Hi, Mel,
I suppose that you do not connect the ADC ananlog pin to a GND or VDD or a divider, you have the ADC analog pin float, which leads to the ADC sample indeterminate.
Pls connect the analog ADC pin to GND or VDD or to a divider, have a try.
If you still have issue, pls post your code so that we have a review.
Hope it can help you
BR
Xiangjun Rong