Thermocouple Signal Conditioning Using the AD594/AD595 - Application Note AN-369

Oct 12, 2026

Upload by ahduag

Thermocouple Signal Conditioning Using the AD594/AD595 explains how thermocouples generate voltage based on the temperature difference between measuring and reference junctions. It covers core thermocouple principles, the need for reference junction temperature, and practical alternatives to an ice-bath reference via cold junction compensation. The document then describes the AD594/AD595 signal conditioner architecture, including differential input amplification, summed compensation and gain-setting through feedback, and open-thermocouple fault detection with alarm output.

LanguageEnglish

CategoryTechnology

FormatPDF

Pages7

Reading Time18 min

AccessFree to download

Introduction

Back to Basics

Thermocouple principles and reference junction

Ice point reference and thermocouple types

Practical Thermocouple Measurement

Cold junction compensation

The AD594/AD595 Circuit Description

Functional block overview

Feedback balance and open-circuit detection

a AN-369 APPLICATION NOTE One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • 781/329-4700 • WorldWide WebSite: [http://www.analog.com](http://www.analog.com) Thermocouple Signal Conditioning Using the AD594/AD595 by Joe Marcin INTRODUCTION One of the most widely used devices for temperature measurement is the thermocouple. Whether in an industrial, commercial or scientific application, a thermocouple offers a cost effective solution to temperature measurements in many environments over wide temperature ranges. Unfortunately, their basic principles are often misunderstood resulting in serious measurement errors. This application note will review thermocouple fundamentals and illustrate circuit designs for thermocouple signal conditioning using the AD594/ AD595 monolithic IC. BACK TO BASICS The basic principles of the thermocouple were discovered in 1821 by Thomas Seebeck. When two dissimilar metals are joined at both ends and one end is heated, a current will flow. If the loop is broken at the center, an open circuit voltage (the Seebeck Voltage) is generated and is proportional to the difference in temperature between the two junctions. Therefore, in determining the temperature of the measuring junction, the reference junction temperature must be known. | MEASURING JUNCTION | T1

VT1 – VT2
T2
A
A VT1 VT2
B B | REFERENCE JUNCTION | | --- | --- | --- | Figure 1a. Thermocouple Loop An ice bath provides a well defined temperature of 0°C for the reference junction. This has become a standard reference point for the thermocouple output voltage vs. temperature tables for various metal combinations. | MEASURING JUNCTION | | | --- | --- | Figure 1b. Ice Point Reference These combinations have been characterized and classified by the National Institute of Standards and Technology (formerly the National Bureau of Standards) . The table below lists the types, composition and characteristics of the more commonly used thermocouples. Table I. Thermocouple Properties | ANSI
Code | Alloy
Combination | Maximum Temperature Range | mV
Output | | --- | --- | --- | --- | | B | Platinum/Rhodium | 0°C to +1700°C | 0 to +12.426 | | E | Chromel/Constantan | –200°C to +900°C | –8.824 to +68.783 | | J | Iron/Constantan | 0°C to +750°C | 0 to +42.283 | | K | Chromel/Alumel | –200°C to +1250°C | –5.973 to +50.633 | | N | Nicrosil/Nisil | –270°C to +1300°C | –4.345 to +47.502 | | R | Platinum/Rhodium
Platinum | 0°C to +1450°C | 0 to +16.741 | | S | Platinum/Rhodium
Platinum | 0°C to +1450°C | 0 to +14.973 | | T | Copper/Constantan | –200°C to +350°C | –5.602 to +17.816 | A voltmeter is commonly used to measure the Seebeck voltage; however, great care must be exercised when interconnecting it to the thermocouple. Referring to Figure 1c, two additional junctions, J2 and J3, are formed atthe connection between the thermocouple and meter. These two junctions produce opposing voltages within the thermocouple loop. Using an isothermal block at the point of connection keeps these junctions in thermal equilibrium and produces equal but opposite emfs. The measured voltage now is the difference in potential between the measuring junction and the isothermal block which serves as the reference junction. MEASURING JUNCTION T2 (VT2 = V BA = V + VB) VT1 – VT2 Figure 1c. Measuring a Thermocouple Voltage with a Voltmeter AN-369 PRACTICAL THERMOCOUPLE MEASUREMENT For most applications, it is impractical to use an ice bath for the reference junction. By compensating for the voltage developed at the reference junction, the ice point reference may be eliminated. This is performed by adding a voltage into the thermocouple loop, equal but opposite to that of the reference junction. A circuit that provides cold junction compensation along with amplification and open thermocouple detection is included in the AD594/AD595 family of thermocouple signal conditioning ICs. MEASURING JUNCT

FAQs

Why must the reference junction temperature be known when measuring a thermocouple output?

The thermocouple voltage (Seebeck voltage) is proportional to the temperature difference between the measuring junction and the reference junction. Without knowing the reference temperature, the measured voltage cannot be correctly converted to the measuring-junction temperature.

How does the AD594/AD595 approach remove the need for an ice bath?

It generates an ice-point-equivalent compensation voltage by using the IC’s local temperature as its reference. That compensation voltage is summed with the locally referenced loop voltage to produce a loop voltage equivalent to an ice-bath referenced measurement.

What happens in case of a broken thermocouple or open circuit at the input?

The differential inputs become unbalanced, the fault is detected, and the overload detection circuit drives a current-limited transistor suitable for interfacing an alarm.

Oct 12, 2026

Upload by ahduag