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FUNCTIONAL BLOCK DIAGRAM

REV. B

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reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.

Thermocouple Conditioner and

Setpoint Controller

AD596*/AD597*

FEATURES
Low Cost
Operates with Type J (AD596) or Type K (AD597)

Thermocouples

Built-In Ice Point Compensation
Temperature Proportional Operation 10 mV/ C
Temperature Setpoint Operation ON/OFF
Programmable Switching Hysteresis
High Impedance Differential Input

GENERAL DESCRIPTION

The AD596/AD597 is a monolithic temperature setpoint con-
troller that has been optimized for use at elevated temperatures
such as those found in oven control applications. The device
cold junction compensates and amplifies a type J or K thermo-
couple input to derive an internal signal proportional to tem-
perature. The internal signal is then compared with an externally
applied setpoint voltage to yield a low impedance switched output
voltage. Dead-Band or switching hysteresis can be programmed
using a single external resistor. Alternately, the AD596/AD597 can
be configured to provide a voltage output (10 mV/

C) directly from

a type J or K thermocouple signal. It can also be used as a stand-
alone voltage output temperature sensor.

The AD596/AD597 can be powered with a single supply from
+5 V to +30 V, or dual supplies up to a total span of 36 V.
Typical quiescent supply current is 160

A, which minimizes

self-heating errors.

The AD596/AD597 H package option includes a thermocouple
failure alarm that indicates an open thermocouple lead when
operated in the temperature proportional measurement mode.
The alarm output has a flexible format which can be used to
drive relays, LEDs or TTL logic.

The device is packaged in a reliability qualified, cost effective
10-pin metal can or SOIC and is trimmed to operate over an
ambient temperature range from +25

C to +100

C. Operation

over an extended ambient temperature range is possible with
slightly reduced accuracy. The AD596 will amplify thermo-
couple signals covering the entire 200

C to +760

C tempera-

ture range recommended for type J thermocouples while the
AD597 can accommodate 200

C to +1250

C type K inputs.

The AD596/AD597 has a calibration accuracy of

C at an

ambient temperature of 60

C and an ambient temperature

stability specification of 0.05

C from +25

C to +100

C. If

higher accuracy, or a lower ambient operating temperature is
required, either the AD594 (J thermocouple) or AD595 (K
thermocouple) should be considered.

*Protected by U.S. Patent No. 4,029,974.

PRODUCT HIGHLIGHTS

1. The AD596/AD597 provides cold junction compensation

and a high gain amplifier which can be used as a setpoint
comparator.

2. The input stage of the AD596/AD597 is a high quality in-

strumentation amplifier that allows the thermocouple to float
over most of the supply voltage range.

3. Linearization not required for thermocouple temperatures

close to 175

C (+100

C to +540

C for AD596).

4. Cold junction compensation is optimized for ambient tem-

peratures ranging from +25

C to +100

5. In the stand-alone mode, the AD596/AD597 produces an

output voltage that indicates its own temperature.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700

World Wide Web Site: http://www.analog.com

Fax: 781/326-8703

© Analog Devices, Inc., 1998

TO-100

OUT

+ALM

+IN

V+

ICE

POINT
COMP

AD596/

AD597

ALM

IN

HYS

GND

SOIC

+IN

IN

TOP VIEW

(Not to Scale)

AD597

ICE POINT

COMP

HYS

V+

GND

OUT

AD596/AD597SPECIFICATIONS

(@ +60 C and V

= 10 V, Type J (AD596), Type K (AD597) Thermocouple,

unless otherwise noted)

REV. B

Model

AD596AH

AD597AH

AD597AR

Min

Typ

Max

Min

Typ

Max

Min

Typ

Max

Units

ABSOLUTE MAXIMUM RATINGS

to V

Volts

Common-Mode Input Voltage

0.15)

Volts

Differential Input Voltage

Volts

Alarm Voltages

+ALM

+36)

Volts

ALM

Volts

Operating Temperature Range

+125

Output Short Circuit to Common

Indefinite

TEMPERATURE MEASUREMENT

(Specified Temperature Range
+25

C to +100

Calibration Error

 4

Stability vs. Temperature

0.02

0.05

0.02

0.05

0.02

0.05

Gain Error

1.5

+1.5

1.5

+1.5

1.5

+1.5

Nominal Transfer Function

mV/

AMPLIFIER CHARACTERISTICS

Closed Loop Gain

180.6

245.5

V/V

Input Offset Voltage

53.21 + 235

41.27 37

Input Bias Current

0.1

Differential Input Range

+50

Common-Mode Range

0.15)

(+V

0.15)

(+V

0.15)

(+V

Volts

Common-Mode SensitivityRTO

mV/V

Power Supply SensitivityRTO

mV/V

Output Voltage Range

Dual Supplies

(V

+ 2.5)

(+V

 2)

(V

+ 2.5)

(+V

 2)

(V

+ 2.5)

(+V

 2)

Volts

Single Supply

(+V

 2)

(+V

 2)

(+V

 2)

Volts

Usable Output Current

3 dB Bandwidth

kHz

ALARM CHARACTERISTICS

Alarm Function Not Pinned Out

CE(SAT)

at 2 mA

0.3

Volts

Leakage Current

Operating Voltage at ALM

(+V

 4)

(+V

 4)

Volts

Short Circuit Current

POWER REQUIREMENTS

Operating

(+V

to V

)

(+V

to V

)

(+V

to V

)

Volts

Quiescent Current

160

300

160

300

160

300

100

200

100

200

100

200

NOTES

This is a measure of the deviation from ideal with a measuring thermocouple junction of 175

C and a chip temperature of 60

C. The ideal transfer function is given by:

AD596: V

OUT

= 180.57

+ (ambient in

53.21

C + 235

AD597: V

OUT

= 245.46

+ (ambient in

41.27

C 37

where V

, and V

represent the measuring and ambient temperatures and are taken from the appropriate J or K thermocouple table. The ideal transfer function minimizes the

error over the ambient temperature range of 25

C to 100

C with a thermocouple temperature of approximately 175

Defined as the slope of the line connecting the AD596/AD597 CJC errors measured at 25

C and 100

C ambient temperature.

Pin 6 shorted to Pin 7.

Current Sink Capability in single supply configuration is limited to current drawn to ground through a 50 k

resistor at output voltages below 2.5 V.

Alarm function available on H package option only.

Specifications subject to change without notice.
Specifications shown in boldface are tested on all production units at final electrical test. Results from those tests are used to calculate outgoing quality levels. All min and max
specifications are guaranteed, although only those shown in boldface

are tested on all production units.

ORDERING GUIDE

Model

Package Description

Package Options

AD596AH

TO-100

H-10A

AD597AH

TO-100

H-10A

AD597AR*

Plastic SOIC

SO-8

*Consult factory for availability.

AD596/AD597

REV. B

Table I. Output Voltage vs. Thermocouple Temperature (Ambient +60 C, V

= 5 V, +15 V)

Thermocouple

Type J

AD596

Type K

AD597

Temperature

Voltage

Output

Voltage

Output

200

7.890

1370

5.891

1446

180

7.402

1282

5.550

1362

160

6.821

1177

5.141

1262

140

6.159

1058

4.669

1146

120

5.426

925

4.138

1016

100

4.632

782

3.553

872

3.785

629

2.920

717

2.892

468

2.243

551

1.960

299

1.527

375

.995

125

.777

191

.501

.392

.507

146

.397

1.019

238

.798

196

1.277

285

1.000

245

1.536

332

1.203

295

2.058

426

1.611

395

2.585

521

2.022

496

3.115

617

2.436

598

4.186

810

3.266

802

100

5.268

1006

4.095

1005

120

6.359

1203

4.919

1207

140

7.457

1401

5.733

1407

160

8.560

1600

6.539

1605

180

9.667

1800

7.338

1801

200

10.777

2000

8.137

1997

220

11.887

2201

8.938

2194

240

12.998

2401

9.745

2392

260

14.108

2602

10.560

2592

280

15.217

2802

11.381

2794

300

16.325

3002

12.207

2996

320

17.432

3202

13.039

3201

340

18.537

3402

13.874

3406

360

19.640

3601

14.712

3611

380

20.743

3800

15.552

3817

400

21.846

3999

16.395

4024

420

22.949

4198

17.241

4232

440

24.054

4398

18.088

4440

460

25.161

4598

18.938

4649

480

26.272

4798

19.788

4857

Thermocouple

Type J

AD596

Type K

AD597

Temperature

Voltage

Output

Voltage

Output

500

27.388

5000

20.640

5066

520

28.511

5203

21.493

5276

540

29.642

5407

22.346

5485

560

30.782

5613

23.198

5694

580

31.933

5821

24.050

5903

600

33.096

6031

24.902

6112

620

34.273

6243

25.751

6321

640

35.464

6458

26.599

6529

660

36.671

6676

27.445

6737

680

37.893

6897

28.288

6944

700

39.130

7120

29.128

7150

720

40.382

7346

29.965

7355

740

41.647

7575

30.799

7560

750

42.283

7689

31.214

7662

760

31.629

7764

780

32.455

7966

800

33.277

8168

820

34.095

8369

840

34.909

8569

860

35.718

8767

880

36.524

8965

900

37.325

9162

920

38.122

9357

940

38.915

9552

960

39.703

9745

980

40.488

9938

1000

41.269

10130

1020

42.045

10320

1040

42.817

10510

1060

43.585

10698

1080

44.439

10908

1100

45.108

11072

1120

45.863

11258

1140

46.612

11441

1160

47.356

11624

1180

48.095

11805

1200

48.828

11985

1220

49.555

12164

1240

50.276

12341

1250

50.633

12428

AD596/AD597

REV. B

TEMPERATURE PROPORTIONAL OUTPUT MODE

The AD596/AD597 can be used to generate a temperature
proportional output of 10 mV/

C when operated with J and K

type thermocouples as shown in Figure 1. Thermocouples pro-
duce low level output voltages which are a function of both the
temperature being measured and the reference or cold junction
temperature. The AD596/AD597 compensates for the cold
junction temperature and amplifies the thermocouple signal to
produce a high level 10 mV/

C voltage output which is a func-

tion only of the temperature being measured. The temperature
stability of the part indicates the sensitivity of the output voltage
to changes in ambient or device temperatures. This is typically
0.02

C over the +25

C to +100

C recommended ambient

temperature range. The parts will operate over the extended
ambient temperature ranges from 55

C to +125

C, but ther-

mocouple nonlinearity at the reference junction will degrade the
temperature stability over this extended range. Table I is a list of
ideal AD596/AD597 output voltages as a function of Celsius
temperature for type J and K ANSI standard thermocouples
with package and reference junction at 60

C. As is normally the

case, these outputs are subject to calibration and temperature
sensitivity errors. These tables are derived using the ideal trans-
fer functions:

AD596 output = (Type J voltage + 301.5

180.57

AD597 output = (Type K voltage)

245.46

0.01 F

+15V

15V

100k

10k

100k

CONSTANTAN
(ALUMEL)

IRON
(CHROMEL)

OPTIONAL

OFFSET

ADJUST

0.01 F

OUT

+5V TO +30V

0 TO 25V

SPAN OF

+5V TO +30V

AD596/

AD597*

*H PACKAGE PINOUT SHOWN

Figure 1. Temperature Proportional Output Connection

The offsets and gains of these devices have been laser trimmed
to closely approximate thermocouple characteristics over mea-
surement temperature ranges centered around 175

C with the

AD596/AD597 at an ambient temperature between 25

C and

100

C. This eliminates the need for additional gain or offset

adjustments to make the output voltage read:

OUT

= 10 mV/

(thermocouple temperature in

C) (within

specified tolerances).

Excluding calibration errors, the above transfer function is accu-
rate to within 1

C from +80

C to +550

C for the AD596 and

C to +350

C for the AD597. The different temperature

ranges are due to the differences in J and K type thermocouple
curves.

European DIN FE-CuNi thermocouple vary slightly from ANSI
type J thermocouples. Table I does not apply when these types
of thermocouples are used. The transfer functions given previ-
ously and a thermocouple table should be used instead.

Figure 1 also shows an optional trimming network which can be
used to change the device's offset voltage. Injecting or sinking
200 nA from Pin 3 will offset the output approximately 10 mV
(1

C).

The AD596/AD597 can operate from a single supply from 5 V
to 36 V or from split supplies totalling 36 V or less as shown.
Since the output can only swing to within 2 V of the positive
supply, the usable measurement temperature range will be re-
stricted when positive supplies less than 15 V for the AD597
and 10 V for the AD596 are used. If the AD596/AD597 is to be
used to indicate negative Celsius temperatures, then a negative
supply is required.

Common-mode voltages on the thermocouple inputs must
remain within the common-mode voltage range of the AD596/
AD597, with a return path provided for the bias currents. If the
thermocouple is not remotely grounded, then the dotted line
connection shown in Figure 1 must be made to one of the ther-
mocouple inputs. If there is no return path for the bias currents,
the input stage will saturate, causing erroneous output voltages.

In this configuration, the AD596/AD597 H package option has
circuitry which detects the presence of an open thermocouple. If
the thermocouple loop becomes open, one or both of the inputs
to the device will be deprived of bias current causing the output
to saturate. It is this saturation which is detected internally and
used to activate the alarm circuitry. The output of this feature
has a flexible format which can be used to source or sink up to
20 mA of current. The collector (+ALM) should not be allowed
to become more positive than (V

+ 36 V), however, it may be

permitted to be more positive than +V

. The emitter voltage

(ALM) should be constrained such that it does not become
more positive than 4 V below +V

. If the alarm feature is not

used, this pin should be connected to Pins 4 or 5 as shown in
Figure 1. The alarm function is unavailable on the AR package
option.

AD596/AD597

REV. B

SETPOINT CONTROL MODE

The AD596/AD597 can be connected as a setpoint controller as
shown in Figure 2. The thermocouple voltage is cold junction
compensated, amplified, and compared to an external setpoint
voltage. The relationship between setpoint voltage and tempera-
ture is given in Table I. If the temperature to be controlled is
within the operating range (55

C to +125

C) of the device, it

can monitor its own temperature by shorting the inputs to
ground. The setpoint voltage with the thermocouple inputs
grounded is given by the expressions:

AD596 Setpoint Voltage =

9.6 mV/

C + 42 mV

AD597 Setpoint Voltage =

10.1 mV/

C 9.1 mV

The input impedance of the setpoint pin of the AD596/AD597
is approximately 50 k

. The temperature coefficient of this

resistance is

15 ppm/

C. Therefore, the 100 ppm/

C 5 k

pot

shown in Figure 2 will only introduce an additional

C degra-

dation of temperature stability over the +25

C to +100

C ambi-

ent temperature range.

AD596/

AD597*

CONSTANTAN
(ALUMEL)

IRON
(CHROMEL)

0.01 F

HYSTERESIS

(OPTIONAL)

TEMPERATURE

CONTROLLED

REGION

HEATER
DRIVER

TEMPERATURE

COMPARATOR

OUTPUT

SET-

POINT

VOLTAGE

REF

100ppm/ C

SET-
POINT
VOLTAGE

*H PACKAGE PINOUT SHOWN

Figure 2. Setpoint Control Mode

Switching hysteresis is often used in setpoint systems of this type
to provide noise immunity and increase system reliability. By
reducing the frequency of on-off cycling, mechanical component
wear is reduced leading to enhanced system reliability. This can
easily be implemented with a single external resistor between
Pins 7 and 3 of the AD596/AD597. Each 200 nA of current
injected into Pin 3 when the output switches will cause about
1

C of hysteresis; that is:

HYST

(

)

OUT

200 nA

HYST

In the setpoint configuration, the AD596/AD597 output is
saturated at all times, so the alarm transistor will be ON regard-
less of whether there is an open circuit or not. However, ALM
must be tied to a voltage below (+V

4 V) for proper operation

of the rest of the circuit.

STAND-ALONE TEMPERATURE TRANSDUCER

The AD596/AD597 may be configured as a stand-alone Celsius
thermometer as shown in Figure 3.

0.01 F

OUT

9.6mV/ C

0.01 F

ICE

POINT
COMP

AD596/

AD597*

*H PACKAGE PINOUT SHOWN

Figure 3. Stand-Alone Temperature Transducer
Temperature Proportional Output Connection

Simply omit the thermocouple and connect the inputs (Pins 1
and 2) to common. The output will now reflect the compensa-
tion voltage and hence will indicate the AD596/AD597 tem-
perature. In this three terminal, voltage output, temperature
sensing mode, the AD596/AD597 will operate over the full
extended 55

C to +125

C temperature range. The output

scaling will be 9.6 mV per

C with the AD596 and 10.1 mV per

C with the AD597. Additionally there will be a 42 mV offset

with the AD596 causing it to read slightly high when used in
this mode.

THERMOCOUPLE CONNECTIONS

The connection of the thermocouple wire and the normal wire
or printed circuit board traces going to the AD596/AD597
forms an effective reference junction as shown in Figure 4. This
junction must be kept at the same temperature as the AD596/
AD597 for the internal cold junction compensation to work
properly. Unless the AD596/AD597 is in a thermally stable
enclosure, the thermocouple leads should be brought in directly
to Pins 1 and 2.

REFERENCE JUNCTION

CONSTANTAN

(ALUMEL)

IRON

(CHROMEL)

NOTE:
A BIAS RETURN PATH
FROM PINS 1 AND 2
OF LESS THAN 1k
IMPEDANCE MUST BE
PROVIDED.

0.01 F

AD596/

AD597*

LIMITING RESISTOR

TO
LED

0.01 F

OUT

*H PACKAGE PINOUT SHOWN

GND

Figure 4. PCB Connections

To ensure secure bonding, the thermocouple wire should be
cleaned to remove oxidization prior to soldering. Noncorrosive
resin flux is effective with iron, constantan, chromel, and
alumel, and the following solders: 95% tin5% silver, or 90%
tin10% lead.

Part Number AD596