38.3.5 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 analog-to-digital conversions. 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]) are set, respectively.

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 analog-to-digital conversions using small reference voltage rails.