Adaptive gain for image sensors

An image sensor, comprising:

• a pixel array arranged in a plurality of rows and a plurality of columns;
• a ramp generator configured to generate a ramp signal;
• a plurality of column analog-to-digital converters (ADCs), wherein each of the plurality of column ADCs is coupled to a plurality of image sensor pixels included in one of the plurality of columns of the pixel array, wherein each of the plurality of column ADCs is configured to:
• apply a first gain to the ramp signal when one of the plurality of image sensor pixels is reset,
• generate a reset comparison signal by comparing the ramp signal to an output of the one of the plurality of image sensor pixels when the one of the plurality of image sensor pixels is reset,
• determine a digital reset value based on the reset comparison signal,
• apply a selected gain to the ramp signal when the one of the plurality of image sensor pixels is read out,
• generate a signal comparison signal by comparing the ramp signal to the output of the one of the plurality of image sensor pixels when the one of the plurality of image sensor pixels is read out, and
• determine a digital signal value based on the signal comparison signal and the digital reset value;
• and
• a controller configured to:
• select, based on the output of the one of the plurality of image sensor pixels when the one of the plurality of image sensor pixels is read out, the selected gain as the first gain or a second gain, and
• determine a pixel value for the one of the plurality of image sensor pixels by adding an offset between the first gain and the second gain to a product of the digital signal value and a ratio between the first gain and the second gain.

A method for reading out an image sensor, the method comprising:

• generating a ramp signal;
• applying a first gain to the ramp signal when an image sensor pixel of the image sensor is reset;
• generating a reset comparison signal by comparing the ramp signal to an output of the image sensor pixel when the image sensor pixel is reset;
• determining a digital reset value based on the reset comparison signal;
• selecting, based on the output of the image sensor pixel when the image sensor pixel is read out, a selected gain as the first gain or a second gain;
• applying the selected gain to the ramp signal when the image sensor pixel is read out;
• generating a signal comparison signal by comparing the ramp signal to the output of the image sensor pixel when the image sensor pixel is read out;
• determining a digital signal value based on the signal comparison signal and the digital reset value;
• and
• determining a pixel value for the image sensor pixel by adding an offset between the first gain and the second gain to a product of the digital signal value and a ratio between the first gain and the second gain.