Adaptive gain comparator for image sensors

An analog-to-digital converter (ADC) coupled with a pixel via a pixel output line, the ADC comprising:

• a first comparison stage, comprising:
• a first output;
• and
• an adaptive gain circuit coupled with a voltage ramp signal, the pixel output line, and the first output, wherein:
• the adaptive gain circuit comprises a high-gain transistor, a low-gain transistor, and a ramp attenuation circuit coupled with the voltage ramp signal and the high-gain transistor;
• the adaptive gain circuit is operable to perform a first comparison, using one of the low-gain transistor or high-gain transistor, of the voltage ramp signal with a pixel output value on the pixel output line;
• and
• the first comparison stage is operable to provide a first output signal on the first output based on the first comparison;
• a second comparison stage coupled with the first output, the second comparison stage comprising a second output, wherein the second comparison stage is operable to provide a second output signal on the second output based on a second comparison of the first output signal to a threshold voltage;
• and
• a counter controllable according to the second output.

A method for converting a pixel output signal of a pixel, comprising:

• receiving, by a first comparison stage of an analog-to-digital converter (ADC), the pixel output signal and a ramp voltage signal;
• selecting a gain level of the first comparison stage;
• performing a first comparison, by the first comparison stage and according to the selected gain level, of the pixel output signal to the ramp voltage signal;
• generating a first output signal based on the first comparison;
• performing a second comparison, by a second comparison stage of the ADC, of the first output signal to a threshold voltage level;
• generating, by the second comparison stage, a second output signal based on the second comparison;
• and
• controlling a counter based on the second output signal.