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This mode is configured for differential inputs. When using the internal reference (1.024V), the input range will be –Input +/- 1.024V. If –Input is connected to 2.048 volts the usable input range is 2.048 +/- 1.024V or 1.024 to 3.072V. For systems where both single ended and differential signals are scanned, connect the –Input to Vssa when scanning a single ended input. This mode does not use the rail-to-rail buffer mode, but it shouldn’t be required for most applications.
Most MCUs do not support neg. supplies. The value of differential input for ADCs is to cancel the common noice while measuring across resistance. Even though there is no neg. supplies, you can simply pull up the voltage or current level above the ground, and still use differential input mode.
There is no need for a negative supply because none of the voltages are negative. What is meant by the explanation is that the input range of the ADC that is digitized is from the reference voltage minus 1.024V to the reference voltage plus 1.024V. If the reference voltage is 1.024V, then the ADC input range is differential and output code is zero when the –Input is 1.024V and the +Input is 0.0V, the ADC output code is mid range when the –Input and +Input are equal to each other (which would generally be at 1.024V) and the ADC output code is full scale when the –Input is 0.0V and the +Input is 1.024V.
The key here is to remember that this is a differential input and the input voltage is the voltage at the +Input minus the voltage at the –Input. When the –Input voltage is more positive than is the +Input voltage, the differential voltage is negative. So, if the –Input is ate 1.124V and the +Input is at 0.924V, then the differential input is 0.924V – 1.124V = –0.200V. All voltages are positive relative to ground, but the differential input is negative.