Most Helpful Member
Inside the ADC there are input protection diodes. (to ground and to supply) There diodes can only take a small amount of current.
The diode on the outside limits the voltage to -0.7V. The internal diode limits the voltage to -0.3V. That puts 0.4 volts across the 1k resistor or 400uA.
The same story on the supply side.
Your scope has a input impedance of 1meg. Add a 10:1 probe with a 9meg resistor and the tip of the probe has a impedance of 10meg and a output of 1meg.