If the ADC does not have any method for integrated VREF monitoring, Figure 2-3 shows how a dedicated set of analog inputs can be used to measure back the reference voltage using the ADC signal path.

The system in Figure 2-3 uses AIN0 and AIN1 to measure the RTD voltage and AIN2 and AIN3 are used to read back the voltage across R_REF. Just like in the periodic VREF monitoring case, a fault is detected if the measured voltage between AIN2 and AIN3 is close to 0 V. At this point, the host can take corrective action if necessary. Similar to the case described in Section 2.2, the challenge with this approach is that the VREF voltage cannot be monitored constantly. Instead, RTD measurements must be halted in order to switch over to the monitoring channel, increasing system latency and complexity compared to using an ADC with an integrated VREF monitor. Moreover, the ADC requires another reference voltage source when using separate analog inputs to measure back the VREF voltage, as the reference voltage source for the RTD is now the measurement channel.

Although increased latency resulting from internal multiplexer switching cannot be overcome, the system can be simplified as shown in Figure 2-3 by using an ADC with multifunction analog inputs. Using such an ADC eliminates the dedicated traces running from R_REF to the input multiplexer because REFx are shared with AINx. As an example, Figure 2-4 illustrates how REFP1 and REFN1 are shared with AIN0 and AIN3, respectively, in the ADS1220.

Table 2-2 summarizes the wire-break detection features integrated into the precision ΔΣ ADCs highlighted in Table 1-1.

For the remainder of this document, the continuous VREF monitor is used to detect RTD wire breaks because this method is the simplest to implement. However, using a periodic VREF monitor or dedicated analog inputs are still acceptable methods for determining wire breaks in all configurations.