The ADC incorporates a sensor bias current source that can be used to apply a small test current to diagnose broken sensor leads or problems existing in the sensor. Figure 9-3 shows the sensor bias block diagram. The sensor bias circuit consists of programmable current sources and bias resistors. The sensor bias circuit connects to the outputs of either the ADC1 or ADC2 multiplexers. Program the sensor bias to either pull-up or pull-down mode. In pull-up mode, the current flows into the positive input and flows out of the negative input. In pull-down mode, the polarities are reversed. Configure the sensor bias either to a 10-MΩ bias resistor, or to current with magnitudes of ±0.5, ±2, ±10, ±50, or ±200 µA.
In pull-up mode, an open sensor results in the positive input pulled to VAVDD, and the negative input pulled to VAVSS. An open sensor in pull-up mode results in a positive full-scale reading. A full-scale reading can also be an indication of sensor overload or that the reference voltage is lower than expected. The sensor bias can remain on while actively converting, or pulsed on periodically to test the sensor. When pulsed on, allow time for settling because external capacitance loads the sensor bias when first enabled. Be aware of offset error as a result of sensor bias current flowing through the multiplexer switch resistance.