"Note that even if the difference between two inputs could be negative, voltages below
GND or above VCC should under no circumstances be applied to any input pin."

“One common misunderstanding is that the ADC can measure negative voltages. The
fact is that the ADC can not measure voltage levels below ground level and not above
the voltage reference level. What is actually meant by having negative results is that
the voltage on the negative ADC input is greater than that on the positive ADC input.”