Logarithmic pixels with correlated double sampling
An imaging pixel may be operated in either a linear mode or a logarithmic mode. In the logarithmic mode, the voltage at a floating diffusion region may be proportional to the logarithm of the intensity of incident light. In order to enable correlated double sampling (CDS) in the logarithmic mode, a transistor may be provided that couples the photodiode to a bias voltage. When the transistor is turned off, the photodiode may be able to operate in a logarithmic mode. When the transistor is turned on, the floating diffusion region may be reset to a baseline voltage level. Images from the linear mode and the logarithmic mode may be combined to form high dynamic range images with flicker mitigation.
1. A method of operating an imaging pixel, wherein the imaging pixel comprises a photodiode, an anti-blooming transistor coupled between the photodiode and a first bias voltage, a transfer transistor coupled between the photodiode and a floating diffusion region, and a control transistor coupled between the floating diffusion region and a second bias voltage, the method comprising:
disabling the anti-blooming transistor;
asserting the transfer transistor;
asserting the control transistor;
after disabling the anti-blooming transistor and asserting the control transistor, sampling a first voltage level at the floating diffusion region, wherein sampling the first voltage level comprises sampling the first voltage level while the anti-blooming transistor is disabled, the control transistor is asserted, and the transfer transistor is asserted;
after sampling the first voltage level at the floating diffusion region, asserting the anti-blooming transistor; and
after asserting the anti-blooming transistor, sampling a second voltage level at the floating diffusion region, wherein sampling the second voltage level at the floating diffusion region comprises sampling the second voltage level while the anti-blooming transistor is asserted, the control transistor is asserted, and the transfer transistor is asserted.
2. The method defined in claim 1 , wherein asserting the anti-blooming transistor results in charge generated in the photodiode flowing through the anti-blooming transistor and exiting the imaging pixel.
3. The method defined in claim 1 , wherein disabling the anti-blooming transistor, asserting the control transistor, and asserting the transfer transistor results in the floating diffusion region having a logarithmic response to incident light.
4. The method defined in claim 1 , wherein disabling the anti-blooming transistor, asserting the control transistor, and asserting the transfer transistor results in a photocurrent flowing through the control and transfer transistors.
5. The method defined in claim 1 , further comprising:
with processing circuitry, subtracting the sample of the second voltage level from the sample of the first voltage level.
6. The method defined in claim 1 , wherein operating the imaging pixel comprises operating the imaging pixel in a logarithmic mode.
7. A method of operating an imaging pixel in a logarithmic mode, wherein the imaging pixel comprises a photodiode, a floating diffusion region, an anti-blooming transistor coupled between the photodiode and a first bias voltage, a transfer transistor coupled between the photodiode and the floating diffusion region, and a control transistor coupled between the floating diffusion region and a second bias voltage, the method comprising:
sampling a first voltage level at the floating diffusion region while the anti-blooming transistor is disabled, the transfer transistor is asserted, and the control transistor is asserted; and
sampling a second voltage level at the floating diffusion region while the anti-blooming transistor is asserted, the transfer transistor is asserted, and the control transistor is asserted.
8. The method defined in claim 7 , the method further comprising:
with processing circuitry, subtracting the sample of the second voltage level from the sample of the first voltage level.
9. The method defined in claim 7 , wherein a gate of the control transistor receives the second bias voltage when the control transistor is asserted.
10. A method of operating an imaging pixel with a photodiode, a floating diffusion region, an anti-blooming transistor coupled between the photodiode and a first bias voltage, a transfer transistor coupled between the photodiode and the floating diffusion region, and a control transistor coupled between the floating diffusion region and a second bias voltage, the method comprising:
capturing a first image using correlated double sampling while the imaging pixel operates in a linear mode, wherein capturing the first image using correlated double sampling while the imaging pixel operates in the linear mode comprises:
disabling the control transistor;
asserting the anti-blooming transistor to reset the photodiode and begin an integration time;
while the transfer transistor is disabled, asserting the control transistor to reset the floating diffusion region to a first voltage;
sampling the first voltage while the transfer transistor, anti-blooming transistor, and control transistors are disabled;
asserting the transfer transistor to end the integration time and transfer charge from the photodiode to the floating diffusion region;
after asserting the transfer transistor to end the integration time and transfer charge from the photodiode to the floating diffusion region, sampling a second voltage while the transfer transistor, anti-blooming transistor, and control transistors are disabled; and
with processing circuitry, subtracting the sample of the first voltage level from the sample of the first second level; and
capturing a second image using correlated double sampling while the imaging pixel operates in a logarithmic mode, wherein capturing the second image using correlated double sampling while the imaging pixel operates in the logarithmic mode comprises:
sampling a third voltage level at the floating diffusion region while the anti-blooming transistor is disabled, the transfer transistor is asserted, and the control transistor is asserted;
sampling a fourth voltage level at the floating diffusion region while the anti-blooming transistor is asserted, the transfer transistor is asserted, and the control transistor is asserted; and
with the processing circuitry, subtracting the sample of the fourth voltage level from the sample of the third voltage level.
11. The method defined in claim 10 , further comprising:
using the first and second captured images to form a high dynamic range image with flicker mitigation.