Measure an external thermocouple by connecting the sensor directly to the J4:1 and J4:2 inputs on the J4 terminal block. The differential filter for this differential input pair has a cutoff frequency of 50 kHz. Additionally, each input has a common-mode filter with a cutoff frequency of 497.36 kHz. J4 is connected through the filter resistors to analog inputs AIN8 and AIN9 on the ADS1263. Figure 3-2 shows the portion of the ADS1263EVM schematic with J4 and the thermocouple input structure.

Figure 3-2 ADS1263EVM Thermocouple Input Structure

Although thermocouples are self-powered, these components must be biased to avoid floating beyond the input range of the PGA integrated into the ADC. Thermocouples are typically biased to AVDD / 2 because AVDD / 2 is also the middle of the PGA common-mode range. Designing a bias circuit at this voltage therefore enables the largest gain for any given thermocouple voltage. The ADS1263EVM offers multiple ways to bias the thermocouple so the output voltage can be successfully read by the ADS1263.

The first method uses pullup and pulldown resistors to center the thermocouple output voltage at AVDD / 2. Figure 3-2 shows that these resistors are designated R3 and R4 on the ADS1263EVM, but are not installed by default. Therefore, 1-MΩ to 10-MΩ resistors must be installed at R3 and R4 to use the pullup and pulldown resistor biasing scheme. One added benefit of using pullup and pulldown resistors is that these resistors enable continuous sensor break detection. If one of the thermocouple wires breaks, AIN8 is pulled up to AVDD and AIN9 is pulled to GND. These conditions cause the ADC to measure a full-scale input that is easily distinguishable from the normal, low-level thermocouple output voltage.

The second thermocouple biasing method that is supported by the ADS1263EVM uses the ADS1263 internal reference from REFOUT (J5:2). Figure 3-3 illustrates how to connect an external jumper between the AIN9 input (J4:2) and REFOUT output (J5:2) to bias the thermocouple input voltage to 2.5-V. One challenge with the configuration in Figure 3-3 is that this option does not offer continuous wire-break detection. Instead, perform wire-break detection using a separate diagnostic measurement or by installing pullup resistor R3.

Figure 3-3 Using REFOUT (input J5:2) to Bias a Thermocouple

To learn more about the different thermocouple biasing schemes, as well as how to measure these sensors with precision ADCs, see the A Basic Guide to Thermocouple Measurements application note. This application note also discusses the need for cold-junction compensation (CJC), which uses a thermistor in conjunction with a thermocouple to derive the measured temperature. This process is described in Section 3.1.2.3.