Analog-to-digital converters for phase-detection autofocus image sensors
Embodiments of the present disclosure provide ADCs particularly suitable for PDAF image sensors, which ADCs may have an increased speed and/or reduced design complexity and power consumption compared to conventional implementations. An example ADC for a PDAF image sensor is configured to implement modified SAR techniques which reduce the number of bit trials required for conversion, and enable increased number of samples in a row-conversion time period of the image sensor. The ADC may implement the modified SAR techniques in combination with CMS in pixel readout signal chain, which may reduce noise without a proportionate increase in ADC sample rate.
1. A phase-detection autofocus (PDAF) image sensor system, comprising:
an array of pixel cells configured to acquire an image, each pixel cell comprising a first and a second light sensor; and
an analog-to-digital converter (ADC) configured to determine, for each of one or more of the pixel cells, a first N-bit digital value representative of a first analog signal value indicative of a charge detected by the first light sensor, where N is an integer greater than 1, and a second N-bit digital value representative of a second analog signal value indicative of a charge detected by the second light sensor, wherein said determination includes:
determining the first N-bit digital value by applying a first successive approximation register (SAR) algorithm to determine N bits of the first N-bit digital value, and
determining the second N-bit digital value by applying a second SAR algorithm to determine only M least significant bits (LSB) of the second N-bit digital value, where M is an integer equal to or greater than 1 and smaller than N, and combining the M LSB of the second N-bit digital value with (N-M) most significant bits (MSB) of the first N-bit digital value to generate the second N-bit digital value so that the (N-M) MSB of the second N-bit digital value are the (N-M) MSB of the first N-bit digital value and the M LSB of the second N-bit digital value are the M LSB determined by applying the second SAR algorithm.
2. The PDAF image sensor according to claim 1 , further comprising a control logic configured to control whether the PDAF image sensor operates in an image readout mode or in an autofocus readout mode.
3. The PDAF image sensor according to claim 2 , wherein, the control logic is configured to determine a digital value for the pixel cell based on the first N-bit digital value and the second N-bit digital value when the PDAF image sensor operates in the image readout mode.
4. The PDAF image sensor according to claim 3 , wherein determining the digital value for the pixel cell based on the first N-bit digital value and the second N-bit digital value comprises determining the digital value for the pixel cell as an average of the first N-bit digital value and the second N-bit digital value.
5. The PDAF image sensor according to claim 2 , wherein, the control logic is configured to determine whether the second N-bit digital value is valid.
6. The PDAF image sensor according to claim 5 , wherein, when the control logic determines that the second N-bit digital value is not valid, the control logic is configured to:
discard the second N-bit digital value, and
enable the ADC to determine a third N-bit digital value representative of the second analog signal value indicative of the charge detected by the second light sensor by applying the SAR algorithm to determine the N bits of the third N-bit digital value.
7. The PDAF image sensor according to claim 6 , wherein the PDAF image sensor is configured to implement correlated multiple sampling (CMS), and wherein the second N-bit digital value is a digital value of a first sample for the image, and the third N-bit digital value is a digital value of a second sample for the same image.
8. The PDAF image sensor according to claim 1 , wherein determining the first N-bit digital value comprises applying the first SAR algorithm to determine N bits of the first N-bit digital value for a first sample and further applying the second SAR algorithm to determine only M LSB of the first N-bit digital value for one or more subsequent samples.
9. The PDAF image sensor according to claim 8 , further comprising combining the M LSB of the first N-bit digital value for at least some of the one or more subsequent samples with (N-M) MSB of the first N-bit digital value from the first sample to generate one or more samples of the first N-bit digital values.
10. The PDAF image sensor according to claim 1 , wherein determining the second N-bit digital value comprises applying the second SAR algorithm to determine only M LSB of the second N-bit digital value for a plurality of samples.
11. The PDAF image sensor according to claim 10 , further comprising combining the M LSB of the second N-bit digital value for each of the plurality of samples with (N-M) MSB of the first N-bit digital value to generate a plurality of samples of the second N-bit digital values.
12. The PDAF image sensor according to claim 1 , wherein the first analog signal value is a first analog voltage value, and the second analog signal value is a second analog voltage value.
13. The PDAF image sensor according to claim 1 , further comprising a power supply.
14. An image sensor system, comprising:
a pixel cell comprising a pair of light sensors; and
one or more circuits configured to determine a first N-bit digital value representative of a first analog signal value indicative of a charge detected by a first light sensor of the pair, where N is an integer greater than 1, and a second N-bit digital value representative of a second analog signal value indicative of a charge detected by a second light sensor of the pair, wherein said determination includes:
applying an adaptive successive approximation register (SAR) algorithm to determine only M least significant bits (LSB) of the first N-bit digital value, where M is an integer equal to or greater than 1 and smaller than N,
applying the adaptive SAR algorithm to determine only M LSB of the second N-bit digital value,
combining the M LSB of the first N-bit digital value with set (N-M) most significant bits (MSB) to generate N bits of the first N-bit digital value so that the (N-M) MSB of the first N-bit digital value are the set (N-M) MSB and the M LSB of the first N-bit digital value are the M LSB of the first N-bit digital value determined by applying the adaptive SAR algorithm, and
combining the M LSB of the second N-bit digital value with the set (N-M) MSB to generate N bits of the second N-bit digital value so that the (N-M) MSB of the second N-bit digital value are the set (N-M) MSB and the M LSB of the second N-bit digital value are the M LSB of the second N-bit digital value determined by applying the adaptive SAR algorithm.
15. The image sensor according to claim 14 , further comprising:
one or more circuits configured to determine a digital value for the pixel cell based on the first N-bit digital value and the second N-bit digital value.
16. The image sensor according to claim 15 , wherein determining the digital value includes determining the digital value as an average of the first N-bit digital value and the second N-bit digital value.
17. The image sensor according to claim 14 , wherein the first analog signal value is a first analog voltage value, and the second analog signal value is a second analog voltage value.
18. A method for implementing analog-to-digital conversion in a phase-detection autofocus image sensor system that includes an array of pixel cells configured to acquire an image, each pixel cell including a first light sensor and a second light sensor, the method comprising:
determining, for each of one or more of the pixel cells, a first N-bit digital value representative of a first analog signal value indicative of a charge detected by the first light sensor, where N is an integer greater than 1, and a second N-bit digital value representative of a second analog signal value indicative of a charge detected by the second light sensor,
wherein said determination includes:
applying an adaptive successive approximation register (SAR) algorithm to determine only M least significant bits (LSB) of the first N-bit digital value,
applying the adaptive SAR algorithm to determine only M LSB of the second N-bit digital value, where M is an integer equal to or greater than 1 and smaller than N,
combining the M LSB of the first N-bit digital value with set (N-M) most significant bits (MSB) to generate N bits of the first N-bit digital value so that the (N-M) MSB of the first N-bit digital value are the set (N-M) MSB and the M LSB of the first N-bit digital value are the M LSB of the first N-bit digital value determined by applying the adaptive SAR algorithm, and
combining the M LSB of the second N-bit digital value with the set (N-M) MSB to generate N bits of the second N-bit digital value so that the (N-M) MSB of the second N-bit digital value are the set (N-M) MSB and the M LSB of the second N-bit digital value are the M LSB of the second N-bit digital value determined by applying the adaptive SAR algorithm.
19. The method according to claim 18 , further comprising:
when the second N-bit digital value is determined to be valid, determining a digital value for the pixel cell as an average of the first N-bit digital value and the second N-bit digital value.
20. The method according to claim 18 , further comprising:
when the second N-bit digital value is determined to not be valid, determining a third N-bit digital value representative of the second analog signal value indicative of the charge detected by the second light sensor by applying the SAR algorithm to determine the N bits of the third N-bit digital value.