For most UNBUFFERED switched-capacitor input ADC's the input impedance is
Zin = 1/(Ch * Fs)
(-- for example see "Design Of Analog Integrated Circuits And Systems", Laker & Sansen, p. 661. )
Ch is switched capacitance = 5pf for the AD9200. Fs is sample rate.
Zin for the AD9200 running at 20Mhz fs is 10k ohms