The CMOS image sensor has developed from passive pixel sensors and active pixel sensors to digital pixel sensors(DPS),which is the trend of image sensors.The advantage of DPS is that the imaging system can be integrated on a chip as imaging system-on-a-chip,decreasing the cost and power consumption,improving the imaging quality.The A/D conversion is the key component of the CMOS sensor.The authors analyze and compare the theories,performances and features of the chip level,column level and pixel level integrated ADC of the image sensor.They present the present and the future trend of the development about digital image sensor in the home and abroad.