Digital pixel sensor with reduced noise

1. An apparatus comprising:

a plurality of pixel sensors, each pixel sensor comprising:

a photodiode,

a transfer gate that connects said photodiode to a floating diffusion node that accumulates noise when said floating diffusion node is floating, said floating diffusion node being connected to said photodiode in response to a transfer gate signal;

a reset gate that connects said floating diffusion node to a reset voltage in response to a reset gate signal;

an analog-to-digital converter (ADC) connected to said floating diffusion node, said ADC converting a voltage on said floating diffusion node to a digital value;

an output circuit that connects said ADC to a bit bus; and

a controller that causes said ADCs to operate in parallel to convert said voltages on said floating diffusion nodes to said digital values in a time that is less than the time needed for said floating diffusion node to acquire ten electron equivalents of noise.

2. The apparatus of claim 1 wherein said time is less than the time needed for said floating diffusion node to acquire three electron equivalents of noise.

3. The apparatus of claim 1 wherein said time is less than the time needed for said floating diffusion node to acquire one electron equivalent of noise.

4. The apparatus of claim 1 wherein each of said ADCs comprises:

a comparator that compares said voltage on said floating diffusion node to a ramp signal and generates a stop clock signal when said ramp signal reaches a predetermined relationship with said voltage on said floating diffusion node; and

a counter that counts clock pulses, said counter being reset by a first counter control signal and said counter stops counting said clock pulses in response to said stop clock signal being generated, wherein said output circuit causes a count generated in said counter to be readout on said bit bus in response to a readout control signal.

5. The apparatus of claim 4 wherein said ADC in each pixel sensor converts said voltage on said floating diffusion node in said pixel sensor after said floating diffusion node has been connected to said reset voltage and before said photodiode in said pixel sensor is connected to said floating diffusion node, said digital value being stored in said pixel sensor as a reset digital value, said ADCs using a first ramp signal as said ramp signal during said conversion.

6. The apparatus of claim 5 wherein said ADC in each of said pixel sensors converts said voltage on said floating diffusion node to a signal digital value in said pixel sensor after said floating diffusion node has been connected to said reset voltage and then connected to said photodiode, said ADC using a second ramp signal as said ramp signal during said conversion.

7. The apparatus of claim 6 wherein each pixel sensor combines said reset signal value and signal digital value to form a corrected digital value for that pixel sensor.

8. The apparatus of claim 6 wherein said first ramp signal has a different slope than said second ramp signal.

9. The apparatus of claim 5 wherein said reset digital value is stored in said counter.

10. The apparatus of claim 5 wherein said reset digital value is stored in said counter.

11. The apparatus of claim 6 wherein both said reset digital value and said signal digital value are readout on said bit bus.

12. A method for generating a digital image, said method comprising:

providing a plurality of pixel sensors, each pixel sensor comprising:

a photodiode corresponding to a predetermined pixel in said digital image; and

an ADC that converts a voltage on a floating diffusion node in said pixel sensor to a digital value;

causing each of said pixel sensors to convert a voltage on said floating diffusion node in that pixel sensor to said digital value in a time that is less than the time needed for said floating diffusion node to acquire ten electron equivalents of noise.

13. The method of claim 12 wherein said time is less than the time needed for said floating diffusion node to acquire three electron equivalents of noise.

14. The method of claim 12 wherein said time is less than the time needed for said floating diffusion node to acquire one electron equivalent of noise.

15. The method of claim 12 wherein said ADCs compare a voltage on said floating diffusion node to a ramp signal during said conversion.

16. The method of claim 15 wherein each of said pixel sensors uses a first ramp signal as said ramp signal to generate a reset digital value that is indicative of a voltage on said floating diffusion node in that pixel sensor generated when said floating diffusion node is connected to a reset voltage prior to said floating diffusion node being connected to said photodiode in that pixel sensor.

17. The method of claim 16 wherein each of said pixel sensors uses a second ramp signal as said ramp signal to generate a signal digital value indicative of a voltage on said floating diffusion node in that pixel sensor after said floating diffusion node is connected to photodiode.

18. The method of 17 wherein said first ramp signal has a different slope than said second ramp signal.

19. The method of claim 17 wherein said reset digital value and said signal digital value are stored in said pixel sensor in which said reset digital value and said signal digital value were generated.

20. The method of claim 17 further comprising combining said reset digital value and said signal digital value to generate a corrected signal value.

21. The method of claim 20 wherein said combining occurs in each of said pixel sensors.