Methods and apparatus for a successive approximation register analog-to-digital converter

1. A calibration circuit, configured to connect to a set of analog-to-digital converters (ADCs) and receive an ADC output code from each ADC, wherein the ADC output code has errors, comprising:

a first calibration sub-circuit configured to perform a first calibration on each ADC output code comprising: compensating for systemic mismatch using a weight-based method, wherein the systemic mismatch comprises a first error type that is shared among each ADC output; and

a second calibration sub-circuit connected to the first calibration circuit and configured to perform a second calibration on each ADC output code comprising: compensating for random mismatch by randomizing the ADC output codes around a switching point, wherein the random mismatch comprises a second error type that occurs among different ADC output codes;

wherein the calibration circuit generates a corrected ADC output code according to the first and second calibrations.

2. The calibration circuit according to claim 1 , wherein the weight-based method comprises computing a plurality of weights, computing a total weight by summing the computed plurality of weights, and subtracting the computed total weight from the ADC output code.

3. The calibration circuit according to claim 1 , wherein randomizing the ADC output codes around a switching point comprises:

comparing a total ADC code length to:

a predetermined first value; and

a predetermined second value;

computing a difference value between the total ADC code length and the predetermined first value; and

adding a random number to the total ADC code length, wherein the random number is in the range of zero to the computed difference value.

4. The calibration circuit according to claim 1 , wherein randomizing the ADC output codes around a switching point comprises:

comparing a total ADC code length to:

a predetermined first value; and

a predetermined third value; and

computing a difference value between the total ADC code length and the predetermined first value; and

subtracting a random number from the total ADC code length, wherein the random number is in the range of zero to the computed difference.

5. The calibration circuit according to claim 1 , wherein the first calibration sub-circuit comprises:

a logic circuit configured to receive a start signal and initiate the first calibration in response to the start signal; and

a first control circuit in communication with the logic circuit and configured to:

receive the ADC output code; and

subtract a switching point code from the ADC output code to generate a subtracted code.

6. The calibration circuit according to claim 5 , wherein the first calibration sub-circuit further comprises a counter in communication with the logic circuit and configured to determine a code count based on the subtracted code and an enable signal.

7. The calibration circuit according to claim 6 , wherein the first calibration sub-circuit further comprises a missing code generator in communication with the logic circuit and configured to compare the code count to an average code count to determine a missing code count.

8. The calibration circuit according to claim 7 , wherein the first calibration sub-circuit further comprises a weight generator in communication with the logic circuit and configured to assign a weight to the missing_code_count and correct the ADC output code according to the assigned weight.

9. A method for calibrating ADC output codes from a set of analog-to-digital converters (ADCs), comprising:

performing a first calibration on each ADC output code comprising: compensating for systemic mismatch using a weight-based method, wherein the systemic mismatch comprises a first error type that is common among all ADC output codes;

performing a second calibration on each ADC output code comprising: compensating for random mismatch by randomizing the ADC output codes around a switching point, wherein the random mismatch comprises a second error type that differs among the ADC output codes;

generating a corrected ADC output code according to the first and second calibrations.

10. The method according to claim 9 , wherein the weight-based method comprises computing a plurality of weights, computing a total weight by summing the computed plurality of weights, and subtracting the computed total weight from the ADC output code.

11. The method according to claim 10 , wherein the plurality of weights are computed during an initialization period.

12. The method according to claim 9 , wherein randomizing the ADC output codes around the switching point comprises:

comparing a total ADC code length to:

a predetermined first value;

a predetermined second value; and

a predetermined third value;

computing a difference value between the total ADC code length and the predetermined first value;

adding a random number to the total ADC code length if the total ADC code length is:

less than the first value; and

greater than or equal to the second value; and

subtracting the second random number from the total ADC code length if the total ADC code length is:

greater than the first value; and

less than or equal to the third value;

wherein the random number is in the range of zero to the computed difference value.

13. The method according to claim 9 , further comprising generating a count value and utilizing the count value to load the ADCs at the switching point.

14. An imaging system, comprising:

a pixel array comprising a plurality of columns of pixels;

a set of radix-2 successive approximation register analog-to-digital converters (SAR ADCs); wherein each SAR ADC:

is connected to a respective column from the plurality of columns; and

generates an ADC output code;

a calibration circuit connected to the set of SAR ADCs and configured to:

perform a first calibration on each ADC output code comprising: compensating for systemic mismatch using a weight-based method, wherein the systemic mismatch is a first error type that is shared among all ADC output codes; and

perform a second calibration on each ADC output comprising: compensating for random mismatch using a dither algorithm, wherein the random mismatch is a second error type that differs among the ADC output codes; and

generate a corrected ADC output code for each ADC output code according to the first and second calibrations; and

an image signal processor connected to the calibration circuit and configured to process image data from the pixel array according to the corrected ADC output codes.

15. The imaging system according to claim 14 , wherein the weight-based method comprises computing a plurality of weights, computing a total weight by summing the computed plurality of weights, and subtracting the computed total weight from the ADC output code.

16. The imaging system according to claim 14 , wherein the calibration circuit comprises:

a logic circuit configured to receive a start signal and initiate the first calibration in response to the start signal; and

a first control circuit in communication with the logic circuit and configured to:

receive the ADC output code; and

subtract a switching point code from the ADC output code to generate a subtracted code.

17. The imaging system according to claim 14 , wherein the calibration circuit further comprises:

a counter in communication with the logic circuit and configured to determine a code count based on the subtracted code and an enable signal;

a missing code generator in communication with the logic circuit and configured to compare the code count to an average code count to determine a missing_code_count; and

a weight generator in communication with the logic circuit and configured to assign a weight to the missing_code_count and correct the ADC output code according to the assigned weight.

18. The imaging system according to claim 14 , further comprising a set of control circuits, wherein each control circuit is connected to and configured to control operation of a respective SAR ADC from the set of SAR ADCs; and transmit the ADC output code to the calibration circuit.

19. The imaging system according to claim 14 , further comprising a counter connected to the set of control circuits and controls a switching point of the SAR ADC.

20. The imaging system according to claim 14 , wherein the calibration circuit performs the first and second calibrations prior to readout of pixel signals from the pixel array.