Recommended thing would be to grab datasheet for your ADC chip and see in chapter named "Data Format". In this chapter it will be explained how the numeric values (data) from the ADC is represented.
It seems that your digital representation is something named 2's complement. Here is the wiki link.
From values you have provided I assume it is 16-bit ADC. So, max value 0x7FFF is your +Vref (positive reference voltage) and 0x8000 is -Vref (negative reference voltage). 0xFFFF in 2's complementary equals to -1.
Let's say your positive reference voltage is 2.5V, and negative reference voltage is -2.5V, then when you see obtained value of ADC equal 0x7FFF it means you have 2.5V on the ADC input. To calculate other positive values you take Vref and divide it by numbers of bits value (0x7FFF equals to dec 32767), so if your ADC gives you 0x0001 is equivalent of
1 * (2.5[V]/32767) = 0.0000763[V]
BTW, this smallest step your ADC is capable to measure is called ADC resolution
Let's have about half range of positive ADC, so now your ADC gives you a number of 0x4000. This is 16384 dec, so calculation here is:
true_voltage_value = 16384 * (2.5[V]/32767) = 1.250099[V]
For negative numbers: if your ADC value is bigger or equal to 0x7FFF then your action may be: negative_measured_value = measured_value - (0xFFFF + 1)
So, when ADC provides you value of 0xFFFF it goes:
negative_measured_value = 0xFFFF - (0xFFFF +1) = -1
true_volt_value = -1 * (2.5[V]/32767) = -0.000076V
Huisman answers it as well.
Note: The values calculated here are theoretical, and how precise your measurement is depend from stability and accuracy of reference voltage(s).