| 摘要 |
An active pixel sensor includes a photosensitive device and a dynamic comparator that when coupled with a voltage ramp will form a digital pixel sensor with pulse width modulated digital output. A number of switches are included in the digital pixel sensor to configure the input of the dynamic comparator to couple with the photosensitive device or the voltage ramp such that the dynamic comparator is free from input transistor mismatch problem, as both input use the same input transistor. A cascade of dynamic comparator is disclosed in this invention, such as to improve the sensitivity and conversion speed of the digital pixel sensor. There are a number of switches that connect and isolate the digital pixel sensor from the bit line, which is shared by a plurality of digital pixel sensors in the sensor array. Photosensitive devices in close proximity can share the dynamic comparator by a number of selection switches, such that each photosensitive device can be read out in a time shared manner. Such configuration reduce the average number of transistors in each digital pixel sensor and thus the total silicon area of the sensor array, and hence the cost of production of the image sensor.
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