The purpose of the analog to digital is to quantize the input signal from the sample and
hold circuit to 2B discrete levels - where B is the number of bits of the analog to digital
converter (ADC). The input voltage can range from 0 to Vref (or -Vref to +Vref for a
bipolar ADC). What this means is that the voltage reference of the ADC is used to set the
range of conversion of the ADC.
For a monopolar ADC, a 0V input will cause the converter to output all zeros. If the
input to the ADC is equal to or larger than 
then the converter will output all
ones. For inputs between these two voltage levels, the ADC will output binary numbers
corresponding to the signal level. For a bipolar ADC, the minimum input is 
not 0V.