27.5.2 ADC Voltage Reference

The reference voltage for the ADC (V_REF) indicates the conversion range for the ADC. Single ended channels that exceed V_REF will result in codes close to 0x3FF. V_REF can be selected as either AV_CC, internal 2.56V reference, or external AREF pin.

AV_CC is connected to the ADC through a passive switch. The internal 2.56V reference is generated from the internal bandgap reference (V_BG) through an internal amplifier. In either case, the external AREF pin is directly connected to the ADC, and the reference voltage can be made more immune to noise by connecting a capacitor between the AREF pin and ground. V_REF can also be measured at the AREF pin with a high impedance voltmeter. Note that V_REF is a high impedance source, and only a capacitive load should be connected in a system.

If the user has a fixed voltage source connected to the AREF pin, the user may not use the other reference voltage options in the application, as they will be shorted to the external voltage. If no external voltage is applied to the AREF pin, the user may switch between AV_CC and 2.56V as reference selection. The first ADC conversion result after switching reference voltage source may be inaccurate, and the user is advised to discard this result.