Input impedance of Arduino Uno analog pins?

I'm using the ACS712 hall-effect based current sensing chip on some small robotic arm motors, and reading in the analog voltage with an Arduino Uno. I've had reasonably good results, but only after putting in an RC filter on the output. However, in the application note on the datasheet, it says not to do that:

"the addition of an RC filter to the output of a sensor IC can result in undesirable device output attenuation — even for DC signals."

Then it gives a formula to calculate attenuation, but it depends on knowing the input impedance of whatever is reading the signal, so that's what I'm after here.