TMP35/TMP36/TMP37

Rev. F | Page 14 of 20

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 mV. 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.

TMP35

0.1µF

GND

OP193

0.1µF

24.9kΩ

R4

4.99kΩ

1.21MΩ

TYPE K

THERMO-

COUPLE

CU

CU

102Ω

0V TO 2.5V

R6

100kΩ

5%

R3

10MΩ

5%

COLD

JUNCTION

CHROMEL

ALUMEL

ISOTHERMAL

BLOCK

7

6

4

3

2

P1

50kΩ

–

+

NOTE:

Figure 31. Single-Supply, Type K Thermocouple Signal Conditioning Circuit with Cold-Junction Compensation