Analog Devices AN369 Application Notes

AN-369

a

APPLICATION NOTE

One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106 • 781/329-4700 • World Wide Web Site: 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.

V

– V

T1

T2

MEASURING

JUNCTION

T1 T2

A

V

T1

B

A

V

T2

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.

V

(V

= 0)

T1

T2

T2 = 08C

V

T2

MEASURING

JUNCTION

T1

A

V

T1

B

A

B

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 Alloy Temperature mV
Code Combination Range 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 0°C to +1450°C 0 to +16.741

Platinum

S Platinum/Rhodium 0°C to +1450°C 0 to +14.973

Platinum

T Copper/Constantan –200°C to +350°C –5.602 to +17.816

Maximum

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 at
the 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.

T2

V

A

V

B

B

(VT2 = VBA = V+VB)

Cu

Cu

– V

V

T1

T2

MEASURING

JUNCTION

T1

V

T1

Figure 1c. Measuring a Thermocouple Voltage with a
Voltmeter

Figure 1b. Ice Point Reference

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.

T2
A
B

Cu

V

COMP

Cu
Cu

VT1 – VT2 + V
(V

– VT2)

COMP

COMP

MEASURING

JUNCTION

T1

V

T1

Figure 1d. Cold Junction Compensation

THE AD594/AD595 CIRCUIT DESCRIPTION

Figure 2 is a block diagram of the AD594/AD595 thermo­couple signal conditioner IC. A Type J (for the AD594) or
Type K (for the AD595) thermocouple is connected to
Pins 1 and 14, the inputs to an instrumentation amplifier
differential stage. This input amplifier is contained in a
loop that uses the local temperature as its reference.
With the IC also at the local temperature, an ice point
compensation circuit develops a voltage equal to the de­ficiency in the locally referenced thermocouple loop.
This voltage is then applied to a second preamplifier
whose output is summed with the output of the input
amplifier. The resultant output is then applied to the in­put of a main output amplifier with feedback to set the
gain of the combined signals. The ice point compensa­tion voltage is scaled to equal the voltage that would be
produced by an ice bath referenced thermocouple mea­suring the IC temperature. This voltage is then summed
with the locally referenced loop voltage, the result being
a loop voltage with respect to an ice point.

–IN –ALM +ALM V+ COMP V

14 13 12 11 10 9 8

OVERLOAD

DETECT

AD594/
AD595

+A

FB

O

Through the feedback path, the main amplifier main­tains a balance at its inputs. In the event of a broken
thermocouple or open circuit at the device’s input, these
inputs become unbalanced, the fault is detected, and the
overload detection circuit drives a current limited n-p-n
transistor that may be interfaced as an alarm.

Although these ICs are specifically calibrated for a Type
J or K thermocouple, other thermocouple types may be
used with recalibration. Pin connections to internal
nodes for the temperature controlled voltages and feed­back are provided to perform recalibration.

INTERPRETING AD594/AD595 OUTPUT VOLTAGES

To produce a temperature proportional output of

10 mV/°C, and provide an accurate reference junction

over the rated operating temperature range, the AD594/
AD595 is gain trimmed at the factory to match the trans­fer characteristics of Type J and K thermocouples at

+25°C. At this calibration temperature, the Seebeck coef-

ficient, the rate of change of thermal voltage with
respect to temperature at a given temperature, is

51.70 µV/°C for a Type J thermocouple and 40.44 µV/°C

for a Type K. This corresponds to a gain of 193.4 for the

AD594 and 247.3 for the AD595 to realize a 10 mV/°C out-

put. Although the device is trimmed for a 250 mV output

at +25°C, an input offset error is induced in the output
amplifier resulting in offsets of 16 µV and 11 µV for the

AD594/AD595 respectively. To determine the actual out­put voltage from the AD594/AD595, the following equa­tions should be used:

AD594 Output

AD595 Output

= (

Type J Voltage

= (

Type K Voltage

+ 16 µV) × 193.4
+ 11 µV) × 247.3

where the Type J and K voltage are taken from the
thermocouple voltage tables referred to zero degrees
Celsius.

It is important to note that a thermocouple’s output is
linear over a narrow temperature range. Over a wide
temperature range, the Seebeck coefficient introduces
nonlinearity. Linearization is not provided by the AD594/
AD595, and any linearization techniques must be per­formed externally. This entails calculating thermo­couple temperature using high order polynomials. The
National Institute of Standards and Technology offers
tables of polynomial coefficients for a given thermo­couple type which may be used in this process.

G

1234567

+IN +C +T COM V––T –C

G

+TC

ICE
POINT
COMP.

–TC

Figure 2. AD594/AD595 Functional Block Diagram

–2–

Table II. Calculated Errors at Various Ambient Temperatures

AN-369

Ambient Temp. Rej. Total Temp. Rej. Total Temp. Rej. Total Temp. Rej. Total

AD594C AD594C AD594A AD594A AD595C AD595C AD595A AD595A

Temp. Error Error Error Error Error Error Error Error
ⴗC ⴗC ⴗC ⴗC ⴗC ⴗC ⴗC ⴗC ⴗC

–55 4.83 5.83 6.83 9.83 5.28 6.28 7.28 10.28
–25 1.98 2.98 3.23 6.23 2.04 3.04 3.29 6.29
0 0.62 1.62 1.25 4.25 0.62 1.62 1.25 4.25
+25 0.00 1.00 0.00 3.00 0.00 1.00 0.00 3.00
+50 0.62 1.62 1.25 4.25 0.62 1.62 1.25 4.25
+70 1.46 2.46 2.59 5.59 1.38 2.38 2.50 5.50
+85 2.25 3.25 3.75 6.75 1.99 2.99 3.49 6.49
+125 4.90 5.90 7.40 10.40 3.38 4.38 5.88 8.88

NOTE
Temp. Rej. Error has two components: (a) Difference between actual reference junction and ice point compensation voltage times the gain; (b) Offset and
gain TCs extrapolated from 0°C to +50°C limits. Total error is temp. rej. plus initial calibration error.

OPTIMIZING PERFORMANCE
Cold Junction Errors

Optimal performance from the AD594/AD595 is
achieved when the thermocouple cold junction and the
device are at thermal equilibrium. Avoid placing heat
generating devices or components near the AD594/
AD595 as this may produce cold junction related errors.
The ambient temperature range for the AD594/AD595 is

specified from 0°C to +50°C, and its cold junction com-

pensation voltage is matched to the best straight line fit
of the thermocouple’s output within this range. Opera­tion outside this range will result in additional error.
Table II shows the maximum calculated errors at various
ambient temperatures.

Circuit Board Layout

The circuit board layout shown in Figure 3 (with the op­tional calibration resistors) achieves thermal equilib­rium between the cold junction and the AD594/AD595.
The package temperature and circuit board are ther­mally contacted in the copper printed circuit board
tracks under Pins 1 and 14. The reference junction is now
composed of a copper-constantan (or copper-alumel)
connection and copper-iron (or copper-chromel) con­nection in thermal equilibrium with the IC.

(CHROMEL)

+C+T

IRON

+IN –IN
1

CONSTANTAN

(ALUMEL)

14

–ALM

+ALM

Soldering

Proper soldering techniques and surface preparation
are necessary to bond the thermocouple to the PC
tracks. Clean the thermocouple wire to remove oxida­tion before soldering. Noncorrosive rosin flux may be
used with the following solders: 95% tin-5% antimony,
95% tin-5% silver, or 90% tin-10% lead.

Bias Current Return

The input instrumentation amplifier of the AD594/AD595
requires a return path for its input bias current and may
not be left “floating.” If the thermocouple measuring
junction is electrically isolated, then Pin 1 of the IC
should be connected to Pin 4, the power supply com­mon. In some applications, tying the thermocouple di­rectly to common is not possible. A resistor from Pin 1 to
common will satisfy the bias current return path but will,
however, generate an additional input offset voltage
due to the 100 nA bias current flowing through it. If the
thermocouple must be grounded at the measuring junc­tion or if a small common mode potential is present, do
not make the connection between Pins 1 and 4.

Noise Suppression

When detecting a low level output voltage from a ther­mocouple, noise reduction is a prime concern. Whether
internally generated or induced by radiation from a
source, noise becomes one of the limiting factors of dy­namic range and resolution. Solving noise problems in­volves eliminating the source and/or shielding. The
latter is more effective when the source cannot be con­trolled or identified.

Noise may be injected into the AD594/AD595 input am­plifier when using a long length of thermocouple. To
determine if this noise path is the culprit, disconnect the

COMMON

AD594
AD595

–T – C V– V

Figure 3. PC Board Layout

OUT

87

COMP

V+

thermocouple from the AD594/AD595 and tie Pins 1 and
14 to Pin 4. The output voltage at Pin 9 of the AD594/
AD595 will now indicate ambient temperature (250 mV

at +25°C). If the noise at the output (Pin 9) disappears,

then shielding on the input is required. Shielded ther­mocouple wire with the shield connected to Pin 4 of the
IC will provide effective noise suppression. If the output

–3–

AN-369

still exhibits noise, it may be entering via the power sup­ply. Proper power supply bypassing and decoupling will
alleviate this condition.

Filtering the thermocouple input will attenuate the noise
before amplification. Figure 4 illustrates an effective in­put filter consisting of a resistor in series with Pin 1 and
a capacitor from this pin to ground. An offset voltage
will result due to the input bias current flowing through
the resistor. Since the input bias current for the inverting
input (Pin 14) varies with input voltage, any resistance in
series with this input would produce an input dependent
offset voltage. Therefore, it is highly recommended to con­nect this pin directly to common. In addition, the capacitor
across the input terminals increases the response time for
the alarm circuit in the event of a broken thermocouple.

Adding capacitance to the frequency compensation pin
(Pin 10) rolls off the bandwidth of the AD594/AD595 output
amplifier thus limiting noise. Without compensation, the

3 dB bandwidth is approximately 10 kHz. A 0.1 µF capacitor

connected between Pins 10 and 11 reduces the 3 dB point
to 120 Hz. This technique, however, is only useful if the
noise does not drive the input stage into saturation.

V

+TC

O

ICE

POINT

COMP.

FB

–TC

–IN –ALM +ALM V+ COMP

14 13

AD594/
AD595

G

1234567

+IN +C +T COM V––T –C

11 10 9 8

12

OVER-

LOAD

DETECT

+A

G

Figure 4. Input Filtering

TRIMMING CALIBRATION ERROR

The AD594/AD595, available in two performance grades,
is factory trimmed to achieve a maximum calibration er­ror of 1°C or 3°C depending on grade. For most applica­tions, this range of error is acceptable; however, by
adding the optional trim circuit shown in Figure 5, this
error may be nulled. A negative offset of approximately
3°C is injected into Pin 5. The trimming potentiometer
provides a balancing current into Pin 3 thus nulling any
calibration error.

+TC

V

OUT

ICE

POINT

COMP.

–TC

CONSTANTAN

(ALUMEL)

IRON

(CHROMEL)

+15V

14 13

AD594/
AD595

G

1234567

8MV 15MV

11 10 9 8

12

OVER-

LOAD

DETECT

+A

G

+T –T

R

CAL

100kV

+15V

Figure 5. Optional Calibration

OFFSETTING AND GAIN CHANGE

The AD594/AD595 is designed to produce a 0 V output at

0°C with a nominal gain of 10 mV/°C, but other ranges

are readily possible. The zero output temperature may
be changed by applying an offset voltage to Pin 8. The
magnitude of this voltage is calculated using the equa­tions for the AD594/AD595 output voltage for a given
thermocouple temperature. Gain changes are easily ac­commodated by adding series resistance to increase

gain or by paralleling the nominal 47 kΩ feedback resis-

tor for gain reduction. The following method illustrates
this principle.

1. Select a temperature range T1–T2.

2. Based on this range, determine an output sensitivity

(mV/°C) that limits the maximum output excursions

from (–V
to (+V

+ 2.5) to (+VS – 2) for dual supplies or from 0

S

– 2) for single supply operation.

S

3. Calculate the average thermocouple sensitivity over
the selected temperature range: (VT1–VT2)/(T1–T2).

4. Divide the desired output sensitivity (mV/°C) by the
average thermocouple sensitivity (mV/°C). This yields

the new gain (G) for the AD594/AD595.

5. Measure the actual feedback resistance between Pins
8 and 5, R

.

FB

6. RIN = RFB/193.4 –1 where RFB is the measured feedback
resistance. NOTE: Use 247.3 for an AD595 instead of

193.4.

7. The new feedback resistance, R

–4–

= (G × 1)(R

EXT

).

IN

AN-369

14 13

12

11 10 9 8

1234567

AD594/
AD595

G

+A

OVER-

LOAD

DETECT

ICE

POINT

COMP.

–TC

+TC

G

–V

S

(0V TO –15V)

CONSTANTAN

(ALUMEL)

IRON

(CHROMEL)

(OPTIONAL)

R

COMP

OSC/

DRIVER

AD654

C

T

R1

R2

CR1

+V

LOGIC

R

PU

F

OUT

+V

S

(+5V TO –VS + 30)

F

OUT

=

V

IN

(10V) (R1 + R2) C

T

+5V

COMMON

IN704A

4.1V

1kV

464V

TEMPERATURE
OFFSET VOLTAGE

FDBK

ICE
POINT
COMP.

+TC

–TC

–5V

V

R

EXT

CONSTANTAN

(ALUMEL)

IRON

(CHROMEL)

+5V

182V

14 13

AD594/
AD595

G

1234567

11 10 9 8

12

OVER-

LOAD

DETECT

+A

G

R

Figure 6. Offsetting and Gain Change

CURRENT MODE TRANSMISSION

In many applications, the AD594/AD595 may be located
in a noisy, remote location with its output driving a long
length of cable. Under these demanding conditions, cur­rent transmission offers better noise immunity and
eliminates errors due to cable resistance. The circuit
shown in Figure 7 converts the AD594/AD595 output to a
current and then converts it back to a voltage at the con­trol point. The feedback voltage at Pin 9 forces the volt­age across R
With the values shown for R

to equal the thermocouple voltage.

SENSE

, this produces a cur-

SENSE

rent output scale factor of 10 µA/°C. Note that the AD594/

AD595 quiescent current flows through the sense resis­tor, thus limiting the minimum measured temperature

to 16°C. The AD711 op amp converts this current back to
a nominal 10 mV/°C at the control point. Total error is

based upon the AD594/AD595 calibration error and the

match between the sense resistor and the 1 kΩ current

to voltage conversion resistor at the control point.

OUT

56kV

ICE

POINT

COMP.

–10mA/8C

2N2222

(OPTIONAL GAIN TRIM)

–TC

–15V

100kV

1kV

AD711

V

OUT

10mV/8C

CONSTANTAN

(ALUMEL)

14 13

1234567

IRON

(CHROMEL)

12

AD594/
AD595

G

R

SETUP

5.11V (4.02V)

11 10 9 8

OVER-

LOAD

DETECT

+A

G

+TC

Figure 7. Current-Mode Transmission

TEMPERATURE-TO-FREQUENCY CONVERSION

A digital output format may be produced by converting
the AD594/AD595 voltage output to a frequency. This
format not only affords noise immunity over long trans­mission paths but also provides information which may
be directly interfaced to a computer. A low cost
voltage-to-frequency converter, the AD654, converts the

10 mV/°C voltage output to a TTL compatible square

wave output. As shown in Figure 8, the entire system is
powered from a single 5 V supply and provides tem-

perature measurements from 0°C to +300°C. Higher

thermocouple temperatures will require a higher power
supply voltage to maintain a maximum AD594/AD595
output swing of 2.5 V below the supply. The AD594/
AD595 output voltage is connected to the AD654 input
through a series resistor to produce a 0 mA to 1 mA
full-scale current. Capacitor C

determines the full-scale

T

output frequency with a maximum usable frequency of
500 kHz resulting in 0.4% nonlinearity. Other tempera­ture ranges and output frequencies are achievable. Re­fer to the AD654 data sheet for additional information.

=

Figure 8. Temperature-to-Frequency Conversion

–5–

AN-369

FAHRENHEIT OUTPUT

The AD594/AD595 may be configured to produce a volt­age proportional to the temperature on a Fahrenheit
scale. Conversion of temperature from a Celsius to Fahr­enheit scale involves multiplying degrees Celsius by 9/5
and adding a 32 degree offset. The offset is produced by

injecting a 200 nA/°C current into Pin 3 while increasing

the feedback resistor to accommodate the gain of 9/5.
Output calibration is as follows:

1. With the thermocouple disconnected, apply a 10 mV
p-p, 100 Hz ac signal to Pins 1 and 14.

2. Adjust R

for a p-p output at Pin 9 of 3.481 V

GAIN

(AD594) or 4.451 V (AD595).

3. With the thermocouple connected and measuring

0°C, adjust R

until the output at Pin 9 reads

OFFSET

320 mV.

The ideal transfer function based on a Fahrenheit output
is:

AD594 Output = (Type J Voltage + 919 µV) × 348.12
AD595 Output = (Type K Voltage + 719 µV) × 445.14

This yields a higher output voltage swing over the useful
range of the thermocouple therefore, requiring a higher
power supply voltage to maintain a maximum output
voltage 2.5 V below the supply.

R

R

GAIN

2kV

9.1kV

OFFSET

2kV

5kV

598kV

CONSTANTAN

(ALUMEL)

IRON

(CHROMEL)

+20V

14 13

11 10 9 8

12

OVER-

LOAD

DETECT

AD594/
AD595

G

1234567

G

10mV/8F

+A

+TC

AD680

ICE

POINT

COMP.

7.5kV

–TC

–5V

Figure 9. Fahrenheit Output

AVERAGE TEMPERATURE

By connecting a number of thermocouples in parallel to
the AD594/AD595 input, an average junction temperature

will be measured. As shown in Figure 10, a 300 Ω resistor

is placed in series with one side of each thermocouple to
limit the current circulating between the thermocouple
branches. Based on a thermocouple temperature that is
either higher or lower than the mean, a positive or nega­tive voltage drop will be developed.

V

AVE

CONSTANTAN

(ALUMEL)

Cu

(R/N)V

14 13

+5V

11 10 9 8

12

OVER-

LOAD

DETECT

AD594/

e

3

N

2

IRON

(CHROMEL)

1

R

Cu

300V

R

Cu

300V

R

Cu

300V

Cu

R

300V

e

e

e

AD595

G

1234567

ISOTHERMAL

REGION

G

(T1, T2, T3 ... T

+A

+TC

OUT

ICE
POINT
COMP.

–TC

)

N

Figure 10. Measuring Average Temperature

–6–

AN-369

MULTIPLEXED THERMOCOUPLES

Multiple thermocouples may be connected to a single
AD595/AD595 via an external CMOS analog multiplexer
such as the ADG507A. For proper operation, all intercon­nects between the thermocouples, multiplexer and
AD594/AD595 inputs are copper and are held in thermal
equilibrium by an isothermal block. As shown in Figure
11, a thermocouple is mounted to measure the IC tem­perature as well as to cancel the reference junction volt­age at the isothermal block. With the multiplexer
enabled, the Constantan (Alumel)—Copper junction
formed by the thermocouple connection at the isother­mal block is in series with a Copper—Constantan
(Alumel) junction formed by the reference thermo­couple connection.

This series combination contributes equal but oppo­site voltages since the block is isothermal. Under this
condition, the AD594/AD595 internal cold junction

Cu
Cu

V

2

+5V

19
20
21
22
23
24
25
26
11
10

9
8
7
6
5
4

18

EN

ADG507A

A

CONSTANTAN

(ALUMEL)

IRON

(CHROMEL)

V

1

ISOTHERMAL

BLOCK

Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu

CONSTANTAN

(ALUMEL)

IRON

(CHROMEL)

28

2

1
27
12

0A1A2

151617

compensation now compensates for the reference junc-

tion at the IC which must remain between 0°C and
+50°C. Note however, that the isothermal block may be

at any convenient temperature or location. Unused mul­tiplexer inputs should be connected to common to mini­mize stray signal pickup. To prevent the AD594/AD595
inputs from “floating” resulting in output saturation, the
multiplexer is permanently enabled by connecting its
enable input to +5 V.

REFERENCES

1. Sheingold, Dan, ed.

Transducer Interface Handbook,

Analog Devices, 1980.

2.

1992 Amplifier Applications Guide,

Analog Devices,

Pub. No. G1646–10–4/92.

3. American Society for Testing and Materials,

Manual
On The Use Of Thermocouples In Temperature Mea­surement,

+15V

–15V

ASTM PCN 04-470020-40.

REFERENCE

JUNCTION

+15V

13

14

AD594/
AD595

G

1234567

11 10 9 8

12

OVER-

LOAD

DETECT

+A

G

ISOTHERMAL

REGION

+TC

V

OUT

ICE

POINT

COMP.

–15V

–TC

E1796a–0–7/98

Figure 11. Multiplexed Inputs

–7–

PRINTED IN U.S.A.