Global shutter image sensor

1. An apparatus comprising:

a photodiode;

a charge sensing unit including a first charge storage device and a second charge storage device;

an analog-to-digital converter (ADC); and

a controller configured to:

within an exposure period:

enable the photodiode to, in response to incident light, accumulate residual charge and transfer overflow charge to the first charge storage device and the second charge storage device when the photodiode saturates;

after the exposure period ends:

disconnect the second charge storage device from the first charge storage device;

transfer the residual charge to the first charge storage device to cause the charge sensing unit to generate a first voltage;

control the ADC to quantize the first voltage to generate a first digital value to measure the residual charge;

connect the first charge storage device with the second charge storage device to cause the charge sensing unit to generate a second voltage;

quantize the second voltage to generate a second digital value to measure the overflow charge; and

generate a digital representation of an intensity of the incident light based on the first digital value, the second digital value, and based on whether the photodiode saturates.

2. The apparatus of claim 1 , wherein the apparatus further comprises:

a first switch coupled between the photodiode and the first charge storage device, the first switch controllable by the controller to transfer the residual charge or the overflow charge from the photodiode; and

a second switch controllable by the controller to connect the first charge storage device and the second charge storage device in parallel, or to disconnect the second charge storage device from the first charge storage device.

3. The apparatus of claim 2 , wherein the controller is configured to, during the exposure period:

control the second switch to connect the first charge storage device and the second charge storage device in parallel; and

control the first switch to enable the photodiode to transfer the overflow charge to the first charge storage device and the second charge storage device connected in parallel if the photodiode saturates.

4. The apparatus of claim 3 , wherein the controller is configured to:

control the second switch to disconnect the second charge storage device from the first charge storage device and from the photodiode to enable the first charge storage device to store a first portion of the overflow charge and the second charge storage device to store a second portion of the overflow charge; and

control the first switch to transfer the residual charge to the first charge storage device to cause the charge sensing unit to generate the first voltage.

5. The apparatus of claim 4 , wherein:

the first voltage is based on a quantity of the residual charge and a quantity of the first portion of the overflow charge and a capacitance of the first charge storage device.

6. The apparatus of claim 5 , wherein the second voltage is based on the quantity of the residual charge, the quantity of the overflow charge, and a total capacitance of the first charge storage device and the second charge storage device.

7. The apparatus of claim 4 , wherein the controller is configured to empty the first charge storage device prior to transferring the residual charge of the each pixel cell to the first charge storage device; and

wherein the first voltage is based on a quantity of the residual charge and a capacitance of the first charge storage device.

8. The apparatus of claim 7 , wherein the second voltage is based on the quantity of the residual charge, a quantity of the second portion of the overflow charge, and a total capacitance of the first charge storage device and the second charge storage device.

9. The apparatus of claim 1 , wherein the controller is configured to:

generate a first digital representation of the residual charge and a second digital representation of the overflow charge based on the first digital value, the second digital value, and capacitances of the first charge storage device and of the second charge storage device; and

based on whether the photodiode saturates, generate the digital representation of the intensity of the incident light based on the first digital representation of the residual charge or the second digital representation of the overflow charge.

10. The apparatus of claim 1 , further comprising a memory and a counter; wherein:

the ADC comprises a comparator configured to:

compare the first voltage against a first ramping voltage to output a first decision;

obtain the first digital value from the counter based on the first decision;

compare the second voltage against a second ramping voltage to output a second decision;

obtain the second digital value from the counter based on the second decision; and

store both the first digital value and the second digital value in the memory.

11. The apparatus of claim 10 , wherein the controller is configured to reset the comparator and the first and second charge storage devices simultaneously prior to comparing the first voltage against the first ramping voltage.

12. The apparatus of claim 11 , wherein the controller is configured to reset the comparator, and then reset the first and second charge storage devices when the comparator is out of reset, prior to comparing the second voltage against the second ramping voltage.

13. The apparatus of claim 12 , wherein the first ramping voltage and the second ramping voltage have opposite ramping directions.

14. The apparatus of claim 12 , wherein a polarity of comparison of the comparator is inverted in the generation of the second digital value with respect to the generation of the first digital value.

15. The apparatus of claim 14 , further comprising an input multiplexor controllable to swap inputs to a first input terminal and a second input terminal of the comparator between the generation of the first digital value and the generation of the second digital value to invert the polarity of the comparison.

16. The apparatus of claim 14 , further comprising an output multiplexor controllable to invert an output of the comparator between the generation of the first digital value and the generation of the second digital value to invert the polarity of the comparison.

17. A method comprising:

within an exposure period, enabling a photodiode to, in response to incident light, accumulate residual charge, and to transfer overflow charge to a first charge storage device and a second charge storage device when the photodiode saturates;

disconnecting the second charge storage device from the first charge storage device;

enabling the photodiode to transfer the residual charge to the first charge storage device to cause the charge sensing unit to output a first voltage;

quantizing the first voltage to generate a first digital value to measure the residual charge;

connecting the second charge storage device with the first charge storage device to cause the charge sensing unit to output a second voltage;

quantizing the second voltage to generate a second digital value to measure the overflow charge; and

generating a digital representation of the incident light intensity based on the first digital value and the second digital value.

18. The method of claim 17 , wherein the method further comprises:

connecting the first charge storage device and the second charge storage device in parallel to receive the overflow charge from the photodiode; and

disconnecting the first charge storage device from the second charge storage device such that the first charge storage device stores a first portion of the overflow charge and the second charge storage device stores a second portion of the overflow charge.

19. The method of claim 18 , wherein the residual charge combines with the first portion of the overflow charge to generate the first voltage.

20. The method of claim 18 , further comprising emptying the first charge storage device prior to transferring the residual charge of to the first charge storage device, wherein the residual charge is stored at the emptied first charge storage device to generate the first voltage.