AD7859 is a 12-bit ADC,which features high speed ,low power and 200ks/s smapling frequency.AD7859 contains a control regiser,ADC output data register,status register,test register and 10 calibration regisers. In this paper, the main function, principle of AD7859 are presented in detail. The circuit diagram and C51 source code of data acquisition are given in the end.