The ADC2 module’s positive and negative voltage references are controlled by the ADC Reference Selection (ADREF) register. The difference between the positive and negative reference voltages is referred to as the reference voltage VREF. Positive reference voltage sources are selected via the PREF<1:0> bits of the ADREF register. Typical positive voltage sources include:
The FVR module produces a fixed 1.024V, 2.048V, or 4.096V level, which can be internally routed as either the positive voltage reference for the ADC or as an input channel to the ADC. It is important to note that the FVR output cannot exceed device VDD, and VDD must be high enough to support the requested FVR voltage. In other words, the 4.096V FVR level is only available on ‘F’ rated devices operating at or above 4.5V, while the 2.048V FVR level is available on both ‘LF’ and ‘F’ rated devices operating at or above 2.5V. If selected as the voltage reference source, the FVR module must also be configured and enabled. See the specific device data sheet for more information on the FVR.
The VREF+ pin allows an external voltage to be used as the positive voltage reference. Positive voltages can range from 0V to device VDD.
Negative reference voltage sources are selected via the NREF bit of the ADREF register. Typical negative voltage sources include:
The VREF- pin allows an external voltage to be used as the negative voltage reference. Negative voltages can range from 0V to 1V.
It is important to note that the positive reference voltage should always be greater than the negative voltage reference by at least 1.8V. Since all ADC conversions are performed with respect to VREF, it is critical to ensure that the VREF sources are stable to avoid incorrect conversion results. Additionally, the VREF source impedance must be extremely low.