Filtering an ADC input with an RC low-pass filter
Why does the ADC draw an input current?

Assume that ADC grabs a voltage sample of the input signal, with that voltage stored on a 10pF capacitor; with these 18 bit ADCs using the over-sampling method, assume 1,000 samples are used to provide a fine 18-bit value. And assume Vin = 3volts.

What do we know

Iinput = F * C * V = 1,000 samples/conversion * 10pF * 3v = 1e+3 * 1e-11 * 3

Iinput = 1e-8 * 3 = 0.1uA * 3 = 0.3uA input current for ONE CONVERSION PER SECOND.