Global shutter image sensor with anti-blooming pixel and knee point self-calibration
An image sensor has global shutter imaging pixel cells, each including an anti-blooming transistor gate for modifying electric charge within a photodiode and for setting the photodiode to a selected potential. The sensor also has a row decoder circuit providing readout signals to each row of the imaging cells during both a readout interval and during a calibration interval for each row and providing to the anti-blooming transistor drain a selectable potential supply voltage. A mode select switch within the row decoder circuit applies either a standard drain supply voltage or an adjustable low voltage to the anti-blooming transistor drain. A programmable function logic circuit determines the timing of operation of the mode select switch to provide knee-point calibration to minimize photo conversion variations that lead to fixed pattern noise.
1. An image sensor comprising:
a plurality of global shutter imaging pixel cells, each including an anti-blooming transistor gate adjacent on one side to a photodiode and adjacent on another side to an anti-blooming transistor drain for modifying electric charge within the photodiode and for setting the photodiode to a selected potential;
a row decoder circuit providing readout signals to each of a plurality of rows of the imaging pixel cells during both a readout interval and a calibration interval for each row and providing to the anti-blooming transistor drain a selectable potential supply voltage, the selectable potential supply voltage being either a standard drain supply voltage or an adjustable low voltage;
a mode select switch within the row decoder circuit being operable to apply the standard drain supply voltage to the drain of the anti-blooming transistor or to apply the adjustable low voltage to the anti-blooming transistor drain; and
a programmable function logic circuit operable to engage the mode select switch to change the potential applied to the drain of the anti-blooming transistor from the standard supply voltage to the adjustable low voltage during an interval following the readout interval and before the calibration interval of each row.
2. The image sensor of claim 1 , wherein the global shutter imaging pixel cells each comprise:
a transfer transistor connected to the photodiode on one side and to an input of a first source follower amplifier transistor on another side;
a first switch transistor connecting an output of the first source follower amplifier transistor to a global shutter signal storage capacitor and a second switch transistor connecting the global shutter signal storage capacitor to a global shutter reset capacitor;
a second source follower transistor wherein an input of a second source follower transistor is connected to the second switch transistor and the global shutter reset capacitor and the output of the second source follower transistor is coupled though a row select transistor to a pixel output column line.
3. A pixel cell of the imaging sensor of claim 1 , wherein a programmable function logic circuit state is determined by a status of a selectable state register setting within the image sensor.
4. A pixel cell of the imaging sensor of claim 1 , wherein the standard drain supply voltage is a highest level compatible with a semiconductor technology by which an image sensor is fabricated and the adjustable low voltage ranges from zero to 0.5 volts.
5. A method for reducing image sensor fixed pattern noise by calibrating a knee point of a two segment piecewise linear response curve representing a pixel image signal output versus increasing incident image light, the method comprising the steps of:
providing a plurality of global shutter imaging pixel cells, each including an anti-blooming transistor gate adjacent on one side to a photodiode and adjacent on another side to an anti-blooming transistor drain for modifying electric charge within the photodiode and for setting the photodiode to a selected potential;
providing a row decoder circuit providing readout signals to each of a plurality of rows of the imaging pixel cells during both a readout interval and a calibration interval for each row and providing to the anti-blooming transistor drain a selectable potential supply voltage, the selectable potential supply voltage being either a standard drain supply voltage or an adjustable low voltage;
providing a mode select switch within the row decoder circuit being operable to apply the standard drain supply voltage to the drain of the anti-blooming transistor or to apply the adjustable low voltage to the anti-blooming transistor drain; and
operating a programmable function logic circuit to engage the mode select switch to change the potential applied to the drain of the anti-blooming transistor from the standard supply voltage to the adjustable low voltage during an interval following the readout interval and before the calibration interval of each row.
6. The method of claim 5 , wherein operating the programmable function logic comprises executing a digital algorithm designed to smooth a transition at the knee point of the two segment piecewise linear response curve representing charging of a pixel photodiode due to increasing image light.
7. An image sensor comprising:
a plurality of global shutter imaging pixel cells, each including an anti-blooming transistor gate adjacent on one side to a photodiode and adjacent on another side to an anti-blooming transistor drain for modifying electric charge within the photodiode and for setting the photodiode to a selected potential and wherein the global shutter imaging pixel cells each comprise:
a transfer transistor connected to the photodiode on one side and to an input of a first source follower amplifier transistor on another side;
a calibration transistor with a gate electrode connected to a calibration enable signal node, a source coupled to a calibration output node, and a drain connected to an output of the first source follower amplifier transistor;
a first switch transistor connecting the output of the first source follower amplifier transistor to a global shutter signal storage capacitor and a second switch transistor connecting the global shutter signal storage capacitor to a global shutter reset capacitor; and
a second source follower transistor wherein an input of the second source follower transistor is connected to the second switch transistor and the global shutter reset capacitor, and an output of the second source follower transistor is coupled though a row select transistor to a pixel output column line;
a row decoder circuit providing readout signals to each of a plurality of rows of the imaging pixel cells simultaneously during both a readout interval and a calibration interval for each row and providing to the anti-blooming transistor drain a selectable potential supply voltage, the selectable potential supply voltage being either a standard drain supply voltage or an adjustable low voltage, wherein the standard drain supply voltage is a highest level compatible with a semiconductor technology by which an image sensor is fabricated and the adjustable low voltage ranges from zero to 0.5 volts;
a mode select switch within the row decoder being operable to apply the standard drain supply voltage to the drain of the anti-blooming transistor or to apply the adjustable low voltage to the anti-blooming transistor drain; and
a programmable function logic circuit operable to engage the mode select switch to change the potential applied to the drain of the anti-blooming transistor from the standard supply voltage to the adjustable low voltage during an interval preceding the readout interval and the calibration interval of each row.
8. A pixel cell of the imaging sensor of claim 7 , wherein a programmable function logic circuit state is determined by a status of a selectable state register setting within the image sensor.