Active pixel sensor with reduced fixed pattern noise using two active regions
View Patent ↗An image sensor includes pixels each of which is designed to transfer charge, accumulated in a photoactive region of the pixel during a first period, through a second active region of the pixel to a power supply node, and to transfer charge, accumulated in the photoactive region during a second period, through the second active region to a sense node in the pixel. Passing charge through the second active region prior to transferring it either to the power supply node or the sense node can help reduce fixed pattern noise. The image sensor can be operated in snap-shot mode.
1. A method of operating a pixel, the method comprising:
setting a first active region of the pixel to a predetermined charge level;
transferring charge accumulated in a photoactive region of the pixel during a non-charge integration period to the first active region of the pixel;
transferring charge from the first active region to a power supply node of the pixel;
setting the first active region to the predetermined charge level;
transferring charge accumulated in the photoactive region during a charge integration period to the first active region of the pixel; and
transferring charge from the first active region to a storage node of the pixel.
2. The method of claim 1 , further comprising:
forming a pixel output signal representing the charge transferred to the storage node; and
sampling and holding the formed pixel output signal in a sample and hold circuit.
3. The method of claim 2 , further comprising:
setting the storage node to a reset voltage level;
setting the first active region to the predetermined charge level; and
transferring charge from the first active region of the pixel to the storage node of the pixel.
4. A method of operating a pixel comprising a photosensor, the method comprising:
preparing the pixel for integration by:
flooding a first active region of the pixel with charge by raising a voltage on a gate between the first active region and a power supply node of the pixel;
lowering the voltage on a gate between the first active region and a charge accumulation region of the pixel;
lowering the voltage on a gate between the first active region and a floating diffusion region of the pixel; and
transferring a charge from the charge accumulation region to the power supply node;
integrating charge with the photosensor to place charge on the charge accumulation region;
transferring the integrated charge from the charge accumulation region to the first active region; and
generating an output signal from the transferred charge.
5. The method of claim 4 , wherein the act of preparing the pixel for integration further comprises:
transferring a charge from the first active region to the power supply node.
6. The method of claim 4 , wherein the act of transferring the integrated charge from the charge accumulation region further comprises:
flooding the first active region with a charge;
transferring the charge from the first active region to the power supply node; and
transferring the charge from the charge accumulation region to the first active region.
7. The method of claim 6 , further comprising:
transferring charge from the first active region to the floating diffusion region.
8. The method of claim 7 , further comprising:
sampling a reset voltage from the pixel by:
resetting the floating diffusion region to a reset voltage level;
flooding the first active region with a charge;
transferring the charge from the first active region to the power supply node; and
transferring the charge from the first active region to the floating diffusion region.