Image sensor and readout circuit thereof

The invention provides an image sensor and a readout circuit thereof, the readout circuit comprises a ramp voltage circuit, a comparison circuit and a counter, the comparison circuit is used for comparing an input pixel signal with a ramp voltage signal; the ramp voltage circuit outputs a first ramp voltage signal at the initial moment of the exposure sampling time period of the pixel unit, and then adaptively selects and outputs a matched ramp voltage signal from the first ramp voltage signal to the nth ramp voltage signal according to the swing of the swing pulse signal. The amplitude of the output signal of the pixel unit is gradually reduced, the voltage range of the ramp voltage signal is gradually reduced and the resolution is gradually increased according to the design of the adaptive multi-sampling technology, and the sampling frequency can be increased by using the saved time so as to reduce random noise, improve the analog-to-digital conversion efficiency and improve the resolution of the ramp signa