When measuring positive external channels or when the SYSMON measures internal sensors, the ADC operates in a unipolar mode. In this mode, the ADC negative input terminal (VN) must always be lower than the ADC positive input terminal (VP). In this mode, the voltage on VP measured with respect to VN must always be positive. The VN input should always be driven by an external analog signal. VN is typically connected to a local ground or common mode signal. The common mode signal on VN can vary from 0V to +0.25V (measured with respect to GND). Because the differential input range is from 0V to 1.0V (VP/VN), the maximum signal on VP is 1.25V. See the following figure.
Figure 1. Unipolar Input Signal Range