CN-0287

Circuit Note

Rev. C | Page 2 of 9

Figure 1. 4-Channel Thermocouple and RTD Circuit (Simplified Schematic: All Connections and Decoupling Not Shown)

CIRCUIT DESCRIPTION

Temperature Measurement Introduction

Thermocouples and RTDs (resistance temperature detectors)

are the most frequently used sensors for temperature

measurement in industrial applications. Thermocouples are

able to measure very high temperatures up to about +2300°C

and also have a fast response time (measured in fractions of a

second). RTDs are capable of higher accuracy and stability than

thermocouples, and the resistance of long wire lengths

(hundreds of meters) to a remote RTD can be compensated for

with 3- or 4-wire connections.

A thermocouple consists of two wires of different metals joined

at one end. This end is placed at the temperature which is to be

measured, refered to as the measurement junction. The other

end is connected to a precision voltage measurement unit, and

this connection is referred to as the reference junction or alternately

the cold junction. The temperature difference between the

measurement junction and the cold junction generates a voltage

(known as the Seebeck effect voltage) that is related to the

difference between the temperatures of the two junctions. The

signal generated is typically from several microvolts to tens of

millivolt depending on the temperature difference.

For example, K-type thermocouples are capable of measuring

−200°C to +1350°C with an output range of approximately −10

mV to +60 mV. It is important for the signal chain to maintain

as high impedance and low leakage as possible to achieve the

highest accuracy for the voltage measurement. In order to

convert this voltage to an absolute temperature, the cold junction

temperature must be accurately known. Traditionally 1°C to 2°C

has been considered sufficient, although since the cold junction

measurement error contributes directly to the absolute

temperature error, a higher accuracy cold junction temperature

measurement is beneficial

An RTD is made from a pure material, such as platinum, nickel

or copper, that has a predictable change in resistance as the

temperature changes.The most widely used RTD is platinum

(Pt100 and Pt1000).

One method used to accurately measure the resistance is to

measure the voltage across the RTD generated by a constant

current source. Errors in the current source can be cancelled by

referring the measurement to the voltage generated across a

AD7193

REFIN2(–)

REFIN2(+)

AIN8

AIN7

AIN2

AIN1

1.69k

Ω

1.69k

Ω

1.69k

Ω

1.69k

Ω

1.69k

Ω

+5V

300

Ω

300

Ω

300

Ω

300

Ω

300

Ω

1.69k

Ω

300

Ω

1.69k

Ω

300

Ω

1.69k

Ω

300

Ω

1.69k

Ω

300

Ω

1nF

1nF

+5V

+5V

27nF

1nF

1nF

+5V

+5V

+5V

+5V

+5V

+5V

27nF

D

S

IN

ADG702

1kΩ

R3

C2

C2

C2

S1

S2

S7

S8

SCLK

D

DIN

SYNC

ADG738

DOUT

PWR-ON

PRESET

A

W

B

SHDN

CS

CLK

SDI

GND

AD5201

LOGIC

CONTROL

+5V

+4.096V

P2

P3

ADR3440

FORCE

SENSE

FORCE

SENSE

GND

+5V

REFIN1(+) REFIN1(0)

+4.096V

AD8603

ADT7310

SCLK

DOUT

DIN

CT

INT

SCLK

DOUT

DIN

CT

INT

COLD JUNCTION COMPENSATION

SCLK

GND

CS

ADT7310_CS

SCLK

DIN

ADG738_CS

SCLK

DOUT

DIN

CS

SCLK

DOUT

DIN

AD7193_CS

DGND

AGND

+5V

SCLK

DIN

+5V

+5V

V–

V+

0.1µF

10µF

+5V

AINCOM

AIN4

AIN3

AIN6

AIN5

JP1

1

2

3

1

2

3

1

2

3

4

1

2

3

4

JP4

+5V

THERMOCOUPLE:

RTD 2,3W

RTD 4W

CH 1

CH 4

4.02k

Ω

0.1%

10ppm

+5V

5.6V

ZENER DIODE

0

Ω

: ANALOG GROUND

: DIGITAL GROUND

ADuM1280

+5V

CT

INT

+5V

ADuM1280

ADT7310_CS

ADG738_CS

SCLK

DOUT

DIN

ADuM5401

AD7193_CS

+5V