Datasheet

TMP35/TMP36/TMP37 Data Sheet
THERMOCOUPLE SIGNAL CONDITIONING WITH
COLD-JUNCTION COMPENSATION
The circuit in Figure 31 conditions the output of a Type K
thermocouple, while providing cold-junction compensation for
temperatures between 0°C and 250°C. The circuit operates from
a single 3.3 V to 5.5 V supply and is designed to produce an
output voltage transfer characteristic of 10 mV/°C.
A Type K thermocouple exhibits a Seebeck coefficient of
approximately 41 µV/°C; therefore, at the cold junction, the
TMP35, with a temperature coefficient of 10 mV/°C, is used
with R1 and R2 to introduce an opposing cold-junction temp-
erature coefficient of −41 µV/°C. This prevents the isothermal,
cold-junction connection between the PCB tracks of the circuit
and the wires of the thermocouple from introducing an error in
the measured temperature. This compensation works extremely
well for circuit ambient temperatures in the range of 20°C to 50°C.
Over a 250°C measurement temperature range, the thermocouple
produces an output voltage change of 10.151 m V. Because the
required output full-scale voltage of the circuit is 2.5 V, the gain
of the circuit is set to 246.3. Choosing R4 equal to 4.99 kΩ sets
R5 equal to 1.22 MΩ. Because the closest 1% value for R5 is
1.21 MΩ, a 50 kΩ potentiometer is used with R5 for fine trim of
the full-scale output voltage. Although the OP193 is a superior
single-supply, micropower operational amplifier, its output stage
is not rail-to-rail; therefore, the 0°C output voltage level is 0.1 V.
If this circuit is digitized by a single-supply ADC, the ADC
common should be adjusted to 0.1 V accordingly.
V
OUT
+V
S
TMP35
0.1µF
GND
OP193
0.1µF
R1
1
24.9kΩ
R4
4.99kΩ
R5
1
1.21MΩ
TYPE K
THERMO-
COUPLE
CU
CU
R2
1
102Ω
V
OUT
0V TO 2.5V
R6
100kΩ
5%
R3
10MΩ
5%
3.3V < +V
S
< 5.5V
COLD
JUNCTION
CHROMEL
ALUMEL
ISOTHERMAL
BLOCK
0°C ≤ T
A
≤ 250°C
7
6
4
3
2
P1
50kΩ
–
+
–
+
NOTE:
1
ALL RESISTORS 1% UNLESS OTHERWISE NOTED.
00337-029
Figure 31. Single-Supply, Type K Thermocouple Signal Conditioning Circuit with Cold-Junction Compensation
Rev. H | Page 14 of 19
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