Column readout multiplexer for CMOS image sensors with multiple readout and fixed pattern noise cancellation
First Claim
1. In an active pixel sensing structure including an array of pixel unit cells adapted to alternate between a light sensing mode wherein said cells output image signals and a reset mode wherein said cells output reset signals, said image signals being proportional to light incident on corresponding cells and said reset signals being proportional to a predefined reference potential, said structure further including readout circuit means connected to signal lines for reading out signals from said cells, an improved readout circuit means comprising:
- first sample and hold means for receiving and storing an image signal developed on said signal lines;
second sample and hold means for receiving and storing a reset signal developed on said signal lines;
first signal amplifying means having a first input terminal and a first output terminal;
second signal amplifying means having a second input terminal and a second output terminal; and
switching means operable between a first mode wherein said first sample and hold means is coupled to said first input terminal and said second sample and hold means is coupled to said second input terminal, and a second mode wherein said first sample and hold means is operatively decoupled from said first input terminal, said second sample and hold means is operatively decoupled from said second input terminal, and said first and second input terminals are connected to a source of predetermined potential;
whereby with said switching means in said second mode, a voltage, D1, taken across said first and second output terminals is proportional to the fixed pattern noise voltage attributable to said first and second amplifying means; and
whereby with said switching means in said first mode, a voltage, D2, taken across said first and second output terminals is proportional to a voltage equal to the difference between said image signal and said reset signal plus said voltage, D1, such that by subtracting said voltage, D1, from said voltage, D2, a signal proportional to the actual light detected by a particular cell can be obtained.
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Abstract
An active pixel sensing structure includes an array of pixel unit cells each of which an adapted to alternate between a light sensing mode wherein the cell outputs an image signal and a reset mode wherein the cell outputs a reset signal. The image signal is proportional to light incident on the cell, and the reset signal is proportional to a predefined reference potential. An improved readout circuit according to the present invention includes a first sample and hold component for receiving and storing the image signal, and second sample and hold component for receiving and storing the reset signal. A signal amplifier is provided for each sample and hold component. A switching circuit is operable between a first mode and a second mode. In the second mode, the first and second sample and hold components are operatively decoupled from the corresponding signal amplifiers while input terminals of the signal amplifiers are connected to a source of predetermined reference potential. With the switching circuit in the second mode, fixed pattern noise (FPN) attributable to the amplifiers is determined. With the switching means in the first mode, a voltage, D2, is determined across outputs of the signal amplifiers. The voltage, D2, is proportional to a voltage equal to the difference between the image signal and the reset signal plus the FPN voltage. A processing means subtracts the voltage, D1, from the voltage, D2, to develop a signal proportional to the light detected by the cell.
176 Citations
18 Claims
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1. In an active pixel sensing structure including an array of pixel unit cells adapted to alternate between a light sensing mode wherein said cells output image signals and a reset mode wherein said cells output reset signals, said image signals being proportional to light incident on corresponding cells and said reset signals being proportional to a predefined reference potential, said structure further including readout circuit means connected to signal lines for reading out signals from said cells, an improved readout circuit means comprising:
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first sample and hold means for receiving and storing an image signal developed on said signal lines; second sample and hold means for receiving and storing a reset signal developed on said signal lines; first signal amplifying means having a first input terminal and a first output terminal; second signal amplifying means having a second input terminal and a second output terminal; and switching means operable between a first mode wherein said first sample and hold means is coupled to said first input terminal and said second sample and hold means is coupled to said second input terminal, and a second mode wherein said first sample and hold means is operatively decoupled from said first input terminal, said second sample and hold means is operatively decoupled from said second input terminal, and said first and second input terminals are connected to a source of predetermined potential; whereby with said switching means in said second mode, a voltage, D1, taken across said first and second output terminals is proportional to the fixed pattern noise voltage attributable to said first and second amplifying means; and whereby with said switching means in said first mode, a voltage, D2, taken across said first and second output terminals is proportional to a voltage equal to the difference between said image signal and said reset signal plus said voltage, D1, such that by subtracting said voltage, D1, from said voltage, D2, a signal proportional to the actual light detected by a particular cell can be obtained. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17)
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18. A method of reading out signals from the cells of an active pixel sensing structure including an array of pixel unit cells adapted to alternate between a light sensing mode wherein each said cell outputs an image signal, and a reset mode wherein each said cell outputs a reset signal, the image signals being proportional to light incident on corresponding cells and the reset signals being proportional to a predetermined reference potential, said method comprising the steps of
sampling and holding an image signal generated by a particular cell; - sampling and holding a reset signal generated by said particular cell;
providing a first signal amplifying means having a first input terminal and a first output terminal; providing a second signal amplifying means having a second input terminal and a second output terminal; providing a switching means operable between a first mode wherein said image signal is coupled to said first input terminal and said reset signal is coupled to said second input terminal, and a second mode wherein said image signal is operatively decoupled from said first input terminal and said reset signal is operatively decoupled from said second input terminal, and said first and second input terminals are coupled to a predefined potential; determining a voltage, D1, across said first and second output terminals with said switching means in said second mode, said voltage, D1, being proportional to a fixed pattern noise voltage attributable to said first and second amplifying means; determining a voltage, D2, across said first and second output terminals with said switching means in said first mode wherein said voltage, D2, is proportional to a voltage equal to the difference between said image signal and said reset signal plus said fixed pattern noise voltage; and subtracting said voltage, D1, from said voltage, D2, to determine a signal proportional to the light detected by said particular cell.
- sampling and holding a reset signal generated by said particular cell;
Specification