Analog read different with external and usb power

Projects Programming
13

Only (according to the datasheet) for a high impedance source where the sample and hold circuit may not have time to settle.

Does the datasheet indicate how long that takes?

int vals[6];

for(byte b=0; b<6; b++)
{
   vals[b] = analogRead(b);
}

will switch analog pins pretty quickly. What characteristics of the circuit indicate that each reading will be valid, and which characteristics indicate that there would not be time for the sample and hold circuit to settle?

I genuinely want to know this so I don't (again) counsel people to read the pin twice and ignore the first value, when it may not be necessary.