Charge packet signal processing using pinned photodiode devices
View Patent ↗An image sensor may include an array of image pixels coupled to analog-to-digital conversion circuitry formed from pinned photodiode charge transfer circuits. Majority charge carriers for the pinned photodiodes in the charge transfer circuits may be electrons for photodiode wells formed from n-type doped regions and may be holes for photodiode formed from p-type doped regions. Pinned photodiodes may be used for charge integration onto a capacitive circuit node. Pinned photodiodes may also be used for charge subtraction from a capacitive circuit node. Comparator circuitry may be used to determine digital values for the pixel output levels in accordance with single-slope conversion, successive-approximation-register conversion, cyclic conversion, and first or second order delta-sigma conversion techniques. The array of image pixels used for imaging may have a conversion mode wherein at least a portion of the pixel circuitry in the array are operated similar to the charge transfer circuits.
1. An image sensor, comprising:
an image pixel comprising a first pinned photodiode of a first dopant type, wherein the image pixel produces an output signal based an amount of charge in the first pinned photodiode of the first dopant type;
an output line coupled to the image pixel, wherein the output line conveys the output signal from the image pixel;
a charge integrator circuit that includes a plurality of charge transfer circuits that are coupled between a summing node and the output line coupled to the image pixel; and
a charge subtraction circuit that is coupled to the summing node and that includes an additional plurality of charge transfer circuits that contain a pinned photodiode of a second dopant type that is different from the first dopant type.
2. The image sensor of claim 1 , wherein the output line is coupled to the charge integrator circuit, the image sensor further comprising:
a level shifting circuit coupled between the output line and the charge integrator circuit, wherein the level shifting circuit raises an image pixel signal to an elevated signal level.
3. The image sensor of claim 2 , wherein the charge integrator circuit comprises a first charge integrator circuit, wherein the charge subtraction circuit comprises a first charge subtraction circuit, wherein the summing node comprises a first summing node, and wherein the image sensor further comprises:
a second charge integrator circuit;
a second charge subtraction circuit; and
a second summing node that is interposed between the second charge integrator circuit and the second charge subtraction circuit.
4. The image sensor of claim 3 , wherein the level shifting circuit comprises a first level shifting circuit, and wherein the image sensor further comprises:
a second level shifting circuit coupled between the first summing node and the second charge integrator circuit; and
a comparator having a first input that is coupled to the second summing node and a second input that is coupled to a reference voltage level.
5. The image sensor of claim 2 , wherein the level shifting circuit comprises a first level shifting circuit that is selectively coupled to the charge integrator circuit, and wherein the image sensor further comprises:
a second level shifting circuit that receives an input from the charge integrator circuit, wherein the second level shifting circuit has an output that is selectively connected to the charge integrator circuit.