hello..
what fpga and adc are you using?..
I tested my ADC implementation with 2 outputs in a FPGA and a voltage source.
I put in one pin, a serial output with the binary number given by the ADC in the other pin, I put a signal "valid" that show me when a data is valid from the ADC (I implemented a control unit for this goal), then a put a voltage in the source, I wrote the binary numbers looking the screen in a oscilloscope and I translated to a Voltage seeing the equation that was used by the ADC.
I hope this could help you.