AD AD8496 Precision thermocouple amplifiers with cold junction compensation Datasheet

Precision Thermocouple Amplifiers
with Cold Junction Compensation
AD8494/AD8495/AD8496/AD8497
FEATURES
FUNCTIONAL BLOCK DIAGRAM
SENSE
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
–IN
1MΩ
ESD AND
OVP
AD8494/AD8495/
AD8496/AD8497
A2
COLD JUNCTION
COMPENSATION
THERMOCOUPLE
+IN
ESD AND
OVP
A3
OUT
A1
REF
08529-020
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 VS = 5 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
Figure 1.
Table 1. Device Temperature Ranges
Part No.
AD8494
AD8495
AD8496
AD8497
Thermocouple
Type
J
K
J
K
Optimized Temperature Range
Ambient Temperature Measurement
(Reference Junction) Junction
0°C to 50°C
Full J type range
0°C to 50°C
Full K type range
25°C to 100°C
Full J type range
25°C to 100°C
Full K type range
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 Table 1 for the optimized ambient temperature
range of each part.
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.
2.
3.
4.
5.
6.
Complete, precision laser wafer trimmed thermocouple
signal conditioning system in a single IC package.
Flexible pinout provides for operation as a setpoint
controller or as a standalone Celsius thermometer.
Rugged inputs withstand 4 kV ESD and provide overvoltage protection (OVP) up to VS ± 25 V.
Differential inputs reject common-mode noise on the
thermocouple leads.
Reference pin voltage can be offset to measure 0°C on
single supplies.
Available in a small, 8-lead MSOP that is fully RoHS compliant.
Rev. 0
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 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.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
www.analog.com
Fax: 781.461.3113
©2010 Analog Devices, Inc. All rights reserved.
AD8494/AD8495/AD8496/AD8497
TABLE OF CONTENTS
Features .............................................................................................. 1
Thermocouples ........................................................................... 11
Applications ....................................................................................... 1
Thermocouple Signal Conditioner .......................................... 11
Functional Block Diagram .............................................................. 1
AD8494/AD8495/AD8496/AD8497 Architecture .................. 11
General Description ......................................................................... 1
Maximum Error Calculation .................................................... 12
Product Highlights ........................................................................... 1
Recommendations for Best Circuit Performance .................. 13
Revision History ............................................................................... 2
Applications Information .............................................................. 14
Specifications..................................................................................... 3
Basic Connection ....................................................................... 14
Absolute Maximum Ratings............................................................ 5
Ambient Temperature Sensor ................................................... 14
Thermal Resistance ...................................................................... 5
Setpoint Controller .................................................................... 15
ESD Caution .................................................................................. 5
Measuring Negative Temperatures .......................................... 15
Pin Configuration and Function Descriptions ............................. 6
Reference Pin Allows Offset Adjustment ................................ 15
Typical Performance Characteristics ............................................. 7
Outline Dimensions ....................................................................... 16
Theory of Operation ...................................................................... 11
Ordering Guide .......................................................................... 16
REVISION HISTORY
7/10—Revision 0: Initial Version
Rev. 0 | Page 2 of 16
AD8494/AD8495/AD8496/AD8497
SPECIFICATIONS
+VS = 5 V, −VS = 0 V, V+IN = V−IN = 0 V, VREF = 0 V, TA = TRJ = 25°C, RL = 100 kΩ, unless otherwise noted. Specifications do not include
gain and offset errors of the thermocouple itself. TA is the ambient temperature at the AD849x; TRJ is the thermocouple reference junction
temperature; TMJ is the thermocouple measurement junction temperature.
Table 2.
Parameter
TEMPERATURE ACCURACY
Initial Accuracy
AD8494/AD8495
AD8496/AD8497
Ambient Temperature
Rejection 1
AD8494/AD8495
AD8496/AD8497
Gain Error 2, 3
AD8494/AD8495
AD8496/AD8497
Transfer Function
INPUTS
Input Voltage Range
Overvoltage Range
Input Bias Current 4
Input Offset Current
Common-Mode Rejection
Power Supply Rejection
NOISE
Voltage Noise
Voltage Noise Density
Current Noise Density
REFERENCE INPUT
Input Resistance
Input Current
Voltage Range
Gain to Output
OUTPUT
Output Voltage Range
Short-Circuit Current 5
DYNAMIC RESPONSE
−3 dB Bandwidth
AD8494
AD8495/AD8497
AD8496
Settling Time to 0.1%
AD8494
AD8495/AD8497
AD8496
POWER SUPPLY
Operating Voltage Range 6
Single Supply
Dual Supply
Quiescent Current
Test Conditions/Comments
Min
A Grade
Typ
Max
Min
C Grade
Typ
Max
Unit
TA = TRJ = TMJ = 25°C
TA = TRJ = 60°C, TMJ = 175°C
3
3
1
1.5
°C
°C
TA = TRJ = 0°C to 50°C
TA = TRJ = 25°C to 100°C
VOUT = 0.125 V to 4.125 V
0.05
0.05
0.025
0.025
°C/°C
°C/°C
0.3
0.3
0.1
0.1
%
%
mV/°C
+VS – 1.6
−VS + 25
50
0.5
0.3
0.5
V
V
nA
nA
°C/V
°C/V
5
−VS – 0.2
+VS – 25
25
VCM = 0 V to 3 V
+VS = 2.7 V to 5 V
f = 0.1 Hz to 10 Hz, TA = 25°C
f = 1 kHz, TA = 25°C
f = 1 kHz, TA = 25°C
5
+VS – 1.6
−VS + 25
50
1.5
1
0.5
−VS – 0.2
+VS – 25
25
0.8
32
100
0.8
32
100
μV p-p
nV/√Hz
fA/√Hz
60
25
60
25
kΩ
μA
V
V/V
−VS
+VS
−VS
1
−VS + 0.025
+VS
1
7
+VS – 0.1
−VS + 0.025
7
+VS – 0.1
V
mA
30
25
31
30
25
31
kHz
kHz
kHz
36
40
32
36
40
32
μs
μs
μs
4 V output step
2.7
±2.7
180
Rev. 0 | Page 3 of 16
36
±18
250
2.7
±2.7
180
36
±18
250
V
V
μA
AD8494/AD8495/AD8496/AD8497
Parameter
TEMPERATURE RANGE (TA)
Specified Performance
AD8494/AD8495
AD8496/AD8497
Operational
Test Conditions/Comments
Min
A Grade
Typ
Max
Min
C Grade
Typ
Max
0
25
−40
50
100
+125
0
25
−40
50
100
+125
1
Unit
°C
°C
°C
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. 0 | Page 4 of 16
AD8494/AD8495/AD8496/AD8497
ABSOLUTE MAXIMUM RATINGS
THERMAL RESISTANCE
Table 3.
Parameter
Supply Voltage
Maximum Voltage at −IN or +IN
Minimum Voltage at −IN or +IN
REF Voltage
Output Short-Circuit Current Duration
Storage Temperature Range
Operating Temperature Range
Maximum IC Junction Temperature
ESD
Human Body Model
Field-Induced Charged Device Model
θJA is specified for a device on a 4-layer JEDEC PCB in free air.
Rating
±18 V
+VS – 25 V
–VS + 25 V
±VS
Indefinite
−65°C to +150°C
−40°C to +125°C
140°C
Table 4.
Package
8-Lead MSOP (RM-8)
ESD CAUTION
4.5 kV
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.
Rev. 0 | Page 5 of 16
θJA
135
Unit
°C/W
AD8494/AD8495/AD8496/AD8497
–IN 1
AD849x
8
+IN
7
+VS
–VS 3
6
OUT
NC 4
5
SENSE
REF 2
–
+
TOP VIEW
(Not to Scale)
NC = NO CONNECT
08529-002
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
Figure 2. Pin Configuration
Table 5. Pin Function Descriptions
Pin No.
1
2
3
4
5
6
7
8
Mnemonic
−IN
REF
−VS
NC
SENSE
OUT
+VS
+IN
Description
Negative Input.
Reference. This pin must be driven by low impedance.
Negative Supply.
No Connect.
Sense Pin. In measurement mode, connect to output; in setpoint mode, connect to setpoint voltage.
Output.
Positive Supply.
Positive Input.
Rev. 0 | Page 6 of 16
AD8494/AD8495/AD8496/AD8497
TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25°C, +VS = 5 V, RL = ∞, unless otherwise noted.
100
1200
AD8495/AD8497
AD8494
AD8496
1000
CONNECTED
THERMOCOUPLE
TEMPERATURE READING (°C)
CMRR (°C/V)
10
1
0.1
800
600
400
200
OPEN THERMOCOUPLE
0
10
100
1k
FREQUENCY (Hz)
10k
100k
–200
TIME (50µs/DIV)
Figure 6. Output Response to Open Thermocouple,
−IN Connected to Ground Through a 1 MΩ Resistor
Figure 3. CMRR vs. Frequency
1000
4.0
INPUT COMMON-MODE VOLTAGE (V)
3.5
100
10
1
+0.05, +3.45
2.0
1.5
1.0
0.5
0
+0.05, –0.36
10k
100k
Figure 4. PSRR vs. Frequency
INPUT BIAS CURRENT (nA)
40
20
10
AD8494
AD8496
AD8495/AD8497
–10
1.5
2.5
3.5
OUTPUT VOLTAGE (V)
4.5
5.5
40
2.00
35
1.75
1.50
30
IBIAS
25
1.25
20
1.00
15
0.75
10
0.50
0.25
5
1k
10k
FREQUENCY (Hz)
100k
Figure 5. Frequency Response
1M
0
–40
08529-018
GAIN (dB)
30
–20
100
+4.91, –0.39
Figure 7. Input Common-Mode Voltage Range vs. Output Voltage,
+VS = 5 V, VREF = 0 V, and VREF = 2.5 V
50
0
0.5
+4.91, –0.37
VREF = 0V
VREF = 2.5V
INPUT OFFSET CURRENT (nA)
100
1k
FREQUENCY (Hz)
+0.05, –0.39
IOS
–20
0
20
40
60
TEMPERATURE (°C)
0
80
100
120
08529-042
10
+4.91, +2.71
–1.0
–0.5
08529-036
1
+0.05, +3.21
2.5
–0.5
0
+4.91, +2.95
3.0
08529-017
AD8495/AD8497
AD8494
AD8496
PSRR (°C/V)
THERMOCOUPLE CONNECTION
AD849x OUTPUT
08529-019
1
08529-035
0.01
0.1
Figure 8. Input Bias Current and Input Offset Current vs. Temperature
Rev. 0 | Page 7 of 16
AD8494/AD8495/AD8496/AD8497
3.00
2.00
16
3.0
12
2.5
2.75
2.50
1.75
1.50
0.50
1.25
IIN
0
0.75
0.50
1.5
4
0
1.0
IIN
–4
0.5
–8
0
–0.50
–0.5
–12
0.25
15
20
25
–1.00
30
Figure 9. AD8494 Input Overvoltage Performance, +VS = 2.7 V (Gain = 96.7)
3.00
–16
–30 –25 –20 –15 –10 –5
0
5
10
INPUT VOLTAGE (V)
08529-021
0
–30 –25 –20 –15 –10 –5
0
5
10
INPUT VOLTAGE (V)
15
20
25
–1.0
30
08529-024
1.00
2.0
VOUT
INPUT CURRENT (mA)
1.00
OUTPUT VOLTAGE (V)
8
2.00
INPUT CURRENT (mA)
OUTPUT VOLTAGE (V)
2.25
1.50
VOUT
Figure 12. AD8494 Input Overvoltage Performance, VS = ±15 V (Gain = 96.7)
2.00
16
3.0
12
2.5
2.75
2.50
1.00
1.75
1.50
0.50
1.25
IIN
0
0.75
0.50
2.0
VOUT
1.5
4
IIN
0
1.0
–4
0.5
–8
0
–0.50
–0.5
–12
0.25
15
20
25
–1.00
30
–16
–30 –25 –20 –15 –10 –5
0
5
10
INPUT VOLTAGE (V)
08529-022
0
–30 –25 –20 –15 –10 –5
0
5
10
INPUT VOLTAGE (V)
Figure 10. AD8495/AD8497 Input Overvoltage Performance,
+VS = 2.7 V (Gain = 122.4)
3.00
15
20
25
–1.0
30
08529-025
1.00
OUTPUT VOLTAGE (V)
8
2.00
INPUT CURRENT (mA)
VOUT
INPUT CURRENT (mA)
OUTPUT VOLTAGE (V)
2.25
1.50
Figure 13. AD8495/AD8497 Input Overvoltage Performance,
VS = ±15 V (Gain = 122.4)
2.00
16
3.0
12
2.5
2.75
2.50
1.50
0.50
1.25
IIN
0
0.75
0.50
2.0
1.5
4
0
IIN
1.0
–4
0.5
–8
0
–0.50
–0.5
–12
0.25
0
–30 –25 –20 –15 –10 –5
0
5
10
INPUT VOLTAGE (V)
VOUT
15
20
25
–1.00
30
Figure 11. AD8496 Input Overvoltage Performance, +VS = 2.7 V
Gain = 90.35)
–16
–30 –25 –20 –15 –10 –5
0
5
10
INPUT VOLTAGE (V)
15
20
25
–1.0
30
08529-026
1.00
INPUT CURRENT (mA)
1.75
OUTPUT VOLTAGE (V)
8
1.00
INPUT CURRENT (mA)
VOUT
2.00
08529-023
OUTPUT VOLTAGE (V)
2.25
1.50
Figure 14. AD8496 Input Overvoltage Performance, VS = ±15 V (Gain = 90.35)
Rev. 0 | Page 8 of 16
AD8494/AD8495/AD8496/AD8497
CL = 0pF
CL = 1000pF
20mV/DIV
20mV/DIV
CL = 0pF
CL = 1000pF
120µs/DIV
08529-029
CL = 4700pF
CL = 10000pF
08529-028
CL = 4700pF
CL = 10000pF
120µs/DIV
Figure 15. AD8494/AD8496 Small-Signal Response
with Various Capacitive Loads
Figure 18. AD8495/AD8497 Small-Signal Response
with Various Capacitive Loads
AD8494/AD8496
AD8495/AD8497
08529-027
0.02%/DIV
120µs/DIV
100µs/DIV
Figure 16. Small-Signal Response, RL = 100 kΩ, CL = 1 nF
Figure 19. AD8494 Large-Signal Step Response and Settling Time
2V/DIV
2V/DIV
100µs/DIV
0.02%/DIV
SETTLING TO 0.1% IN 32µs
100µs/DIV
Figure 17. AD8495/AD8497 Large-Signal Step Response and Settling Time
Rev. 0 | Page 9 of 16
08529-041
SETTLING TO 0.1% IN 40µs
08529-040
0.02%/DIV
SETTLING TO 0.1% IN 36µs
08529-039
20mV/DIV
2V/DIV
Figure 20. AD8496 Large-Signal Step Response and Settling Time
TIME (1.5ms/DIV)
Figure 24. Output Voltage Start-Up
+VS
4
(+) –40°C
(+) +25°C
(+) +85°C
(+) +125°C
3
2
1
0
–1
(–) –40°C
(–) +25°C
(–) +85°C
(–) +125°C
–2
–3
–5
1k
10k
LOAD RESISTANCE (Ω)
100k
Figure 22. Output Voltage Swing vs. Load Resistance, VS = ±5 V
90
80
70
60
50
40
30
10
100
1k
FREQUENCY (Hz)
10k
100k
08529-031
20
1
(+) –40°C
(+) +25°C
(+) +85°C
(+) +125°C
+1.2
(–) –40°C
(–) +25°C
(–) +85°C
(–) +125°C
+0.8
+0.4
100µ
1m
OUTPUT CURRENT (A)
5m
Figure 25. Output Voltage Swing vs. Output Current, VS = ±5 V
100
10
–1.2
–0.8
–VS
10µ
08529-033
–4
–0.4
Figure 23. Voltage Noise Spectral Density vs. Frequency
Rev. 0 | Page 10 of 16
08529-034
OUTPUT VOLTAGE SWING (V)
REFERRED TO SUPPLY VOLTAGES (VS = ±5V)
5
OUTPUT VOLTAGE SWING (V)
08529-032
08529-030
1s/DIV
Figure 21. 0.1 Hz to 10 Hz RTI Voltage Noise
NOISE (nV/ Hz)
OUTPUT VOLTAGE
5V POWER-UP
OUTPUT VOLTAGE
(50mV/DIV)
200nV/DIV
SUPPLY VOLTAGE
(1.25V/DIV)
AD8494/AD8495/AD8496/AD8497
AD8494/AD8495/AD8496/AD8497
THEORY OF OPERATION
THERMOCOUPLES
Table 6. J Type Thermocouple Voltages and AD8494 Readings
A thermocouple is a rugged, low cost temperature transducer
whose output is proportional to the temperature difference
between a measurement junction and a reference junction. It
has a very wide temperature range. Its low level output (typically
tens of microvolts per °C) requires amplification. Variation in
the reference junction temperature results in measurement
error unless the thermocouple signal is properly compensated.
Measurement
Junction
Temperature
(TMJ)
50°C
50°C
0°C
0°C
A thermocouple consists of two dissimilar metals. These metals
are connected at one end to form the measurement junction,
also called the hot junction. The other end of the thermocouple
is connected to the metal lines that lead to the measurement
electronics. This connection forms a second junction: the
reference junction, also called the cold junction.
Thermocouple
Voltage
+2.585 mV
0 mV
0 mV
−2.585 mV
AD8494/AD8495/AD8496/AD8497 ARCHITECTURE
Figure 27 shows a block diagram of the AD849x circuitry. The
AD849x consists of a low offset, fixed-gain instrumentation
amplifier and a temperature sensor.
SENSE
REFERENCE
JUNCTION
–IN
AD849x
THERMOCOUPLE WIRES
1MΩ
08529-004
PCB
TRACES
AD8494
Reading
250 mV
250 mV
0 mV
0 mV
ESD AND
OVP
AD8494/AD8495/
AD8496/AD8497
A2
COLD JUNCTION
COMPENSATION
THERMOCOUPLE
Figure 26. Thermocouple Junctions
To derive the temperature at the measurement junction (TMJ),
the user must know the differential voltage created by the thermocouple. The user must also know the error voltage generated by
the temperature at the reference junction (TRJ). Compensating
for the reference junction error voltage is typically called cold
junction compensation. The electronics must compensate for
any changes in temperature at the reference (cold) junction so
that the output voltage is an accurate representation of the hot
junction measurement.
THERMOCOUPLE SIGNAL CONDITIONER
The AD8494/AD8495/AD8496/AD8497 thermocouple amplifiers
provide a simple, low cost solution for measuring thermocouple
temperatures. These amplifiers simplify many of the difficulties
of measuring thermocouples. An integrated temperature sensor
performs cold junction compensation. A fixed-gain instrumentation amplifier amplifies the small thermocouple voltage to
provide a 5 mV/°C output. The high common-mode rejection
of the amplifier blocks common-mode noise that the long
thermocouple leads can pick up. For additional protection, the
high impedance inputs of the amplifier make it easy to add
extra filtering.
Table 6 shows an example of a J type thermocouple voltage for
various combinations of 0°C and 50°C on the reference and
measurement junctions. Table 6 also shows the performance
of the AD8494 amplifying the thermocouple voltage and
compensating for the reference junction temperature changes,
thus eliminating the error.
+IN
ESD AND
OVP
A3
OUT
A1
REF
08529-020
MEASUREMENT
JUNCTION
Reference
Junction
Temperature
(TRJ)
0°C
50°C
0°C
50°C
Figure 27. Block Diagram
The AD849x output is a voltage that is proportional to the temperature at the measurement junction of the thermocouple (TMJ).
To derive the measured temperature from the AD849x output
voltage, use the following transfer function:
TMJ = (VOUT − VREF)/(5 mV/°C)
An ideal AD849x achieves this output with an error of less than
±2°C, within the specified operating ranges listed in Table 7.
Instrumentation Amplifier
A thermocouple signal is so small that considerable gain is
required before it can be sampled properly by most ADCs. The
AD849x has an instrumentation amplifier with a fixed gain that
generates an output voltage of 5 mV/°C for J type and K type
thermocouples.
VOUT = (TMJ × 5 mV/°C) + VREF
To accommodate the nonlinear behavior of the thermocouple,
each amplifier has a different gain so that the 5 mV/°C is accurately maintained for a given temperature measurement range.
•
•
Rev. 0 | Page 11 of 16
The AD8494 and AD8496 (J type) have an instrumentation
amplifier with a gain of 96.7 and 90.35, respectively.
The AD8495 and AD8497 (K type) have an instrumentation
amplifier with a gain of 122.4.
AD8494/AD8495/AD8496/AD8497
The small thermocouple voltages mean that signals are quite
vulnerable to interference, especially when measured with
single-ended amplifiers. The AD849x addresses this issue in
several ways. Low input bias currents and high input impedance
allow for easy filtering at the inputs. The excellent common-mode
rejection of the AD849x prevents variations in ground potential
and other common-mode noise from affecting the measurement.
Temperature Sensor (Cold Junction Compensation)
The AD849x also includes a temperature sensor for cold junction compensation. This temperature sensor is used to measure
the reference junction temperature of the thermocouple and to
cancel its effect.
•
•
The AD8494/AD8495 cold junction compensation is
optimized for operation in a lab environment, where the
ambient temperature is around 25°C. The AD8494/AD8495
are specified for an ambient range of 0°C to 50°C.
The AD8496/AD8497 cold junction compensation is
optimized for operation in a less controlled environment,
where the temperature is around 60°C. The AD8496/AD8497
are specified for an ambient range of 25°C to 100°C.
Application examples for the AD8496/AD8497 include
automotive applications, autoclave, and ovens.
Thermocouple Break Detection
The AD849x offers open thermocouple detection. The inputs
of the AD849x are PNP type transistors, which means that the
bias current always flows out of the inputs. Therefore, the input
bias current drives any unconnected input high, which rails the
output. Connecting the negative input to ground through a
1 MΩ resistor causes the AD849x output to rail high in an open
thermocouple condition (see Figure 6, Figure 28, and the
Ground Connection section).
As is normally the case, the AD849x outputs are subject to
calibration, gain, and temperature sensitivity errors. The user
can calculate the maximum error from the AD849x using the
following information.
The five primary sources of AD849x error are described in this
section.
AD849x Initial Calibration Accuracy
Error at the initial calibration point can be easily calibrated out
with a one-point temperature calibration. See Table 2 for the
specifications.
AD849x Ambient Temperature Rejection
The specified ambient temperature rejection represents the
ability of the AD849x to reject errors caused by changes in the
ambient temperature/reference junction. For example, with
0.025°C/°C ambient temperature rejection, a 20°C change in the
reference junction temperature adds less than 0.5°C error to the
measurement. See Table 2 for the specifications.
AD849x Gain Error
Gain error is the amount of additional error when measuring away
from the measurement junction calibration point. For example,
if the part is calibrated at 25°C and the measurement junction is
100°C with a gain error of 0.1%, the gain error contribution is
(100°C − 25°C) × (0.1%) = 0.075°C. This error can be calibrated
out with a two-point calibration if needed, but it is usually small
enough to ignore. See Table 2 for the specifications.
Manufacturing Tolerances of the Thermocouple
Consult the data sheet for your thermocouple to find the
specified tolerance of the thermocouple.
Linearity Error of the Thermocouple
Each part in the AD849x family is precision trimmed to optimize
a linear operating range for a specific thermocouple type and
for the widest possible measurement and ambient temperature
ranges. The AD849x achieves a linearity error of less than ±2°C,
within the specified operating ranges listed in Table 7. This error
is due only to the nonlinearity of the thermocouple.
08529-008
1MΩ
MAXIMUM ERROR CALCULATION
Figure 28. Ground the Negative Input Through a 1 MΩ Resistor
for Open Thermocouple Detection
Input Voltage Protection
The AD849x has very robust inputs. Input voltages can be up
to 25 V from the opposite supply rail. For example, with a +5 V
positive supply and a −3 V negative supply, the part can safely
withstand voltages at the inputs from −20 V to +22 V. Voltages
at the reference and sense pins should not go beyond 0.3 V of
the supply rails.
Table 7. AD849x ±2°C Accuracy Temperature Ranges
Part
AD8494
AD8495
AD8496
AD8497
Thermocouple
Type
J
K
J
K
Max
Error
±2°C
±2°C
±2°C
±2°C
Ambient
Temperature
Range
0°C to 50°C
0°C to 50°C
25°C to 100°C
25°C to 100°C
Measurement
Temperature
Range
−35°C to +95°C
−25°C to +400°C
+55°C to +565°C
−25°C to +295°C
For temperature ranges outside those listed in Table 7 or for
instructions on how to correct for thermocouple nonlinearity
error with software, see the product page for the AD8494,
AD8495, AD8496, or AD8497, or contact an Analog Devices
representative.
Rev. 0 | Page 12 of 16
AD8494/AD8495/AD8496/AD8497
RECOMMENDATIONS FOR BEST CIRCUIT
PERFORMANCE
Keeping the AD849x at the Same Temperature
as the Reference Junction
Input Filter
The AD849x compensates for thermocouple reference junction
temperature by using an internal temperature sensor. It is
critical to keep the reference junction (thermocouple-to-PCB
connection) as close to the AD849x as possible. Any difference
in temperature between the AD849x and the reference junction
appears directly as temperature error. Temperature difference
between the device and the reference junction may occur if the
AD849x is not physically close to the reference junction or if the
AD849x is required to supply large amounts of output power.
The filter should be set to a low corner frequency that still
allows the input signal to pass through undiminished. The
primary purpose of the filter is to remove RF signals, which,
if allowed to reach the AD849x, can be rectified and appear
as temperature fluctuations.
CC
R
CD
KEEP JUNCTION AND
AD849x AT SAME
TEMPERATURE
1MΩ
AD849x
KEEP
TRACES
SHORT
THERMOCOUPLE WIRES
AD849x
CC
FILTER FREQUENCYDIFF =
REFERENCE
JUNCTION
PCB
TRACES
Figure 31. Compensating for Thermocouple Reference Junction Temperature
Driving the Reference Pin
1
2πR(2C D + CC)
1
FILTER FREQUENCYCM =
2πRC C
WHERE CD ≥ 10CC
The AD849x comes with a reference pin, which can be used
to offset the output voltage. This is particularly useful when
reading a negative temperature in a single-supply system.
08529-011
CONNECT WHEN
THERMOCOUPLE TIP
TYPE IS UNKNOWN
R
MEASUREMENT
JUNCTION
08529-010
A low-pass filter before the input of the AD849x is strongly
recommended (see Figure 29), especially when operating in an
electrically noisy environment. Long thermocouple leads can
function as an excellent antenna and pick up many unwanted
signals.
Figure 29. Filter for Any Thermocouple Style
INCORRECT
CORRECT
To prevent input offset currents from affecting the measurement
accuracy, the filter resistor values should be less than 50 kΩ.
Ground Connection
AD849x
REF
It is always recommended that the thermocouple be connected
to ground through a 100 kΩ to 1 MΩ resistor placed at the
negative (inverting) input of the amplifier on the PCB (see
Figure 30). This solution works well regardless of the thermocouple tip style.
AD849x
REF
V
V
+
–
08529-006
AD8613
Figure 32. Driving the Reference Pin
For best performance, the reference pin should be driven with a
low output impedance source, not a resistor divider. The AD8613
and the OP777 are good choices for the buffer amplifier.
08529-038
1MΩ
Figure 30. Ground the Thermocouple with a 1 MΩ Resistor
If there is no electrical connection at the measurement junction
(insulated tip), the resistor value is small enough that no meaningful common-mode voltage is generated. If there is an electrical
connection through a grounded or exposed tip, the resistor value
is large enough that any current from the measurement tip to
ground is very small, preventing measurement errors.
The AD849x inputs require only one ground connection or source
of common-mode voltage. Any additional ground connection is
detrimental to performance because ground loops can form
through the thermocouple, easily swamping the small
thermocouple signal. Grounding the thermocouple through a
resistor as recommended prevents such problems.
Debugging Tip
If the AD849x is not providing the expected performance, a
useful debugging step is to implement the ambient temperature
configuration in Figure 34. If the ambient temperature sensor
does not work as expected, the problem is likely with the AD849x
or with the downstream circuitry. If the ambient temperature
sensor configuration is working correctly, the problem typically
lies with how the thermocouple is connected to the AD849x.
Common errors include an incorrect grounding configuration
or lack of filtering.
Rev. 0 | Page 13 of 16
AD8494/AD8495/AD8496/AD8497
APPLICATIONS INFORMATION
BASIC CONNECTION
AMBIENT TEMPERATURE SENSOR
Figure 33 shows an example of a basic connection for the
AD849x, with a J type or K type thermocouple input.
The AD849x can be configured as a standalone Celsius thermometer with a 5 mV/°C output, as shown in Figure 34. The
thermocouple sensing functionality is disabled by shorting both
AD849x inputs to ground; the AD849x simply outputs the value
from the on-board temperature sensor.
5V
0.1µF
+VS
10µF
7
As a temperature sensor, the AD8494 has a measurement temperature range of −40°C to +125°C with a precision output of
COLD JUNCTION
COMPENSATION
6
IN-AMP
–IN
5V
+VS
OUT
5
1
AD849x
1MΩ
2
REF
7
SENSE
COLD JUNCTION
COMPENSATION
3
–VS
0.1µF
10µF
+IN
8
6
IN-AMP
Figure 33. Basic Connection for the AD849x
–IN
5
1
AD849x
To measure negative temperatures, apply a voltage at the reference pin to offset the output voltage at 0°C. The output voltage
of the AD849x is
OUT
2
REF
SENSE
3
–VS
08529-013
THERMOCOUPLE
VOUT = TA × 5 mV/°C
8
08529-012
+IN
Figure 34. Ambient Temperature Sensor
VOUT = (TMJ × 5 mV/°C) + VREF
A filter at the input is recommended to remove high frequency
noise. The 1 MΩ resistor to ground enables open thermocouple
detection and proper grounding of the thermocouple. The sense
pin should be connected to the output pin of the AD849x.
Decoupling capacitors should be used to ensure clean power
supply voltages on +VS and, if using dual supplies, on −VS, also.
A 0.1 μF capacitor should be placed as close as possible to each
AD849x supply pin. A 10 μF tantalum capacitor can be used
farther away from the part and can be shared.
The AD8494 is the best choice for use as an ambient temperature sensor. The AD8495, AD8496, and AD8497 can also be
configured as ambient temperature sensors, but their output
transfer functions are not precisely 5 mV/°C. For information
about the exact transfer functions of the AD8494/AD8495/
AD8496/AD8497, see the product page for the AD8494,
AD8495, AD8496, or AD8497, or contact an Analog Devices
representative.
The thermometer mode can be particularly useful for debugging
a misbehaving circuit. If the basic connection is not working,
disconnect the thermocouple and short both inputs to ground.
If the system reads the ambient temperature correctly, the
problem is related to the thermocouple. If the system does not
read the ambient temperature correctly, the problem is with
the AD849x or with the downstream circuitry.
Rev. 0 | Page 14 of 16
AD8494/AD8495/AD8496/AD8497
SETPOINT CONTROLLER
MEASURING NEGATIVE TEMPERATURES
The AD849x can be used as a temperature setpoint controller,
with a thermocouple input from a remote location or with the
AD849x itself being used as a temperature sensor. When the
measured temperature is below the setpoint temperature, the
output voltage goes to −VS. When the measured temperature is
above the setpoint temperature, the output voltage goes to +VS.
For best accuracy and CMRR performance, the setpoint voltage
should be created with a low impedance source. If the setpoint
voltage is generated with a voltage divider, a buffer is
recommended.
The AD849x can measure negative temperatures on dual
supplies and on a single supply. When operating on dual
supplies with the reference pin grounded, a negative output
voltage indicates a negative temperature at the thermocouple
measurement junction.
5V
+VS
VOUT = (TMJ × 5 mV/°C) + VREF
When operating the AD849x on a single supply, level-shift
the output by applying a positive voltage (less than +VS) on
the reference pin. An output voltage less than VREF indicates
a negative temperature at the thermocouple measurement
junction.
7
REFERENCE PIN ALLOWS OFFSET ADJUSTMENT
COLD JUNCTION
COMPENSATION
+IN
8
6
IN-AMP
–IN
5
1
AD849x
1MΩ
2
OUT
SENSE
SETPOINT
VOLTAGE
3
08529-014
THERMOCOUPLE
The reference pin can be used to level-shift the AD849x output
voltage. This is useful for measuring negative temperatures on a
single supply and to match the AD849x output voltage range to
the input voltage range of the subsequent electronics in the
signal chain.
–VS
REF
Figure 35. Setpoint Controller
The reference pin can also be used to offset any initial calibration errors. Apply a small reference voltage proportional to the
error to nullify the effect of the calibration error on the output.
Hysteresis can be added to the setpoint controller by using a
resistor divider from the output to the reference pin, as shown
in Figure 36. The hysteresis in °C is
THYST 
VS  R1 /(R1  R2)
5 mV/  C
5V
+VS
7
COLD JUNCTION
COMPENSATION
8
6
IN-AMP
–IN
5
1
AD849x
1MΩ
2
REF
R1
1kΩ
OUT
SENSE
3
–VS
R2
100kΩ
R1
1kΩ
SETPOINT
VOLTAGE
08529-015
+IN
THERMOCOUPLE
Figure 36. Adding 10 Degrees of Hysteresis
A resistor equivalent to the output resistance of the divider should
be connected to the sense pin to ensure good CMRR.
Rev. 0 | Page 15 of 16
AD8494/AD8495/AD8496/AD8497
OUTLINE DIMENSIONS
3.20
3.00
2.80
8
3.20
3.00
2.80
1
5.15
4.90
4.65
5
4
PIN 1
IDENTIFIER
0.65 BSC
0.95
0.85
0.75
15° MAX
1.10 MAX
0.40
0.25
6°
0°
0.23
0.13
COMPLIANT TO JEDEC STANDARDS MO-187-AA
0.70
0.55
0.40
091709-A
0.15
0.05
COPLANARITY
0.10
Figure 37. 8-Lead Mini Small Outline Package [MSOP]
(RM-8)
Dimensions shown in millimeters
ORDERING GUIDE
Model 1, 2
AD8494ARMZ
AD8494ARMZ-R7
AD8494CRMZ
AD8494CRMZ-R7
AD8495ARMZ
AD8495ARMZ-R7
AD8495CRMZ
AD8495CRMZ-R7
AD8496ARMZ
AD8496ARMZ-R7
AD8496CRMZ
AD8496CRMZ-R7
AD8497ARMZ
AD8497ARMZ-R7
AD8497CRMZ
AD8497CRMZ-R7
1
2
Temperature Range
0°C to 50°C
0°C to 50°C
0°C to 50°C
0°C to 50°C
0°C to 50°C
0°C to 50°C
0°C to 50°C
0°C to 50°C
25°C to 100°C
25°C to 100°C
25°C to 100°C
25°C to 100°C
25°C to 100°C
25°C to 100°C
25°C to 100°C
25°C to 100°C
Package Description
8-Lead MSOP
8-Lead MSOP, 7” Tape and Reel
8-Lead MSOP
8-Lead MSOP, 7” Tape and Reel
8-Lead MSOP
8-Lead MSOP, 7” Tape and Reel
8-Lead MSOP
8-Lead MSOP, 7” Tape and Reel
8-Lead MSOP
8-Lead MSOP, 7” Tape and Reel
8-Lead MSOP
8-Lead MSOP, 7” Tape and Reel
8-Lead MSOP
8-Lead MSOP, 7” Tape and Reel
8-Lead MSOP
8-Lead MSOP, 7” Tape and Reel
Z = RoHS Compliant Part.
The AD8494 and AD8496 models are prereleased.
©2010 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D08529-0-7/10(0)
Rev. 0 | Page 16 of 16
Package Option
RM-8
RM-8
RM-8
RM-8
RM-8
RM-8
RM-8
RM-8
RM-8
RM-8
RM-8
RM-8
RM-8
RM-8
RM-8
RM-8
Branding
Y36
Y36
Y37
Y37
Y33
Y33
Y34
Y34
Y3C
Y3C
Y3D
Y3D
Y39
Y39
Y3A
Y3A