Differential inputs ADC:
Yes, the AD6644 has a differential input stage. Citing its datasheet, the maximum damage free input voltage is your analog supply voltage. Which doesn't mean that's the maximum usable voltage, i.e. a point that isn't already clipping. That would be, as per the datasheet, p.3 a 2.2V betweeen AIN and /AIN. 

To stress this further (emphasis by me):


  The AD6644 input voltage range is offset from ground by 2.4 V.
  Each analog input connects through a 500 Ω resistor to a 2.4 V
  bias voltage and to the input of a differential buffer (Figure 21).
  The resistor network on the input properly biases the followers
  for maximum linearity and range. Therefore, the analog source
  driving the AD6644 should be ac-coupled to the input pins.


In other words: you don't use a 65MS/s ADC to measure a DC voltage. You use it with capacitors in series to its inputs, and the ADC itself establishes the 2.4V bias. It doesn't really matter what absolute voltage offset your analog signal has – that offset doesn't pass through the capacitors – it's only important that the peak-to-peak voltage doesn't exceed 2.2V.