An image sensor comprises an active pixel sensor (APS) array, a first analog-to-digital converter (ADC) and a ramp signal generator. The APS array includes a plurality of pixels arranged in a two-dimensional matrix, wherein the APS array generates a reset signal and an image signal for each selected column of the APS array. The first ADC includes a correlated double sampling (CDS) circuit array comprising CDS circuits that are arranged for each column of the APS array, wherein the first ADC generates a digital code from a signal corresponding to the difference between the reset signal and the image signal which are generated by the CDS circuit using a ramp signal. The ramp signal generator generates the ramp signal, wherein a second ADC receives a feedback of the generated ramp signal and generates a feedback reference code, and wherein the ramp signal generator calibrates the ramp signal based on a comparison using the feedback reference code.