Reducing smear through digital cancellation in image sensor

1. An apparatus of reading out an image sensor including a plurality of pixels arranged in rows and columns, the apparatus comprising:

a plurality of analog-to-digital converters (ADCs) coupled to respective columns and configured to output digital codes representative of received pixel signals of a set of pixels coupled to the respective columns; and

a smear cancellation circuit configured to:

determine an average value of the digital codes;

multiply the average value by at least one factor to obtain an offset value; and

subtract the offset value from the digital codes to obtain smear-cancelled digital codes.

2. The apparatus of claim 1 , wherein the at least one factor is a predetermined value of a computer simulation result of the image sensor.

3. The apparatus of claim 1 , wherein the at least one factor is a characterization result of the image sensor.

4. The apparatus of claim 1 , wherein the at least one factor comprises:

a first factor associated with a subset of the set of pixels disposed in a same row, the subset of the set of pixels being concurrently read out to the ADCs; and

the smear cancellation circuit is configured to multiply the average value by the first factor to obtain the offset value.

5. The apparatus of claim 1 , wherein the at least one factor comprises:

a second factor associated with the set of pixels disposed in a same row, the set of pixels being concurrently read out to the ADCs; and

the smear cancellation circuit is configured to multiply the average value by the second factor to obtain the offset value.

6. The apparatus of claim 1 , wherein the at least one factor comprises a plurality of factors, and the smear cancellation circuit is further configured to:

determine a number of the digital codes having a value greater than a predetermined threshold value to obtain a determination result;

select a factor of the plurality of factors in response to the determination result; and

multiply the average value by the selected factor to obtain the offset value.

7. The apparatus of claim 6 , wherein the number of the digital codes are associated with a number of bright pixels of the set of pixels.

8. The apparatus of claim 1 , wherein the at least one factor is a function of a capacitive coupling between adjacent lines of the set of pixels or adjacent pixels of the set of pixels disposed in a same row.

9. The apparatus of claim 8 , wherein the adjacent lines are one of a column line, a row select line, a power line, a ground line, and a reset line.

10. The apparatus of claim 1 , further comprising a data storage configured to store the digital codes.

11. An image sensor comprising the apparatus of claim 1 .

12. A method of cancelling smears in an image sensor comprising a pixel array having a plurality of pixels arranged in rows and columns, the method comprising:

providing pixel signals of a set of pixels to a plurality of analog-to-digital converters (ADCs);

converting the pixel signals to digital codes representative of the pixel signals by the ADCs;

determining an average value of the digital codes by a smear cancellation circuit;

multiplying the average value by at least one factor to obtain an offset value; and

subtracting the offset value from the digital codes to obtain smear-cancelled digital codes.

13. The method of claim 12 , prior to determining the average value, further comprising:

storing the digital codes in a data storage.

14. The method of claim 12 , wherein the at least one factor comprises a plurality of factors, the method further comprising:

determining a number of the digital codes having a value greater than a predetermined threshold value to obtain a determination result;

selecting a factor of the plurality of factors in response to the determination results; and

multiplying the average value by the selected factor to obtain the offset value.

15. The method of claim 14 , wherein the number of the digital codes are associated with a number of bright pixels of the set of pixels.

16. The method of claim 12 , further comprising storing the smear-cancelled digital codes in a storage device for further processing.

17. The method of claim 12 , wherein the at least one factor comprises a first factor associated with a subset of pixels in the set of pixels and a second factor associated with the entire set of pixels, the method further comprising:

determining whether the subset of pixels are concurrently read out to the ADCs;

when the subset of pixels are determined to be concurrently readout to the ADCs:

selecting the first factor to be the at least one factor to obtain the offset value;

when the entire set of pixels are determined to be concurrently readout to the ADCs:

selecting the second factor to be the at least one factor to obtain the offset value.

18. The method of claim 12 , wherein the at least one factor is a predetermined value of a computer simulation result of the image sensor.

19. The method of claim 12 , wherein the at least one factor is a characterization result of the image sensor.

20. The method of claim 12 , wherein the at least one factor is a function of a capacitive coupling between adjacent lines of the set of pixels or adjacent pixels of the set of pixels disposed in a same row.