Abstract: A high precision CMOS bandgap voltage reference source fabricated in TSMC 40 nm CMOS is presented in this paper. Error source of bandgap and the way to eliminate it are analyzed. By using chopper technology and notch filter, not only offset voltage is removed but also ripple wave caused by chopper technology is lessened. Based on simulation, compared to bandgap without chopper technology, 3σ inaccuracy of the bandgap with chopper technology increased by 7.7 times with a maximum 3σ inaccuracy of 0.54% and a temperature coefficient of 14×10^-6/℃.