The methods that I used to measure the input impedance of the AD2/AD are as follows:
1. While powering the device, I used a FLUKE 289 (50 000 count meter) in ohm mode to measure the impedance between the digital channels and GND.
2. Applied a 3.1v from a battery to a Digital I/O pin and inserted the FLUKE in series between the batt and I/O pin in μA range to measure the current passing into the I/O pin while the AD is running as a Logic analyzer. I used a battery to eliminate the noise from the switching power supply, in order to get clean and accurate readings as much as possible. input impedance = Batt Voltage / input current
Also, measured the PTC (used to protect the FPGA pins) inside the the AD, to verify that its value remains relatively constant during operation. I got readings around 221Ω from the PTC, which is consistent with the datasheet.
3. Applied a low frequency of 50 Hz square wave (did sin wave as well) from the built in Wave Gen, with a series 100Ω resistor between the I/O pin and Wave Gen. input impedance = WavGen voltage / (voltage across the 100Ω /100)
for this measurement, I set the FLUKE to AC+DC mode. Also viewed the voltage across the 100Ω (which is a function of the input current) and the WavGen output on the AD scope to get an idea about the input currents when changing the frequency/amplitude of the input.
I hope this info is helpful to provide some answer.