You won't be able to calculate a specific input impedance for the ADC inputs; "high impedance" is about as close as you'll get.
The electrical characteristics of the ADC are listed in Table 6-26 on page 76 of the TMS320F28027 datasheet. ADC inputs have a typical parasitic input capacitance of 5 pF and typical leakage current of ±5 μA. Figure 6-20 on page 78 gives the equivalent circuit for an ADC input pin.