These devices require a reference voltage for operation. Both ADCs default to the same internal reference, however, the reference voltage of ADC1 is independent of the ADC2 reference voltage. The reference voltage is provided internally by the internal 2.5-V reference, or externally, by one of the three external reference inputs. The specified external reference voltage range is 0.9 V to 5 V. The reference voltage is defined as VREF = VREFP – VREFN, where VREFP and VREFN are the absolute positive and absolute negative reference voltages, respectively. The polarity of the reference voltage internal to the ADC must be always positive. The magnitude of the reference voltage together with the PGA gain establishes the ADC full-scale differential input range as defined by VIN = ±VREF / gain. Figure 9-12 shows the block diagram of the ADC1 reference multiplexer. Use the reference multiplexer to select the internal reference, one of three external reference inputs, or the analog power supply.
The ADC1 reference multiplexer consists of a positive multiplexer and a negative multiplexer. The positive and negative multiplexers are programmed by the RMUXP[2:0] and RMUXN[2:0] bits, respectively, of the REFMUX register. The positive reference input is either internal (2.5 V), external (pins AIN0, AIN2, AIN4), or the analog power-supply voltage (VAVDD). The negative reference input is either internal (2.5 V), external (pins AIN1, AIN3, AIN5), or the analog power-supply voltage (VAVSS). A reference polarity-reversal switch changes the reference polarity from negative to positive. The polarity switch allows either positive or negative external reference polarity. Set the reversal switch to the normal position (REFREV = 0) when using the internal reference or analog power supplies.
The ADC also contains and integrated low-reference voltage monitor. This monitor provides continuous detection of a low or missing reference during the conversion cycle. The low reference alarm is appended to the data output status byte (REF_ALM, bit 4 of the status byte).