peak-to-peak voltage+signal
You can either restrict your digital range by using a lower ADC reference voltage, just digitize it and double the numeric result or a better method is to double the amplitude with a x2 analog amplifier then digitize it. The second method gives more resolution (accuracy) and costs very little more.
Exactly what circuitry you use depends on other factors, for example, what frequency is your signal, does it have a DC offset and does any part of the waveform pass below 0 volts.
Brian.