The Reference Voltage (VREF) is the standard voltage against which the Analog Input Voltage must be measured. VREF can be an input voltage provided through external pin. Some ADCs are capable of generating VREF for the ADC module from the Analog VCC of the MCU device. The range of VREF varies among different devices and the respective device datasheet must be referred to know the exact value. Typically, VREF can be selected by configuring the bit field of the corresponding register.