Image sensor with calibrated column analog-to-digital converters

• US 9,912,883 B1
• Filed: 05/09/2017
• Issued: 03/06/2018
• Est. Priority Date: 05/10/2016
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Apple Inc.
• Inventors: Lee, Bumha
• Primary Examiner(s): Nguyen, Linh
• Application Number:
  US15/590,775
• Time in Patent Office: 301 Days
• Field of Search: 341118, 341120, 341155, 341167-170
• CPC Class Codes: H03M 1/0658 by calculating a running average of a number of subsequent samples
• CPC Class Codes: H03M 1/188 Multi-path, i.e. having a separate analogue/digital converter for each possible range
• CPC Class Codes: H03M 1/56 Input signal compared with linear ramp
• CPC Class Codes: H04N 25/75 Circuitry for providing, modifying or processing image signals from the pixel array
• CPC Class Codes: H04N 25/78 Readout circuits for addressed sensors, e.g. output amplifiers or A/D converters

Abstract

Image sensors using multiple-ramp single slope analog to digital converters (ADCs) and method of their operation are disclosed. The images sensors use additional column ADCs to detect offset errors in the fine ramp signals and feedback in the analog domain to correct the errors. Averaging errors over multiple analog-to-digital conversion cycles allows for improved error correction.

Claims

1. An image sensor comprising:

an array of pixels configured in multiple columns;

a multiple-ramp single slope analog-to-digital conversion circuit operatively connected to the array of pixels and comprising;

a respective column analog-to-digital converter (ADC) for each of the multiple columns, each column ADC configured to receive a signal generated from a pixel in its column as a first input;

a coarse ramp generator operative to apply a coarse ramp signal as a second input to each column ADC during a coarse conversion step within an analog to digital conversion period; and

a plurality of fine ramp generators, each fine ramp generator operative to apply a respective fine ramp signal as the second input to each column ADC during a fine conversion step within the analog to digital conversion period;

a plurality of additional column ADCs configured to measure the fine ramp signals;

an error detection circuit operative to determine an error in at least one fine ramp signal, based on outputs from the plurality of additional column ADCs; and

feedback circuitry operative to modify an operation of at least one fine ramp generator based on the error determined by the error detection circuit.

2. The image sensor of claim 1, wherein the error detection circuit is operative to:

apply a first fine ramp signal and a trip level of the coarse ramp signal to a first comparator;

apply a second fine ramp signal and the trip level of the coarse ramp signal to a second comparator;

measure a first time between triggering of the first comparator and triggering of the second comparator;

apply the second fine ramp signal and the trip level of the coarse ramp signal to the first comparator;

apply the first fine ramp signal and the trip level of the coarse ramp signal to the second comparator;

measure a second time between triggering of the first comparator and triggering of the second comparator; and

average the first measured time and the second measured time;

wherein;

the first fine ramp signal and the second fine ramp signal are adjacent fine ramp signals.

3. The image sensor of claim 2, wherein:

the first comparator is a component of a first of the additional column ADCs;

the second comparator is a component of a second of the additional column ADCs;

the first measured time is during a first fine conversion step; and

the second measured time is during a next fine conversion step after the first fine conversion step.

4. The image sensor of claim 1, wherein the feedback circuitry is operative to modify an offset voltage of at least one of the plurality of fine ramp generators.

5. The image sensor of claim 1, wherein the feedback circuitry is operative to modify a current source of at least one of the plurality of fine ramp generators.

6. The image sensor of claim 1, wherein the error detection circuit is operative to average errors in at least one ramp signal determined during each of multiple analog to digital conversion periods.

7. The image sensor of claim 6, wherein the averaged errors are used to modify digital output of a column ADC.

8. The image sensor of claim 6, wherein the averaged errors are used to modify operation of the fine ramp generators.

9. A method of operating an image sensor that includes multiple-ramp single slope analog-to-digital converters (ADCs), and additional ADCs, the method comprising:

receiving an initiation signal;

using the additional column ADCs to perform an initial error measurement of ramp signals produced by a set of ramp generators;

performing an initial analog error correction on at least one of the set of ramp generators;

performing a subsequent error measurement of the ramp signals, using the additional column ADCs, during operation of the image sensor; and

performing a subsequent analog error correction on at least one of the set of ramp generators;

wherein;

the initial and subsequent analog error corrections comprise at least one of adjusting an offset voltage applied to one of the set of ramp generators or adjusting a current source within one of the set of ramp generators.

10. The method of operating an image sensor of claim 9, further comprising:

receiving a signal from an analog front end associated with the image sensor; and

changing a property of at least one of the set of ramp generators based on the received signal value.

11. The method of operating an image sensor of claim 10, wherein the signal from the analog front end comprises is of a change in a gain of an amplifier in the analog front end.

12. The method of operating an image sensor of claim 10, wherein changing the property of at least one of the set of ramp generators comprises adjusting one of a setting for a spacing between ramp signals or a setting for a slope of the ramp signals.

13. The method of operating an image sensor of claim 9, further comprising:

receiving a signal indicating an end of error measurement of the ramp signals; and

reducing power applied to the additional column ADCs.

14. The method of operating an image sensor of claim 9, wherein, during the initial error measurement, a first ramp signal exhibits a nonlinear abrupt change.

15. The method of operating an image sensor of claim 9, further comprising:

performing subsequent error measurements of the ramp signals using the additional column ADCs during each of multiple analog to digital conversion periods;

averaging the error measurements; and

modifying an operation of at least one of the set of ramp generators based on the averaged error measurements.

16. An electronic device comprising:

a camera comprising;

an image sensor operative to capture a digital representation of an image; and

an image processor operative to control an operation of the image sensor;

wherein the image sensor comprises;

an array of pixels;

a multiple-ramp single slope analog-to-digital conversion circuit linked with the array of pixels comprising;

a first set of column analog-to-digital converters (ADCs);

a coarse ramp generator operative to apply a coarse ramp signal to the first set of column ADCs; and

a plurality of fine ramp generators, each fine ramp generator operative to apply a respective fine ramp signal to the first set of column ADCs;

a second set of column ADCs configured to measure the fine ramp signals;

an error detection circuit operative to determine an error in at least one fine ramp signal based on outputs from the second set of column ADCs; and

feedback circuitry operative to provide an analog signal, based on the error determined by the error detection circuit, to at least one of the plurality of fine ramp generators to modify the fine ramp signal applied by the at least one fine ramp generator.

17. The electronic device of claim 16, wherein each of the second set of column ADCs comprise a comparator operative to:

form a first comparison between a trip level of the coarse ramp signal to a first fine ramp signal; and

form a second comparison between the trip level of the coarse ramp signal to a second fine ramp signal;

wherein the first fine ramp signal and the second fine ramp signal are adjacent.

18. The electronic device of claim 16, wherein the feedback circuitry is operative to perform at least one of modifying an offset voltage of at least one of the plurality of fine ramp generators or modifying a current source of at least one of the plurality of fine ramp generators.

19. The electronic device of claim 16, wherein the error detection circuit is operative to average errors in at least one fine ramp signal determined during each of multiple analog to digital conversion periods.

20. The electronic device of claim 19, wherein the feedback circuitry is operative to modify the operation of the fine ramp generators based on the averaged errors.