A How Integrated PGA Rail Detection Helps Identify Wire Breaks

Although wire-break detection can be implemented using IDACs and VREF monitoring as described throughout this document, some ADCs integrate additional features to make wire-break detection easier.

One such feature is PGA rail detection, which allows the user to detect if the PGA output (or input, in some cases) is operating too close to the supply rails. This feature can be used to identify wire breaks because AINP or AINN can be pulled to AVDD when the IDAC no longer has a path to ground, as shown in Figure A-1. This feature can be employed during a diagnostic measurement routine to more accurately determine which pin has broken.

The ADS124S08 is an example of an ADC that integrates PGA rail detection features. If the level of the ADS124S08 PGA output voltage exceeds AVDD – 0.15 V, or drops below AVSS + 0.15 V, the ADC sets a flag to indicate that the output has gone beyond the output range of the PGA. Each PGA output (OUTN and OUTP) can trigger an overvoltage or undervoltage flag, giving a total of four flags:

• FL_P_RAILP (bit 5 of the STATUS byte): V_OUTP has exceeded AVDD – 0.15 V
• FL_P_RAILN (bit 4 of the STATUS byte): V_OUTP dropped below AVSS + 0.15 V
• FL_N_RAILP (bit 3 of the STATUS byte): V_OUTN has exceeded AVDD – 0.15 V
• FL_N_RAILN (bit 2 of the STATUS byte): V_OUTN dropped below AVSS + 0.15 V

For more information about the PGA rail detection features in a specific ADC, see that ADC data sheet. Table A-1 can also be referenced for a quick comparison of available features integrated into the precision delta-sigma ADCs highlighted in Table 1-1.