FUNCTIONAL BLOCK DIAGRAM

OVERLOAD
DETECT

ICE

POINT

COMP.

TC

+TC

AD594/AD595

+IN

COM

IN

ALM

+ALM

V+

COMP

Monolithic Thermocouple Amplifiers

with Cold Junction Compensation

AD594/AD595

FEATURES
Pretrimmed for Type J (AD594) or

Type K (AD595) Thermocouples

Can Be Used with Type T Thermocouple Inputs
Low Impedance Voltage Output: 10 mV/ C
Built-In Ice Point Compensation
Wide Power Supply Range: +5 V to 15 V
Low Power: <1 mW typical
Thermocouple Failure Alarm
Laser Wafer Trimmed to 1 C Calibration Accuracy
Setpoint Mode Operation
Self-Contained Celsius Thermometer Operation
High Impedance Differential Input
Side-Brazed DIP or Low Cost Cerdip

PRODUCT DESCRIPTION

The AD594/AD595 is a complete instrumentation amplifier and
thermocouple cold junction compensator on a monolithic chip.
It combines an ice point reference with a precalibrated amplifier
to produce a high level (10 mV/

C) output directly from a ther-

mocouple signal. Pin-strapping options allow it to be used as a
linear amplifier-compensator or as a switched output setpoint
controller using either fixed or remote setpoint control. It can
be used to amplify its compensation voltage directly, thereby
converting it to a stand-alone Celsius transducer with a low
impedance voltage output.

The AD594/AD595 includes a thermocouple failure alarm that
indicates if one or both thermocouple leads become open. The
alarm output has a flexible format which includes TTL drive
capability.

The AD594/AD595 can be powered from a single ended supply
(including +5 V) and by including a negative supply, tempera-
tures below 0

C can be measured. To minimize self-heating, an

unloaded AD594/AD595 will typically operate with a total sup-
ply current 160

A, but is also capable of delivering in excess of

5 mA to a load.

The AD594 is precalibrated by laser wafer trimming to match
the characteristic of type J (iron-constantan) thermocouples and
the AD595 is laser trimmed for type K (chromel-alumel) inputs.
The temperature transducer voltages and gain control resistors

are available at the package pins so that the circuit can be
recalibrated for the thermocouple types by the addition of two
or three resistors. These terminals also allow more precise cali-
bration for both thermocouple and thermometer applications.

The AD594/AD595 is available in two performance grades. The
C and the A versions have calibration accuracies of

C and

C, respectively. Both are designed to be used from 0

C to

+50

C, and are available in 14-pin, hermetically sealed, side-

brazed ceramic DIPs as well as low cost cerdip packages.

PRODUCT HIGHLIGHTS

1. The AD594/AD595 provides cold junction compensation,

amplification, and an output buffer in a single IC package.

2. Compensation, zero, and scale factor are all precalibrated by

laser wafer trimming (LWT) of each IC chip.

3. Flexible pinout provides for operation as a setpoint control-

ler or a stand-alone temperature transducer calibrated in
degrees Celsius.

4. Operation at remote application sites is facilitated by low

quiescent current and a wide supply voltage range +5 V to
dual supplies spanning 30 V.

5. Differential input rejects common-mode noise voltage on the

thermocouple leads.

REV. C

Information furnished by Analog Devices is believed to be accurate and
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.

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

Š Analog Devices, Inc., 1999

AD594/AD595SPECIFICATIONS

REV. C

Model

AD594A

AD594C

AD595A

AD595C

Min

Typ

Max

Min

Typ

Max

Min

Typ

Max

Min

Typ

Max

Units

ABSOLUTE MAXIMUM RATING

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
0

C to +50

Calibration Error at +25

Stability vs. Temperature

0.05

0.025

0.05

0.025

Gain Error

1.5

0.75

1.5

0.75

Nominal Transfer Function

mV/

AMPLIFIER CHARACTERISTICS

Closed Loop Gain

193.4

247.3

Input Offset Voltage

(Temperature in

51.70

40.44

Input Bias Current

0.1

Differential Input Range

+50

Common-Mode Range

 0.15

 4

 0.15

 4

 0.15

 4

 0.15

 4

Volts

Common-Mode Sensitivity RTO

mV/V

Power Supply Sensitivity RTO

mV/V

Output Voltage Range

Dual Supply

+ 2.5

 2

+ 2.5

 2

+ 2.5

 2

+ 2.5

 2

Volts

Single Supply

+ 2

Volts

Usable Output Current

3 dB Bandwidth

kHz

ALARM CHARACTERISTICS

CE(SAT)

at 2 mA

0.3

Volts

Leakage Current

A max

Operating Voltage at ALM

 4

Volts

Short Circuit Current

POWER REQUIREMENTS

Specified Performance

= 5, V

= 0

= 5, V

= 0

= 5, V

= 0

= 5, V

= 0

Volts

Operating

to V

Volts

Quiescent Current (No Load)

160

300

160

300

160

300

160

300

100

PACKAGE OPTION

TO-116 (D-14)

AD594AD

AD594CD

AD595AD

AD595CD

Cerdip (Q-14)

AD594AQ

AD594CQ

AD595AQ

AD595CQ

NOTES

Calibrated for minimum error at +25

C using a thermocouple sensitivity of 51.7

C. Since a J type thermocouple deviates from this straight line approximation, the AD594 will normally

read 3.1 mV when the measuring junction is at 0

C. The AD595 will similarly read 2.7 mV at 0

Defined as the slope of the line connecting the AD594/AD595 errors measured at 0

C and 50

C ambient temperature.

Pin 8 shorted to Pin 9.

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.

must not exceed 16.5 V.

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.
Specifications subject to change without notic

(@ +25 C and V

= 5 V, Type J (AD594), Type K (AD595) Thermocouple,

unless otherwise noted)

INTERPRETING AD594/AD595 OUTPUT VOLTAGES

To achieve a temperature proportional output of 10 mV/

C and

accurately compensate for the reference junction over the rated
operating range of the circuit, the AD594/AD595 is gain trimmed
to match the transfer characteristic of J and K type thermocouples
at 25

C. For a type J output in this temperature range the TC is

51.70

C, while for a type K it is 40.44

C. The resulting

gain for the AD594 is 193.4 (10 mV/

C divided by 51.7

and for the AD595 is 247.3 (10 mV/

C divided by 40.44

C).

In addition, an absolute accuracy trim induces an input offset to
the output amplifier characteristic of 16

V for the AD594 and

V for the AD595. This offset arises because the AD594/

AD595 is trimmed for a 250 mV output while applying a 25

thermocouple input.

Because a thermocouple output voltage is nonlinear with respect
to temperature, and the AD594/AD595 linearly amplifies the

compensated signal, the following transfer functions should be
used to determine the actual output voltages:

AD594 output = (Type J Voltage + 16

193.4

AD595 output = (Type K Voltage + 11

247.3 or conversely:

Type J voltage = (AD594 output/193.4) 16

Type K voltage = (AD595 output/247.3) 11

Table I lists the ideal AD594/AD595 output voltages as a func-
tion of Celsius temperature for type J and K ANSI standard
thermocouples, with the package and reference junction at
25

C. As is normally the case, these outputs are subject to cali-

bration, gain and temperature sensitivity errors. Output values
for intermediate temperatures can be interpolated, or calculated
using the output equations and ANSI thermocouple voltage
tables referred to zero degrees Celsius. Due to a slight variation
in alloy content between ANSI type J and DIN F

-C

AD594/AD595

REV. C

Thermocouple

Type J

AD594

Type K

AD595

Temperature

Voltage

Output

Voltage

Output

200

7.890

1523

5.891

1454

180

7.402

1428

5.550

1370

160

6.821

1316

5.141

1269

140

6.159

1188

4.669

1152

120

5.426

1046

4.138

1021

100

4.632

893

3.553

876

3.785

729

2.920

719

2.892

556

2.243

552

1.960

376

1.527

375

.995

189

.777

189

.501

.392

3.1

2.7

.507

101

.397

101

1.019

200

.798

200

1.277

250

1.000

250

1.536

300

1.203

300

2.058

401

1.611

401

2.585

503

2.022

503

3.115

606

2.436

605

4.186

813

3.266

810

100

5.268

1022

4.095

1015

120

6.359

1233

4.919

1219

140

7.457

1445

5.733

1420

160

8.560

1659

6.539

1620

180

9.667

1873

7.338

1817

200

10.777

2087

8.137

2015

220

11.887

2302

8.938

2213

240

12.998

2517

9.745

2413

260

14.108

2732

10.560

2614

280

15.217

2946

11.381

2817

300

16.325

3160

12.207

3022

320

17.432

3374

13.039

3227

340

18.537

3588

13.874

3434

360

19.640

3801

14.712

3641

380

20.743

4015

15.552

3849

400

21.846

4228

16.395

4057

420

22.949

4441

17.241

4266

440

24.054

4655

18.088

4476

460

25.161

4869

18.938

4686

480

26.272

5084

19.788

4896

Thermocouple

Type J

AD594

Type K

AD595

Temperature

Voltage

Output

Voltage

Output

500

27.388

5300

20.640

5107

520

28.511

5517

21.493

5318

540

29.642

5736

22.346

5529

560

30.782

5956

23.198

5740

580

31.933

6179

24.050

5950

600

33.096

6404

24.902

6161

620

34.273

6632

25.751

6371

640

35.464

6862

26.599

6581

660

36.671

7095

27.445

6790

680

37.893

7332

28.288

6998

700

39.130

7571

29.128

7206

720

40.382

7813

29.965

7413

740

41.647

8058

30.799

7619

750

42.283

8181

31.214

7722

760

31.629

7825

780

32.455

8029

800

33.277

8232

820

34.095

8434

840

34.909

8636

860

35.718

8836

880

36.524

9035

900

37.325

9233

920

38.122

9430

940

38.915

9626

960

39.703

9821

980

40.488

10015

1000

41.269

10209

1020

42.045

10400

1040

42.817

10591

1060

43.585

10781

1080

44.439

10970

1100

45.108

11158

1120

45.863

11345

1140

46.612

11530

1160

47.356

11714

1180

48.095

11897

1200

48.828

12078

1220

49.555

12258

1240

50.276

12436

1250

50.633

12524

Table I. Output Voltage vs. Thermocouple Temperature (Ambient +25

C, V

= 5 V, +15 V)

thermocouples Table I should not be used in conjunction with
European standard thermocouples. Instead the transfer function
given previously and a DIN thermocouple table should be used.
ANSI type K and DIN N

-N

thermocouples are composed

OVERLOAD
DETECT

TC

+TC

AD594/
AD595

CONSTANTAN
(ALUMEL)

IRON
(CHROMEL)

+5V

COMMON

ICE

POINT

COMP.

10mV/ C

Figure 1. Basic Connection, Single Supply Operation

of identical alloys and exhibit similar behavior. The upper tem-
perature limits in Table I are those recommended for type J and
type K thermocouples by the majority of vendors.

SINGLE AND DUAL SUPPLY CONNECTIONS

The AD594/AD595 is a completely self-contained thermocouple
conditioner. Using a single +5 V supply the interconnections
shown in Figure 1 will provide a direct output from a type J
thermocouple (AD594) or type K thermocouple (AD595) mea-
suring from 0

C to +300

Any convenient supply voltage from +5 V to +30 V may be
used, with self-heating errors being minimized at lower supply
levels. In the single supply configuration the +5 V supply con-
nects to Pin 11 with the V connection at Pin 7 strapped to
power and signal common at Pin 4. The thermocouple wire in-
puts connect to Pins 1 and 14 either directly from the measuring
point or through intervening connections of similar thermo-
couple wire type. When the alarm output at Pin 13 is not used it
should be connected to common or V. The precalibrated feed-
back network at Pin 8 is tied to the output at Pin 9 to provide a
10 mV/

C nominal temperature transfer characteristic.

By using a wider ranging dual supply, as shown in Figure 2, the
AD594/AD595 can be interfaced to thermocouples measuring
both negative and extended positive temperatures.

AD594/AD595

REV. C

CONSTANTAN
(ALUMEL)

IRON
(CHROMEL)

+5V TO +30V

COMMON

SPAN OF

5V TO 30V

0V TO 25V

OVERLOAD
DETECT

TC

+TC

AD594/
AD595

ICE

POINT

COMP.

Figure 2. Dual Supply Operation

With a negative supply the output can indicate negative tem-
peratures and drive grounded loads or loads returned to positive
voltages. Increasing the positive supply from 5 V to 15 V ex-
tends the output voltage range well beyond the 750

temperature limit recommended for type J thermocouples
(AD594) and the 1250

C for type K thermocouples (AD595).

Common-mode voltages on the thermocouple inputs must remain
within the common-mode range of the AD594/AD595, with a
return path provided for the bias currents. If the thermocouple
is not remotely grounded, then the dotted line connections in
Figures 1 and 2 are recommended. A resistor may be needed in
this connection to assure that common-mode voltages induced
in the thermocouple loop are not converted to normal mode.

THERMOCOUPLE CONNECTIONS

The isothermal terminating connections of a pair of thermo-
couple wires forms an effective reference junction. This junction
must be kept at the same temperature as the AD594/AD595 for
the internal cold junction compensation to be effective.

A method that provides for thermal equilibrium is the printed
circuit board connection layout illustrated in Figure 3.

IRON

(CHROMEL)

CONSTANTAN

(ALUMEL)

+IN

IN

ALM

+ALM

COMP

COMMON

OUT

V+

Figure 3. PCB Connections

Here the AD594/AD595 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 com-
posed of a copper-constantan (or copper-alumel) connection
and copper-iron (or copper-chromel) connection, both of which
are at the same temperature as the AD594/AD595.

The printed circuit board layout shown also provides for place-
ment of optional alarm load resistors, recalibration resistors and
a compensation capacitor to limit bandwidth.

To ensure secure bonding the thermocouple wire should be
cleaned to remove oxidation prior to soldering. Noncorrosive
rosin flux is effective with iron, constantan, chromel and alumel
and the following solders: 95% tin-5% antimony, 95% tin-5%
silver or 90% tin-10% lead.

FUNCTIONAL DESCRIPTION

The AD594 behaves like two differential amplifiers. The out-
puts are summed and used to control a high gain amplifier, as
shown in Figure 4.

OVERLOAD
DETECT

ICE

POINT

COMP.

TC

+TC

AD594/AD595

+IN

COM

IN

ALM

+ALM

V+

COMP

Figure 4. AD594/AD595 Block Diagram

In normal operation the main amplifier output, at Pin 9, is con-
nected to the feedback network, at Pin 8. Thermocouple signals
applied to the floating input stage, at Pins 1 and 14, are ampli-
fied by gain G of the differential amplifier and are then further
amplified by gain A in the main amplifier. The output of the
main amplifier is fed back to a second differential stage in an in-
verting connection. The feedback signal is amplified by this
stage and is also applied to the main amplifier input through a
summing circuit. Because of the inversion, the amplifier causes
the feedback to be driven to reduce this difference signal to a
small value. The two differential amplifiers are made to match
and have identical gains, G. As a result, the feedback signal that
must be applied to the right-hand differential amplifier will pre-
cisely match the thermocouple input signal when the difference
signal has been reduced to zero. The feedback network is trim-
med so that the effective gain to the output, at Pins 8 and 9, re-
sults in a voltage of 10 mV/

C of thermocouple excitation.

In addition to the feedback signal, a cold junction compensation
voltage is applied to the right-hand differential amplifier. The
compensation is a differential voltage proportional to the Celsius
temperature of the AD594/AD595. This signal disturbs the dif-
ferential input so that the amplifier output must adjust to restore
the input to equal the applied thermocouple voltage.

The compensation is applied through the gain scaling resistors
so that its effect on the main output is also 10 mV/

C. As a

result, the compensation voltage adds to the effect of the ther-
mocouple voltage a signal directly proportional to the difference
between 0

C and the AD594/AD595 temperature. If the thermo-

couple reference junction is maintained at the AD594/AD595
temperature, the output of the AD594/AD595 will correspond
to the reading that would have been obtained from amplification
of a signal from a thermocouple referenced to an ice bath.

AD594/AD595

REV. C

The AD594/AD595 also includes an input open circuit detector
that switches on an alarm transistor. This transistor is actually a
current-limited output buffer, but can be used up to the limit as
a switch transistor for either pull-up or pull-down operation of
external alarms.

The ice point compensation network has voltages available with
positive and negative temperature coefficients. These voltages
may be used with external resistors to modify the ice point com-
pensation and recalibrate the AD594/AD595 as described in the
next column.

The feedback resistor is separately pinned out so that its value
can be padded with a series resistor, or replaced with an external
resistor between Pins 5 and 9. External availability of the feedback
resistor allows gain to be adjusted, and also permits the AD594/
AD595 to operate in a switching mode for setpoint operation.

CAUTIONS:

The temperature compensation terminals (+C and C) at Pins 2
and 6 are provided to supply small calibration currents only. The
AD594/AD595 may be permanently damaged if they are
grounded or connected to a low impedance.

The AD594/AD595 is internally frequency compensated for feed-
back ratios (corresponding to normal signal gain) of 75 or more.
If a lower gain is desired, additional frequency compensation
should be added in the form of a 300 pF capacitor from Pin 10
to the output at Pin 9. As shown in Figure 5 an additional 0.01

capacitor between Pins 10 and 11 is recommended.

AD594/
AD595

COMP

300pF

0.01 F

Figure 5. Low Gain Frequency Compensation

RECALIBRATION PRINCIPLES AND LIMITATIONS

The ice point compensation network of the AD594/AD595
produces a differential signal which is zero at 0

C and corre-

sponds to the output of an ice referenced thermocouple at the
temperature of the chip. The positive TC output of the circuit is
proportional to Kelvin temperature and appears as a voltage at
+T. It is possible to decrease this signal by loading it with a
resistor from +T to COM, or increase it with a pull-up resistor
from +T to the larger positive TC voltage at +C. Note that
adjustments to +T should be made by measuring the voltage which
tracks it at T. To avoid destabilizing the feedback amplifier the
measuring instrument should be isolated by a few thousand
ohms in series with the lead connected to T.

AD594/

AD595

+IN

IN

COM

Figure 6. Decreased Sensitivity Adjustment

Changing the positive TC half of the differential output of the
compensation scheme shifts the zero point away from 0

C. The

zero can be restored by adjusting the current flow into the nega-
tive input of the feedback amplifier, the T pin. A current into

this terminal can be produced with a resistor between C and
T to balance an increase in +T, or a resistor from T to COM
to offset a decrease in +T.

If the compensation is adjusted substantially to accommodate a
different thermocouple type, its effect on the final output volt-
age will increase or decrease in proportion. To restore the
nominal output to 10 mV/

C the gain may be adjusted to match

the new compensation and thermocouple input characteristics.
When reducing the compensation the resistance between T
and COM automatically increases the gain to within 0.5% of the
correct value. If a smaller gain is required, however, the nominal
47 k

internal feedback resistor can be paralleled or replaced

with an external resistor.

Fine calibration adjustments will require temperature response
measurements of individual devices to assure accuracy. Major
reconfigurations for other thermocouple types can be achieved
without seriously compromising initial calibration accuracy, so
long as the procedure is done at a fixed temperature using the
factory calibration as a reference. It should be noted that inter-
mediate recalibration conditions may require the use of a
negative supply.

EXAMPLE: TYPE E RECALIBRATION--AD594/AD595

Both the AD594 and AD595 can be configured to condition the
output of a type E (chromel-constantan) thermocouple. Tem-
perature characteristics of type E thermocouples differ less from
type J, than from type K, therefore the AD594 is preferred for
recalibration.

While maintaining the device at a constant temperature follow
the recalibration steps given here. First, measure the device
temperature by tying both inputs to common (or a selected
common-mode potential) and connecting FB to VO. The AD594
is now in the stand alone Celsius thermometer mode. For this
example assume the ambient is 24

C and the initial output VO

is 240 mV. Check the output at VO to verify that it corresponds
to the temperature of the device.

Next, measure the voltage T at Pin 5 with a high impedance
DVM (capacitance should be isolated by a few thousand ohms
of resistance at the measured terminals). At 24

C the T voltage

will be about 8.3 mV. To adjust the compensation of an AD594
to a type E thermocouple a resistor, R1, should be connected
between +T and +C, Pins 2 and 3, to raise the voltage at T by
the ratio of thermocouple sensitivities. The ratio for converting a
type J device to a type E characteristic is:

r (AD594) =(60.9

C)/(51.7

C)= 1.18

Thus, multiply the initial voltage measured at T by r and ex-
perimentally determine the R1 value required to raise T to that
level. For the example the new T voltage should be about 9.8 mV.
The resistance value should be approximately 1.8 k

The zero differential point must now be shifted back to 0

This is accomplished by multiplying the original output voltage
VO by r and adjusting the measured output voltage to this value
by experimentally adding a resistor, R2, between C and T,
Pins 5 and 6. The target output value in this case should be
about 283 mV. The resistance value of R2 should be approxi-
mately 240 k

Finally, the gain must be recalibrated such that the output VO
indicates the device's temperature once again. Do this by adding
a third resistor, R3, between FB and T, Pins 8 and 5. VO should
now be back to the initial 240 mV reading. The resistance value

AD594/AD595

REV. C

of R3 should be approximately 280 k

. The final connection

diagram is shown in Figure 7. An approximate verification of
the effectiveness of recalibration is to measure the differential
gain to the output. For type E it should be 164.2.

AD594/

AD595

+IN

IN

COM

Figure 7. Type E Recalibration

When implementing a similar recalibration procedure for the
AD595 the values for R1, R2, R3 and r will be approximately
650

, 84 k

, 93 k

and 1.51, respectively. Power consump-

tion will increase by about 50% when using the AD595 with
type E inputs.

Note that during this procedure it is crucial to maintain the
AD594/AD595 at a stable temperature because it is used as the
temperature reference. Contact with fingers or any tools not at
ambient temperature will quickly produce errors. Radiational
heating from a change in lighting or approach of a soldering iron
must also be guarded against.

USING TYPE T THERMOCOUPLES WITH THE AD595

Because of the similarity of thermal EMFs in the 0

C to +50

range between type K and type T thermocouples, the AD595
can be directly used with both types of inputs. Within this ambi-
ent temperature range the AD595 should exhibit no more than
an additional 0.2

C output calibration error when used with

type T inputs. The error arises because the ice point compensa-
tor is trimmed to type K characteristics at 25

C. To calculate

the AD595 output values over the recommended 200

C to

+350

C range for type T thermocouples, simply use the ANSI

thermocouple voltages referred to 0

C and the output equation

given on page 2 for the AD595. Because of the relatively large
nonlinearities associated with type T thermocouples the output
will deviate widely from the nominal 10 mV/

C. However, cold

junction compensation over the rated 0

C to +50

C ambient

will remain accurate.

STABILITY OVER TEMPERATURE

Each AD594/AD595 is tested for error over temperature with
the measuring thermocouple at 0

C. The combined effects of

cold junction compensation error, amplifier offset drift and gain
error determine the stability of the AD594/AD595 output over
the rated ambient temperature range. Figure 8 shows an AD594/
AD595 drift error envelope. The slope of this figure has units
of

TEMPERATURE OF AD594C/AD595C

0.6 C

DRIFT ERROR

50 C

25 C

+0.6 C

Figure 8. Drift Error vs. Temperature

THERMAL ENVIRONMENT EFFECTS

The inherent low power dissipation of the AD594/AD595 and
the low thermal resistance of the package make self-heating
errors almost negligible. For example, in still air the chip to am-
bient thermal resistance is about 80

C/watt (for the D package).

At the nominal dissipation of 800

W the self-heating in free air

is less than 0.065

C. Submerged in fluorinert liquid (unstirred)

the thermal resistance is about 40

C/watt, resulting in a self-

heating error of about 0.032

SETPOINT CONTROLLER
The AD594/AD595 can readily be connected as a setpoint
controller as shown in Figure 9.

CONSTANTAN
(ALUMEL)

IRON
(CHROMEL)

+5V

COMMON

HEATER

20M
(OPTIONAL)
FOR
HYSTERESIS

SETPOINT
VOLTAGE
INPUT

TEMPERATURE
CONTROLLED
REGION

LOW = > T < SETPOINT
HIGH = > T > SETPOINT

TEMPERATURE
COMPARATOR OUT

HEATER

DRIVER

OVERLOAD
DETECT

TC

+TC

AD594/
AD595

ICE

POINT

COMP.

Figure 9. Setpoint Controller

The thermocouple is used to sense the unknown temperature
and provide a thermal EMF to the input of the AD594/AD595.
The signal is cold junction compensated, amplified to 10 mV/

and compared to an external setpoint voltage applied by the
user to the feedback at Pin 8. Table I lists the correspondence
between setpoint voltage and temperature, accounting for the
nonlinearity of the measurement thermocouple. If the setpoint
temperature range is within the operating range (55

C to

+125

C) of the AD594/AD595, the chip can be used as the

transducer for the circuit by shorting the inputs together and
utilizing the nominal calibration of 10 mV/

C. This is the centi-

grade thermometer configuration as shown in Figure 13.

In operation if the setpoint voltage is above the voltage corre-
sponding to the temperature being measured the output swings
low to approximately zero volts. Conversely, when the tempera-
ture rises above the setpoint voltage the output switches to
the positive limit of about 4 volts with a +5 V supply. Figure
9 shows the setpoint comparator configuration complete with a
heater element driver circuit being controlled by the AD594/
AD595 toggled output. Hysteresis can be introduced by inject-
ing a current into the positive input of the feedback amplifier
when the output is toggled high. With an AD594 about 200 nA
into the +T terminal provides 1

C of hysteresis. When using a

single 5 V supply with an AD594, a 20 M

resistor from V

+T will supply the 200 nA of current when the output is forced
high (about 4 V). To widen the hysteresis band decrease the
resistance connected from VO to +T.

AD594/AD595

REV. C

ALARM CIRCUIT

In all applications of the AD594/AD595 the ALM connection,
Pin 13, should be constrained so that it is not more positive
than (V+) 4 V. This can be most easily achieved by connect-
ing Pin 13 to either common at Pin 4 or V at Pin 7. For most
applications that use the alarm signal, Pin 13 will be grounded
and the signal will be taken from +ALM on Pin 12. A typical
application is shown in Figure 10.

In this configuration the alarm transistor will be off in normal
operation and the 20 k pull up will cause the +ALM output on
Pin 12 to go high. If one or both of the thermocouple leads are
interrupted, the +ALM pin will be driven low. As shown in Fig-
ure 10 this signal is compatible with the input of a TTL gate
which can be used as a buffer and/or inverter.

CONSTANTAN
(ALUMEL)

IRON
(CHROMEL)

+5V

ALARM
TTL GATE

20k

ALARM

OUT

GND

OVERLOAD
DETECT

TC

+TC

AD594/
AD595

ICE

POINT

COMP.

10mV/ C

Figure 10. Using the Alarm to Drive a TTL Gate
("Grounded'' Emitter Configuration)

Since the alarm is a high level output it may be used to directly
drive an LED or other indicator as shown in Figure 11.

CONSTANTAN
(ALUMEL)

IRON
(CHROMEL)

COMMON

LED

270

V+

OVERLOAD
DETECT

TC

+TC

AD594/
AD595

ICE

POINT

COMP.

10mV/ C

Figure 11. Alarm Directly Drives LED

A 270

series resistor will limit current in the LED to 10 mA,

but may be omitted since the alarm output transistor is current
limited at about 20 mA. The transistor, however, will operate in
a high dissipation mode and the temperature of the circuit will
rise well above ambient. Note that the cold junction compensa-
tion will be affected whenever the alarm circuit is activated. The
time required for the chip to return to ambient temperature will
depend on the power dissipation of the alarm circuit, the nature
of the thermal path to the environment and the alarm duration.

The alarm can be used with both single and dual supplies. It
can be operated above or below ground. The collector and emit-
ter of the output transistor can be used in any normal switch
configuration. As an example a negative referenced load can be
driven from ALM as shown in Figure 12.

CONSTANTAN
(ALUMEL)

IRON
(CHROMEL)

+10V

GND

ALARM
RELAY

12V

OVERLOAD
DETECT

TC

+TC

AD594/
AD595

ICE

POINT

COMP.

10mV/ C

Figure 12. ALM Driving A Negative Referenced Load

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 so that it does not become more positive than 4
volts below the V+ applied to the circuit.

Additionally, the AD594/AD595 can be configured to produce
an extreme upscale or downscale output in applications where
an extra signal line for an alarm is inappropriate. By tying either
of the thermocouple inputs to common most runaway control
conditions can be automatically avoided. A +IN to common
connection creates a downscale output if the thermocouple opens,
while connecting IN to common provides an upscale output.

CELSIUS THERMOMETER

The AD594/AD595 may be configured as a stand-alone Celsius
thermometer as shown in Figure 13.

+5V TO +15V

OUTPUT

10mV/ C

GND

0 TO 15V

OVERLOAD
DETECT

TC

+TC

AD594/
AD595

ICE

POINT

COMP.

Figure 13. AD594/AD595 as a Stand-Alone Celsius
Thermometer

Simply omit the thermocouple and connect the inputs (Pins 1
and 14) to common. The output now will reflect the compensa-
tion voltage and hence will indicate the AD594/AD595
temperature with a scale factor of 10 mV/

C. In this three termi-

nal, voltage output, temperature sensing mode, the AD594/
AD595 will operate over the full military 55

C to +125

C tem-

perature range.

AD594/AD595

REV. C

C731g011/99

PRINTED IN U.S.A.

OUTLINE DIMENSIONS

Dimensions shown in inches and (mm).

TO-116 (D) Package

0.100
(2.54)

BSC

0.700

0.010

(17.78

0.25)

0.180

0.030

(4.57

0.76)

0.085 (2.16)

0.035

0.010

(0.89

0.25)

0.047

0.007

(1.19

0.18)

0.017 +0.003

0.002

0.43 +0.08

0.05

(

0.125

(3.18)

MIN

0.31

0.01

(7.87

0.25)

0.01

0.002

(0.25

0.05)

0.095 (2.41)

0.30 (7.62) REF

0.430 (10.92)

0.040

(1.02) R

PIN 1

0.265
(6.73)

0.290

0.010

(7.37

0.25)

Cerdip (Q) Package

PIN 1

0.77

0.015

(19.55

0.39)

0.310
(7.87)

0.260

0.020

(6.6

0.51)

SEATING
PLANE

0.100
(2.54)
BSC

0.035

0.010

(0.889

0.254)

0.180

0.030

(4.57

0.76)

0.125

3.175)

MIN

0.032

(0.812)

0.300 (7.62)

REF

0.148

0.015

(3.76

0.38)

0.010

0.001

(0.254

0.025)

0.018

(0.457)

0.600 (15.24)

BSC

THERMOCOUPLE BASICS

Thermocouples are economical and rugged; they have reason-
ably good long-term stability. Because of their small size, they
respond quickly and are good choices where fast response is im-
portant. They function over temperature ranges from cryogenics
to jet-engine exhaust and have reasonable linearity and accuracy.

Because the number of free electrons in a piece of metal de-
pends on both temperature and composition of the metal, two
pieces of dissimilar metal in isothermal and contact will exhibit
a potential difference that is a repeatable function of tempera-
ture, as shown in Figure 14. The resulting voltage depends on
the temperatures, T1 and T2, in a repeatable way.

CONSTANTAN

IRON

UNKNOWN

TEMPERATURE

ICE POINT

REFERENCE

CONSTANTAN

Figure 14. Thermocouple Voltage with 0

C Reference

Since the thermocouple is basically a differential rather than
absolute measuring device, a know reference temperature is
required for one of the junctions if the temperature of the other
is to be inferred from the output voltage. Thermocouples made
of specially selected materials have been exhaustively character-
ized in terms of voltage versus temperature compared to primary
temperature standards. Most notably the water-ice point of 0

is used for tables of standard thermocouple performance.

An alternative measurement technique, illustrated in Figure 15,
is used in most practical applications where accuracy requirements
do not warrant maintenance of primary standards. The reference
junction temperature is allowed to change with the environment
of the measurement system, but it is carefully measured by some
type of absolute thermometer. A measurement of the thermo-
couple voltage combined with a knowledge of the reference
temperature can be used to calculate the measurement junction
temperature. Usual practice, however, is to use a convenient
thermoelectric method to measure the reference temperature

and to arrange its output voltage so that it corresponds to a ther-
mocouple referred to 0

C. This voltage is simply added to the

thermocouple voltage and the sum then corresponds to the stan-
dard voltage tabulated for an ice-point referenced thermocouple.

CONSTANTAN

IRON

CuNi

= V

FOR PROPERLY
SCALED V

= f(T

)

Figure 15. Substitution of Measured Reference
Temperature for Ice Point Reference

The temperature sensitivity of silicon integrated circuit transis-
tors is quite predictable and repeatable. This sensitivity is
exploited in the AD594/AD595 to produce a temperature re-
lated voltage to compensate the reference of "cold" junction of a
thermocouple as shown in Figure 16.

CONSTANTAN

IRON

Figure 16. Connecting Isothermal Junctions

Since the compensation is at the reference junction temperature,
it is often convenient to form the reference "junction" by connect-
ing directly to the circuit wiring. So long as these connections
and the compensation are at the same temperature no error will
result.

Part Number AD594