OK, thank you for your replies, they helped me to find what was wrong.
There were two reasons for the problem.
First, Indeed, I should use the receiver XBee in API mode.I configured the API mode in X-CTU v6.2.0, in the Serial menu.
Second,I should not read the values of the receiver in ASCII because I receive a sequence containing much more data than the actual value of the ADC of the transmitter XBee. Instead, I should read the the data in Hex and look at the bits corrresponding to the data coming from the ADC.