A kind of 3-bit flash ADC core is presented, which adopts CMOS inverter as comparators and PLA encoding scheme, and is very suitable for standard digital CMOS process and easy to be integrated with digital circuits, such as CPU, memory, etc. The optimization of the 3-bit ADC core is given and the simulation shows that the proposed ADC core can achieve 2Gsps sampling rate while consuming only 0.56mW power.