Texas Instruments ADS1263EVM-PDK User Manual page 10

Analog Interface
3.1.2.3 Using Thermistor RT1 for Thermocouple Cold-Junction Compensation
The ADS1263EVM includes provisions for using a PCB-mounted thermistor to measure the cold junction of
a thermocouple connected to the AIN8 and AIN9 inputs on J4.
copper pours create an isothermal bridge between the AIN8 and AIN9 inputs. This bridge helps make sure
that thermistor RT1 measures the same temperature as the thermocouple cold junction at inputs J4:1 and J4:2,
enabling a more accurate thermocouple measurement.
Figure 3-6. PCB Layout for J4 Showing Copper Pours for CJC Measurements
As discussed in Section 3.1.2.2, thermistor RT1 is not populated by default on the ADS1263EVM. Choose a
10-kΩ NTC thermistor in a 0603 package for this component. Additionally, populate the 10-kΩ bias resistor (R34)
to create a valid resistor divider as well as the 0-Ω resistor (R36) to directly bias the sensor using the 2.5-V
reference output of the ADS1263. Populating the linearization resistor (R33) is optional.
Measure the thermistor voltage as described in
multiplexed ADC that can only measure one single-ended or differential input per conversion cycle. As a
result, the thermocouple and CJC measurements must be taken separately and then manipulated in software to
determine the resulting temperature. To learn more about CJC as well as how to use this value to determine the
true measured temperature, see the
3.1.2.4 Connecting an RTD to J3 on the ADS1263EVM
The ADS1263EVM provides the flexibility to measure multiple RTD types and configurations using differential
pairs AIN0 and AIN1, AIN2 and AIN3, or AIN6 and AIN7 through terminal block J3. Each of these differential
pairs has a differential filter with a cutoff frequency of 50 kHz. Additionally, each input has a common-mode
filter with a cutoff frequency of 497.36 kHz. See
connections.
The ADS1263EVM also includes a placeholder for a precision reference resistor (R1) in series with the RTD.
The ADS1263 integrated IDACs provide a constant current through both the RTD and R1 to establish a
ratiometric reference voltage between AIN4 and AIN5 on the EVM. Any noise or drift in the IDAC occurs equally
in the input and reference voltage, and therefore cancels out.
Choose resistor R1 to accommodate the RTD resistance across the entire temperature range. For example, a
Pt1000 at 850°C has a typical impedance of approximately 3.9-kΩ. A 0-Ω resistor must also be installed at R2 to
provide the IDAC a path to ground.
Choose the IDAC current magnitude and the total circuit impedance to make sure that the ADC IDAC
compliance voltage is met. For additional information regarding ratiometric RTD measurement circuits using
precision ADCs and for important specifications including compliance voltage, see the
Measurements application note.
The following sections detail how to connect a 2-wire, 3-wire, or 4-wire RTD to the ADS1263EVM.
10 ADS1263EVM-PDK Evaluation Module
Section 3.1.2.2. However, note that the ADS1263 is a
A Basic Guide to Thermocouple Measurements
Figure 3-1 for an overview of the ADS1263EVM analog input
Copyright © 2023 Texas Instruments Incorporated
Figure 3-6 shows how top- and bottom-layer
application note.
A Basic Guide to RTD
SBAU206B – APRIL 2015 – REVISED MAY 2023
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