JAJSEG5A January 2018 – October 2018 ADS112U04
To help detect a possible sensor malfunction, the device provides internal 10-µA, burn-out current sources. When enabled by setting the respective bit (BCS) in the configuration register, one current source provides current to the positive analog input (AINP) currently selected and the other current source sinks current from the selected negative analog input (AINN).
In case of an open circuit in the sensor, these burn-out current sources pull the positive input towards AVDD and the negative input towards AVSS, resulting in a full-scale reading. A full-scale reading can also indicate that the sensor is overloaded or that the reference voltage is absent. A near-zero reading can indicate a shorted sensor. The absolute value of the burn-out current sources typically varies by ±5% and the internal multiplexer adds a small series resistance. Therefore, distinguishing a shorted sensor condition from a normal reading can be difficult, especially if an RC filter is used at the inputs. In other words, even if the sensor is shorted, the voltage drop across the external filter resistance and the residual resistance of the multiplexer causes the output to read a value higher than zero.
Keep in mind that ADC readings of a functional sensor may be corrupted when the burn-out current sources are enabled. Disable the burn-out current sources when preforming the precision measurement, and only enable these sources to test for sensor fault conditions.