"converts the output of ADC 16-bit bipolar into a voltage value"

To do that, you'd need a voltage-output DAC (Digital-to-Analogue Converter)

"output to appear on Hyperterminal as 0 Volt, not 8000H"

So you don't actually want a Voltage output at all; you want a text message?

Depending on your resolution, you could just use a lookup-table.

Otherwise, you will need to scale the hex value from ADC to give you a numerical Volts value, then convert that number into its ASCII text representation, then send the ASCII text out of the serial port.
There is a current thread dealing with converting numbers to text.