ADC Converter problem


We have a ConnectCore 9P 9215 Digi EL 4.2 Jumpstart kit. We use internal add channel in our applications and we note that de A/D samples of a a simple DC signal varies so much. I have tried change the ADC reference voltage to external source without success.
How we change directly the ADC registers, to change clock configurations and other parameters??? You have others sugestions???

Do you help me???