The ADC pre-scaler controls the internal ADC clock that controls the conversion process. By default, the successive approximation circuitry requires an input clock frequency between 50kHz and 200kHz. If a lower resolution than 10 bits is needed, the input clock frequency to the ADC can be as high as 1000kHz to get a higher sample rate. The conversion process requires 13-14 ADC clock cycles and this sets an upper limit on the sampling frequency. The pre-scaler options are shown in table 2.