fat16lib:
MarkT,These tests are not appropriate for dynamic ADC performance.
Here is a good article ATmega ADC tutorial | Open Music Labs.
You just can't use the AVR Arduino at high sample rates with high impedance sources. If the ADC clock is 1MHz, the sample and hold time is on the order of a microsecond.
My point is to see the real-world degradation of performance (perhaps at several impedance levels), I'm sure the datasheet isn't far wrong, but it would be nice to see "typical" behaviour for faster operation at high impedance - explore the error against the 2 dimensions of speed and input impedance...