an output terminal driving a first input of a comparator circuit, the plurality of pixel cells coupled to the ramp signal generator so that analog signals from the ramp signal generator drive a second input of a comparator circuit, each comparator circuit delivering an output when the analog signals from the ramp signal generator are equal to an output from a photodetector;
a plurality of M row select lines, each row select line coupled to said comparator of pixel cells of one of said M rows, one of said row select lines activated sequentially to enable said comparators of said corresponding one of said M rows;
a plurality of N registers, each register having a data input receiving a count from said counter circuit and an enable input connected to said output terminals of comparators of pixels cells in one of said N columns, each register storing said count of said counter circuit upon receipt of said output of a corresponding comparator.

2 more elements

Dependencies are inferred from the claim text.