A design of a reference voltage source and its output buffer circuit for pipeline analog to digital Converter (ADC) which is based on Global Foundry 0.18um 1.8V CMOS mixed signal process is presented in this paper. An improved Brokaw structure is adopted in the key circuit of this current-mode bandgap reference (BGR) to provide a low voltage reference of 0.5V for the design of low supply voltage circuit. The proposed configuration circuit for this voltage reference source can achieve a variety of values by adjusting reference voltage output, and the output buffer circuit will transfer the output voltage into differential reference voltage source for the application of differential input signal in ADC. The simulation results of Spectre show that, the temperature coefficient is 9.3ppm/ o C in the range form -40 o C to 125 o C, Power Supply Rejection Ration (PSRR) is 107.3dB, the setting time of output buffer for multiplying digital to analog converter(MDAC) reference voltage and subADC reference voltage is 1.257ns and 1.549ns, separately.