ADW75001Z-0REEL7 View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

ADW75001Z-0REEL7

Low Voltage Temperature Sensors

Analog Devices 

TMP35/TMP36/TMP37

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.

3.3V < +VS < 5.5V

0.1µF

+VS

VOUT

TMP35

R3

10MΩ R4

5%

4.99kΩ

R51

1.21MΩ

P1

50kΩ

GND

TYPE K

THERMO-

COUPLE

CHROMEL

ALUMEL

COLD

JUNCTION

R11

24.9kΩ

CU

0.1µF

7

2

–

6

OP193

3

+

4

CU

NOTE:

1 ALL RESISTORS 1% UNLESS OTHERWISE NOTED.

VOUT

0V TO 2.5V

R6

100kΩ

5%

0°C ≤ TA ≤ 250°C

ISOTHERMAL

BLOCK

R21

102Ω

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

Rev. F | Page 14 of 20

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