ANALOG DEVICES AD 8494 AR Datasheet

Precision Thermocouple Amplifiers

FEATURES

Low cost and easy to use
Pretrimmed for J or K type thermocouples
Internal cold junction compensation
High impedance differential input
Standalone 5 mV/°C thermometer
Reference pin allows offset adjustment
Thermocouple break detection
Laser wafer trimmed to 1°C initial accuracy and

0.025°C/°C ambient temperature rejection
Low power: <1 mW at V
Wide power supply range

Single supply: 2.7 V to 36 V
Dual supply: ±2.7 V to ±18 V

Small, 8-lead MSOP

APPLICATIONS

J or K type thermocouple temperature measurement
Setpoint controller
Celsius thermometer
Universal cold junction compensator
White goods (oven, stove top) temperature measurements
Exhaust gas temperature sensing
Catalytic converter temperature sensing

= 5 V

S

with Cold Junction Compensation

AD8494/AD8495/AD8496/AD8497

FUNCTIONAL BLOCK DIAGRAM

REF

AD8494/AD8495/

+IN

ESD AND

OVP

THERMO­COUPLE

–IN

ESD AND

1MΩ

OVP

Table 1. Device Temperature Ranges

Thermo­Coupl e

Part No.

Typ e

AD8494 J 0°C to 50°C Full J type range
AD8495 K 0°C to 50°C Full K type range
AD8496 J 25°C to 100°C Full J type range
AD8497 K 25°C to 100°C Full K type range

AD8496/AD8497

A2

COLD JUNCTIO N
COMPENSATION

A1

SENSE

Figure 1.

Optimized Temperature Range

Ambient Temperature
(Reference Junction)

Measurement
Junction

A3

OUT

08529-001

GENERAL DESCRIPTION

The AD8494/AD8495/AD8496/AD8497 are precision
instrumentation amplifiers with thermocouple cold junction
compensators on an integrated circuit. They produce a high
level (5 mV/°C) output directly from a thermocouple signal by
combining an ice point reference with a precalibrated amplifier.
They can be used as standalone thermometers or as switched
output setpoint controllers using either a fixed or remote
setpoint control.

The AD8494/AD8495/AD8496/AD8497 can be powered from a
single-ended supply (less than 3 V) and can measure temperatures
below 0°C by offsetting the reference input. To minimize self­heating, an unloaded AD849x typically operates with a total
supply current of 180 µA, but it is also capable of delivering in
excess of ±5 mA to a load.

The AD8494 and AD8496 are precalibrated by laser wafer
trimming to match the characteristics of J type (iron-constantan)
thermocouples; the AD8495 and AD8497 are laser trimmed to
match the characteristics of K type (chromel-alumel) thermo­couples. See Tabl e 1 for the optimized ambient temperature
range of each part.

Rev. C

Information furnished by Analog Devices is believed to be accurate and reliable. However, no
responsibility is assumed by Anal og Devices for its use, nor for any infringements of patents or ot her
rights of third parties that may result from its use. Specifications subject to change without notice. No
license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
Trademarks and registered trademarks are the property of their respective owners.

The AD8494/AD8495/AD8496/AD8497 allow a wide variety of
supply voltages. With a 5 V single supply, the 5 mV/°C output
allows the devices to cover nearly 1000 degrees of a thermo­couple’s temperature range.

The AD8494/AD8495/AD8496/AD8497 work with 3 V supplies,
allowing them to interface directly to lower supply ADCs. They
can also work with supplies as large as 36 V in industrial systems
that require a wide common-mode input range.

PRODUCT HIGHLIGHTS

1. Complete, precision laser wafer trimmed thermocouple

signal conditioning system in a single IC package.

2. Flexible pinout provides for operation as a setpoint

controller or as a standalone Celsius thermometer.

3. Rugged inputs withstand 4 kV ESD and provide over-

voltage protection (OVP) up to V

4. Differential inputs reject common-mode noise on the

thermocouple leads.

5. Reference pin voltage can be offset to measure 0°C on

single supplies.

6. Available in a small, 8-lead MSOP that is fully RoHS compliant.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: [phone] www.analog.com
Fax: [phone] ©2010–2011 Analog Devices, Inc. All rights reserved.

± 25 V.

S

AD8494/AD8495/AD8496/AD8497

TABLE OF CONTENTS

Features.............................................................................................. 1

Applications....................................................................................... 1

Functional Block Diagram .............................................................. 1

General Description ......................................................................... 1

Product Highlights ........................................................................... 1

Revision History ............................................................................... 2

Specifications..................................................................................... 3

Absolute Maximum Ratings............................................................ 5

Thermal Resistance ...................................................................... 5

ESD Caution.................................................................................. 5

Pin Configuration and Function Descriptions............................. 6

Typical Performance Characteristics ............................................. 7

Theory of Operation ...................................................................... 11

REVISION HISTORY

6/11—Rev. B to Rev. C

Changes to Figure 35 and Figure 36............................................. 15

4/11—Rev. A to Rev. B

Changes to Figure 1.......................................................................... 1

Changes to Figure 33 and Figure 34............................................. 14

Changes to Figure 35 and Figure 36............................................. 15

Changes to Ordering Guide.......................................................... 16

10/10—Rev. 0 to Rev. A

Changes to Linearity Error of the Thermocouple Section........ 12

Changes to Ambient Temperature Sensor Section .................... 14

Changes to Ordering Guide.......................................................... 16

7/10—Revision 0: Initial Version

Thermocouples........................................................................... 11

Thermocouple Signal Conditioner.......................................... 11

AD8494/AD8495/AD8496/AD8497 Architecture .................. 11

Maximum Error Calculation.................................................... 12

Recommendations for Best Circuit Performance .................. 13

Applications Information.............................................................. 14

Basic Connection ....................................................................... 14

Ambient Temperature Sensor................................................... 14

Setpoint Controller .................................................................... 15

Measuring Negative Temperatures .......................................... 15

Reference Pin Allows Offset Adjustment................................ 15

Outline Dimensions....................................................................... 16

Ordering Guide .......................................................................... 16

Rev. C | Page 2 of 16

AD8494/AD8495/AD8496/AD8497

SPECIFICATIONS

+VS = 5 V, −VS = 0 V, V
gain and offset errors of the thermocouple itself. T
temperature; T

is the thermocouple measurement junction temperature.

MJ

Table 2.

A Grade C Grade
Parameter Test Conditions/Comments Min Typ Max Min Typ Max Unit

TEMPERATURE ACCURACY

Initial Accuracy

AD8494/AD8495 TA = TRJ = TMJ = 25°C 3 1 °C
AD8496/AD8497 TA = TRJ = 60°C, TMJ = 175°C 3 1.5 °C

Ambient Temperature

Rejection

1

AD8494/AD8495 TA = TRJ = 0°C to 50°C 0.05 0.025 °C/°C
AD8496/AD8497 TA = TRJ = 25°C to 100°C 0.05 0.025 °C/°C

Gain Error2, 3 V

AD8494/AD8495 0.3 0.1 %
AD8496/AD8497 0.3 0.1 %

Transfer Function 5 5 mV/°C

INPUTS

Input Voltage Range −VS – 0.2 +VS – 1.6 −VS – 0.2 +VS – 1.6 V
Overvoltage Range +VS – 25 −VS + 25 +VS – 25 −VS + 25 V
Input Bias Current4 25 50 25 50 nA
Input Offset Current 1.5 0.5 nA
Common-Mode Rejection VCM = 0 V to 3 V 1 0.3 °C/V
Power Supply Rejection +VS = 2.7 V to 5 V 0.5 0.5 °C/V

NOISE

Voltage Noise f = 0.1 Hz to 10 Hz, TA = 25°C 0.8 0.8 μV p-p
Voltage Noise Density f = 1 kHz, TA = 25°C 32 32 nV/√Hz
Current Noise Density f = 1 kHz, TA = 25°C 100 100 fA/√Hz

REFERENCE INPUT

Input Resistance 60 60 kΩ
Input Current 25 25 μA
Voltage Range −VS +VS −VS +VS V
Gain to Output 1 1 V/V

OUTPUT

Output Voltage Range −VS + 0.025 +VS – 0.1 −VS + 0.025 +VS – 0.1 V
Short-Circuit Current5 7 7 mA

DYNAMIC RESPONSE

−3 dB Bandwidth

AD8494 30 30 kHz
AD8495/AD8497 25 25 kHz
AD8496 31 31 kHz

Settling Time to 0.1% 4 V output step

AD8494 36 36 μs
AD8495/AD8497 40 40 μs
AD8496 32 32 μs

POWER SUPPLY

Operating Voltage Range6

Single Supply 2.7 36 2.7 36 V
Dual Supply ±2.7 ±18 ±2.7 ±18 V

Quiescent Current 180 250 180 250 μA

+IN

= V

= 0 V, V

−IN

= 0 V, TA = TRJ = 25°C, RL = 100 k, unless otherwise noted. Specifications do not include

REF

is the ambient temperature at the AD849x; TRJ is the thermocouple reference junction

A

= 0.125 V to 4.125 V

OUT

Rev. C | Page 3 of 16

AD8494/AD8495/AD8496/AD8497

A Grade C Grade
Parameter Test Conditions/Comments Min Typ Max Min Typ Max Unit

TEMPERATURE RANGE (TA)

Specified Performance

AD8494/AD8495 0 50 0 50 °C
AD8496/AD8497 25 100 25 100 °C

Operational −40 +125 −40 +125 °C

1

Ambient temperature rejection specifies the change in the output measurement (in °C) for a given change in temperature of the cold junction. For the AD8494 and

AD8495, ambient temperature rejection is defined as the slope of the line connecting errors calculated at 0°C and 50°C ambient temperature. For the AD8496 and
AD8497, ambient temperature rejection is defined as the slope of the line connecting errors calculated at 25°C and 100°C ambient temperature.

2

Error does not include thermocouple gain error or thermocouple nonlinearity.

3

With a 100 kΩ load, measurement junction temperatures beyond approximately 880°C for the AD8494 and AD8496 and beyond approximately 960°C for the AD8495

and AD8497 require supply voltages larger than 5 V or a negative voltage applied to the reference pin. Measurement junction temperatures below 5°C require either a
positive offset voltage applied to the reference pin or a negative supply.

4

Input stage uses PNP transistors, so bias current always flows out of the part.

5

Large output currents can increase the internal temperature rise of the part and contribute to cold junction compensation (CJC) error.

6

Unbalanced supplies can also be used. Care should be taken that the common-mode voltage of the thermocouple stays within the input voltage range of the part.

Rev. C | Page 4 of 16

AD8494/AD8495/AD8496/AD8497

ABSOLUTE MAXIMUM RATINGS

Table 3.

Parameter Rating

Supply Voltage ±18 V
Maximum Voltage at −IN or +IN +VS – 25 V
Minimum Voltage at −IN or +IN –VS + 25 V
REF Voltage ±VS
Output Short-Circuit Current Duration Indefinite
Storage Temperature Range −65°C to +150°C
Operating Temperature Range −40°C to +125°C
Maximum IC Junction Temperature 140°C
ESD

Human Body Model 4.5 kV
Field-Induced Charged Device Model 1.5 kV

Stresses above those listed under Absolute Maximum Ratings
may cause permanent damage to the device. This is a stress
rating only; functional operation of the device at these or any
other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute
maximum rating conditions for extended periods may affect
device reliability.

THERMAL RESISTANCE

θJA is specified for a device on a 4-layer JEDEC PCB in free air.

Table 4.

Package θJA Unit

8-Lead MSOP (RM-8) 135 °C/W

ESD CAUTION

Rev. C | Page 5 of 16