The design is presented of a kind of an Analog to Digital Converter (ADC) with configurable resolution based on single chip C8051F020. Its main feature was made use of the mostinternal resource of the single chip C8051F020, that there was no or a little need of external hardware, applying the method of successive approximation. The order of the ADC provided was higher than 12 bits, which was composed of a lowerresolution ADC and Digital to Analog Converter (DAC) inside the single chip. Its precision can be set and modified easily, and the circuit is easy to achieve with high conversion rate.