Texas Instruments ADS1x48EVM, User Manual, Page: 13 / 48

3.5.1 Connecting a Thermocouple to J5 on the ADS1x48EVM
Connect an external thermocouple directly to the J5:1 and J5:2 inputs on the J5 terminal block, which represent
the TC+ and TC– nets, respectively. The differential filter for this differential input pair has a cutoff frequency of
1.93 kHz. Additionally, each input has a common-mode filter with a cutoff frequency of 38.63 kHz. TC+ and TC–
are connected through the filter resistors to two analog inputs (AIN4 and AIN5, respectively) on the ADS1x48.
Figure 3-7 shows the portion of the ADS1x48EVM schematic with J5 and the thermocouple input structure.
10.0M
R24
DNP
AVDD
10.0M
R23
DNP
4.12k
R16
4.12k
R13
0.01uF
C10 1000pF
C3
1000pF
C4
1
2
3
4
5
6
J5
REFOUT
REF5025
TC+
TC-
NTC+
NTC-
TC+
TC-
AIN4
AIN5
GND
GND
Figure 3-7. Thermocouple Input Structure on the ADS1x48EVM
Although thermocouples are self-powered, these components must be biased in order to avoid floating beyond
the input range of the PGA integrated into the ADC. The ADS1x48EVM offers multiple ways to bias the
thermocouple so that the output voltage can be successfully read by the ADC.
The first method uses pullup and pulldown resistors, which are labeled in Figure 3-7 as R24 and R23,
respectively. These resistors are do not place (DNP) and therefore do not come installed on the EVM. To use
the pullup and pulldown resistor method, install 1-MΩ to 10-MΩ resistors in the aforementioned locations. With
these resistors installed, the thermocouple output voltage is centered at AVDD / 2, which puts the output voltage
in the middle of the common-mode range of the PGA integrated into the ADS1x48. Additionally, using pullup and
pulldown resistors enables continuous sensor break detection. If one of the thermocouple wires breaks, AIN4
is pulled to AVDD and AIN5 is pulled to GND, resulting in a full-scale input that can be detected in a separate
firmware routine.
The second thermocouple-biasing method supported by the ADS1x48EVM uses one of the two reference
voltage output pins to bias TC–. For example,
Figure 3-8 shows that connecting an external jumper between the
TC– input (J5:2) and the REF5025 output (J5:3) biases the thermocouple voltage to 2.5 V.
REFOUT
REF5025
TC+
TC
±
Thermocouple
Exte rnal
Jumper
AIN4
AIN5
+2 .5 V
+2 .04 8 V
Figure 3-8. Using REF5025 (input J5:3) to Bias a Thermocouple
If AVDD = 5 V in the configuration shown in Figure 3-8 , the thermocouple output voltage is now centered in the
middle of common-mode range of the PGA integrated in the ADS1x48. If AVDD is instead chosen to be 3.3 V,
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SBAU378A – SEPTEMBER 2021 – REVISED JANUARY 2022
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ADS1x48EVM Evaluation Module 13
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