High dynamic range imaging pixels with improved readout
An imaging system may include an image sensor having an array of dual gain pixels. Each pixel may be operated using an improved three read method and an improved four read method such that all signals are read in a high gain configuration in order to prevent electrical offset in signal levels. Each pixel may be operated using an improved three read, two analog to digital conversion (ADC) method in which a frame buffer is used to store calibration data. Each pixel may be operated using an improved three read, three ADC method in which no frame buffer is required. A high dynamic range image signal may be produced for each pixel based on signals read from the pixel and on light conditions.
1. A method of operating an imaging system, comprising:
with a photodiode in a pixel, accumulating charge in response to incident light;
with readout circuitry, reading out a first signal while the pixel is in a high gain configuration, wherein the first signal is based on a first portion of the accumulated charge that overflows from the photodiode into a floating diffusion node and a gain select storage node
with a transfer transistor, transferring a second portion of the accumulated charge remaining in the photodiode to the floating diffusion node while the pixel is in the high gain configuration;
with the readout circuitry, reading out a second signal while the pixel is in the high gain configuration, wherein the second signal is based on the first and second portions of the accumulated charge at the floating diffusion node;
asserting a gate signal for a gain select transistor to distribute the first and second portions of the accumulated charge between the floating diffusion node and a gain select storage node; and
with the readout circuitry, reading out a third signal while the pixel is in the high gain configuration, wherein the third signal is based on the distributed charge at the floating diffusion node, wherein reading out the third signal while the pixel is in the high gain configuration comprises:
deasserting the gate signal for the gain select transistor, and
with the readout circuitry, reading out a third signal while the gate signal of the gain select transistor is deasserted such that the floating diffusion node is electrically isolated from the gain select storage node, wherein the third signal is based on the distributed charge at the floating diffusion node.
2. The method defined in claim 1 , wherein the gain select transistor is interposed between the floating diffusion node and the gain select storage node.
3. The method defined in claim 1 , wherein the pixel is a dual gain pixel.
4. The method defined in claim 1 , further comprising:
resetting the pixel by setting the floating diffusion node and the gain select storage node to a pixel voltage.
5. The method defined in claim 4 , further comprising:
after resetting the pixel, with readout circuitry, reading out a fourth signal while the pixel is in the high gain configuration, wherein the fourth signal is based on the pixel voltage at the floating diffusion node.
6. A method of operating an imaging system, comprising:
with a photodiode in a dual gain pixel, accumulating charge in response to incident light;
with readout circuitry, reading out a first signal while the dual gain pixel is in a high gain configuration, wherein the first signal is based on a first portion of the accumulated charge that overflows from the photodiode into a floating diffusion node and a gain select storage node;
after reading out the first signal, resetting the floating diffusion node and a gain select storage region to a pixel voltage;
with the readout circuitry, reading out a second signal while the dual gain pixel is in the high gain configuration, wherein the second signal is based on the pixel voltage at the floating diffusion node;
with a transfer transistor, transferring a second portion of the accumulated charge from the photodiode to the floating diffusion node while the dual gain pixel is in the high gain configuration;
with the readout circuitry, reading out a third signal while the dual gain pixel is in the high gain configuration, wherein the third signal is based on the second portion of the accumulated charge at the floating diffusion node and wherein the high gain configuration comprises deasserting a gate signal for a gain select transistor to isolate the floating diffusion node from the gain select storage region.
7. The method defined in claim 6 , further comprising:
with a frame buffer, storing a dark offset calibration voltage.
8. The method defined in claim 7 , further comprising:
performing analog to digital conversion on a selected one of the first and second signals based on a detected light condition; and
performing analog to digital conversion on the third signal.
9. The method defined in claim 8 , wherein performing analog to digital conversion on the selected one of the first and second signals based on the detected light condition further comprises:
performing analog to digital conversion on the first signal in response to detecting that light intensity is above a threshold value.
10. The method defined in claim 9 , further comprising:
amplifying the first signal by a gain ratio between the third signal and the first signal; and
generating a high dynamic range image signal based on the amplified first signal, the calibration voltage, and the third signal.
11. The method defined in claim 8 , wherein performing analog to digital conversion on the selected one of the first and second signals based on the detected light condition further comprises:
performing analog to digital conversion on the second signal in response to detecting that light intensity is below a threshold value.
12. The method defined in claim 11 , further comprising:
generating a high dynamic range image signal based on the difference between the third signal and the second signal.
13. The method defined in claim 6 , further comprising:
performing analog to digital conversion on the first, second, and third signals.
14. The method defined in claim 13 , further comprising:
generating a high dynamic range image signal based on the difference between the third signal and the second signal in response to detecting that light intensity is below a threshold value.
15. The method defined in claim 13 , further comprising:
amplifying the first signal by a gain ratio between the third signal and the first signal; and
generating a high dynamic range image signal based on an amplified first signal, and the second and third signals.
16. A method of operating an imaging system comprising:
reading a first reset signal from a dual gain pixel when the dual gain pixel is in a high gain configuration, wherein the first reset signal is based on overflow charge in a floating diffusion node of the dual gain pixel;
resetting the floating diffusion node and a gain select storage region to a reset voltage;
reading a second reset signal from the dual gain pixel when the dual gain pixel is in the high gain configuration, wherein the second reset signal is based on the reset voltage at the floating diffusion node;
reading a first high gain signal from the dual gain pixel when the dual gain pixel is in the high gain configuration;
performing analog to digital conversion on the first and second reset signals and on the high gain signal; and
generating a high dynamic range image based on the first and second reset signals and the high gain signal.
17. The method defined in claim 16 , further comprising:
storing a dark offset calibration value in a frame buffer.