High Linearity CMOS Ambient Light Sensor

A battery-operated mobile device, comprising:

• a backlit display having a power level adjustable in accordance with an output signal, the output signal being representative of an ambient light intensity, the power level being adjustable such that the backlit display is visible by a user as the ambient light intensity varies;
• a light sensor configured to measure the ambient light intensity and provide the output signal to an external circuit operably connected to the backlit display and configured to vary the power level of the backlit display in accordance with the output signal, the light sensor comprising;
• a first photodetector configured to receive a light signal characterized by the ambient light intensity to be measured, said first photodetector generating an output current comprising a light current proportional to said ambient light intensity and a dark current that is independent of said light signal, the output current being provided to a node;
• a second photodetector connected in series with said first photodetector at the node, said second photodetector having a covering that prevents light from reaching said second photodetector and generating a sinking current equal to said dark current, said second photodetector being connected to the node and sinking the dark current at the node;
• and
• a high impedance cascode current mirror amplifier circuit connected to the said cascode current mirror amplifier circuit generating a cascode current mirror output current proportional to said light current through said node and providing the cascode current mirror output current to the external circuit;
• wherein the first photodectector, the second photodetector, the node and the cascode current mirror amplifier circuit are further configured in respect of one another to compensate for non-linearities associated with the dark current while not amplifying the dark current and noise associated therewith.