Using ADC output in VHDL

Hi everyone !

I made an ADC and I would like to work with the output data.
The output data from the ADC is :
Data : std_logic_vector(13 downto 0)
My objective is to do something like :
Interesting_output = A x Data(n-1) + B x Data(n) + C x Data(n+1)
But I don't know how to use the output from my ADC and convert it into a 
number (for example 11111111111111 = 2 [Volts] and 00000000000000 = 0,4 
[Volts])

Can someone help me?