36.5.5 Selecting the Voltage Reference Source

The user application can select the voltage reference for the ADC module, which can be internal or external. The Voltage Reference Input Selection bits, VREFSEL[2:0] (ADCCON3[15:13]), select the voltage reference for ADC. The upper voltage reference (V_REFH) and the lower voltage reference (V_REFL) may be the internal AV_DD and AV_SS voltage rails or the band gap reference generator or the external V_REFH+ and V_REF- input pins. When the voltage reference and band gap reference are ready, the BGVRRDY (ADCCON2[31]) bit is set. If a fault occurs in the voltage reference (such as a brown-out), the REFFLT bit (ADCCON2[30]) is set. The BGVRRDY and REFFLT bits can also generate interrupts if the BGVRIEN bit (ADCCON2[15]) and REFFLTIEN bit (ADCCON2[14]), respectively, are set.

The voltages applied to the external reference pins must comply with certain specifications. See Electrical Characteristics from Related Links.

The Analog Input Charge Pump Enable bit, AICPMPEN (ADCCON1[12]), must be set when the difference between the selected reference voltages (V_REFH - V_REFL) is less than 0.65 * (AV_DD - AV_SS). Setting this bit does not increase the magnitude of the reference voltage; however, setting this bit reduces the series source resistance to the sampling capacitors. This maximizes the SNR for ADC using small reference voltage rails.