9.3.10.4 Reference Monitor

The user can select to continuously monitor the ADC reference inputs for a shorted or missing reference voltage. The reference detection circuit offers two thresholds, the first threshold is 300 mV and the second threshold is
1/3 · (AVDD – AVSS). The reference detection circuit measures the differential reference voltage and sets a flag latched after each conversion in the STATUS byte if the voltage is below the threshold. A reference voltage less than 300 mV can indicate a potential short on the reference inputs or, in case of a ratiometric RTD measurement, a broken wire between the RTD and the reference resistor. A reference voltage between 300 mV and
1/3 · (AVDD – AVSS) can indicate a broken sensor excitation wire in a 3-wire RTD setup.

Additionally, a resistor of 10 MΩ can be connected between the selected REFPx and REFNx inputs. The resistor can be used to detect a floating reference input. With a floating input, the resistor pulls both reference inputs to the same potential so that the reference detection circuit can detect this condition. The pull-together reference resistor is not recommended to be continuously connected to active reference inputs. This resistor lowers the input impedance of the reference inputs and can contribute gain error to the measurement.

The reference detection circuits must be enabled with the FL_REF_EN[1:0] bits of the reference control register (05h). The FL_REF_L0 flag (bit 0 of the STATUS byte) indicates if the reference voltage is lower than 0.3 V. The FL_REF_L1 flag (bit 1 of the STATUS byte) indicates if the reference voltage is lower than
1/3 · (AVDD – AVSS). A diagram of the reference detection circuit is shown in Figure 79. A reference monitor fault is latched at each conversion cycle and the flags in the status register are updated at the falling edge of DRDY.

Figure 79. Reference Monitor Block Diagram