Texas Instruments ADS1x48EVM, User Manual, Page: 15 / 48

3.5.2 Connecting a Thermistor to J5 on the ADS1x48EVM
Unlike a thermocouple, thermistors are not self-powered and require a constant voltage or current source to
operate. Typically, constant voltage is preferred because the thermistor impedance can vary from hundreds of
ohms at low temperature to hundreds of thousands of ohms at high temperature (or vice versa for a thermistor
with a negative temperature coefficient). A resistor is then added in series with the thermistor to create a resistor
divider that can be measured by an ADC.
Connect an external thermistor directly to the J5:5 and J5:6 pins on the J5 terminal block, which represent the
NTC+ and NTC– 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. NTC+
and NTC– are connected through the filter resistors to two analog inputs (AIN6 and AIN7, respectively) on
the ADS1x48.
Figure 3-9 shows the portion of the ADS1x48EVM schematic with J5 and the thermistor input
structure.
0.01uF
C12 1000pF
C7
1000pF
C8
4.12k
R21
4.12k
R22
1
2
3
4
5
6
J5
10.0k
R17
REFOUT
REF5025
NTC+
NTC-
NTC+
NTC-
TC+
TC-
AIN6
AIN7
10.0k
R14
DNP
GND
GND
10k
t°
RT1
DNP
Figure 3-9. Thermistor Input Structure on the ADS1x48EVM
Figure 3-9 shows two DNP components: a thermistor (RT1) and a 10-kΩ linearization resistor (R14). RT1 can
be used for CJC for thermocouple measurements (see Section 3.5.2.1 ). R14 helps linearize the thermistor
output voltage over a smaller temperature range. See section 2.8.2 in the A Basic Guide to Thermocouple
Measurements application report to learn more about the benefits of using a linearization resistor when
measuring a thermistor. Finally, resistor R17 in Figure 3-9 is the bias resistor used in conjunction with the
external resistor to form a resistor divider. Resistor R17 is chosen to be 10 kΩ because 10 kΩ is a commonly-
used nominal thermistor impedance. Choosing both resistors to have the same nominal impedance balances the
resistor divider at 25°C.
As stated earlier in this section, thermistors are not self-powered and require a bias source to operate. As Figure
3-10 shows, connect the constant voltage output from REFOUT to NTC+ via an external jumper to bias the
sensor. Alternatively, NTC+ can be connected to the REF5025 output instead. In either case, ensure that the
reference output used to bias the resistor divider is the same reference selected for the ADC measurements.
Exte rnal
Jumper
R17
NTC
±
NTC+
REF5025
REFOUT
NTC
AIN6
AIN7
+2 .5 V
+2 .04 8 V
Figure 3-10. Connecting a Thermistor to the J5 Terminal Block on the ADS1x48EVM
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SBAU378A – SEPTEMBER 2021 – REVISED JANUARY 2022
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