You need to acondition your signal to the ADC input.
What you need to do is to condition the miminum and maximum values of the signal to be measured to the corresponding minimum and maximum signal that you can input on the ADC.
For example you want to measure a signal which the lower value is 50 and the maximum value is 100, and let's say your ADC range is from 0 to 1.2 volts you need to create a circuit that will transfer this values.
When your input singal is 50 the output of this circuit you need to create should be 0 and when the input signal is 100 the output of this signal should be 1.2
You need to consider all the details related to linearity, noise, temperature changes, etc.
I hope this can help you out.
If you are using a AC signal you can mount it on a DC component to shift the values to be measured on the ADC