Image sensor and analog-to-digital converter

A method for operating an image sensor, comprising:

• generating a pixel signal;
• generating a ramp voltage that changes at a first slope in a first section;
• generating a digital code by counting a clock in the first section;
• initializing, when a level of the ramp voltage does not reach a voltage level of the pixel signal during the first section, the digital code;
• generating a ramp voltage that changes at a second slope having a greater absolute value than the first slope in a second section;
• generating the digital code by counting the clock in the second section;
• and
• stopping, when the level of the ramp voltage reaches the voltage level of the pixel signal, the counting of the clock in response to the level of the ramp voltage and providing the digital code corresponding to the pixel signal.

An analog-to-digital converter, comprising:

• a ramp voltage generation circuit suitable for generating a ramp voltage that changes at a first slope in a first section and generating the ramp voltage that changes at a second slope having a greater absolute value than the first slope in a second section following the first section;
• an operation amplifier suitable for comparing a voltage to be converted with the ramp voltage in the first section and the second section;
• and
• a counter circuit suitable for generating a digital code corresponding to the voltage to be converted in response to an output of the operation amplifier,
• wherein the counter circuit generates the digital code by counting a clock until the output of the operation amplifier transitions in the first section, and
• wherein when the output of the operation amplifier does not transition in the first section, the counter circuit generates the digital code by counting the clock until the output of the operation amplifier transitions in the second section.

An image sensor comprising:

• a ramp voltage generation circuit suitable for sequentially generating first to Nth ramp voltages of an initial level during first to Nth time sections, respectively, an Mth ramp voltage having a greater ratio of decrease than a (M−1)th ramp voltage among the first to Nth ramp voltages, M being between 2 to N;
• an operation amplifier suitable for generating indication of when any ramp voltage first reaches a level of a pixel signal among the first to Nth ramp voltages;
• and
• a counter circuit suitable for generating an image signal corresponding to the pixel signal and having a resolution that depends on the indication.