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PDF ADC8497 Data sheet ( Hoja de datos )
| Número de pieza | ADC8497 | |
| Descripción | (ADC8494 - ADC8497) Precision Thermocouple Amplifiers | |
| Fabricantes | Analog Devices | |
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Precision Thermocouple Amplifiers
with Cold Junction Compensation
AD8494/AD8495/AD8496/AD8497
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 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
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
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 self-
heating, 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) thermo-
couples. See Table 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 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.
Trademarksandregisteredtrademarksarethepropertyoftheirrespectiveowners.
FUNCTIONAL BLOCK DIAGRAM
REF
+IN ESD AND
OVP
AD8494/AD8495/
AD8496/AD8497
A2
THERMO-
COUPLE
COLD JUNCTION
COMPENSATION
A3
–IN
1MΩ
ESD AND
OVP
A1
Figure 1.
SENSE
OUT
Table 1. Device Temperature Ranges
Thermo-
Optimized Temperature Range
Couple Ambient Temperature Measurement
Part No. Type
(Reference Junction) Junction
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
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 thermo-
couple’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 VS ± 25 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: 781.329.4700
www.analog.com
Fax: 781.461.3113 ©2010–2011 Analog Devices, Inc. All rights reserved.
Free Datasheet http://www.datasheet4u.com/
1 page  
ABSOLUTE MAXIMUM RATINGS
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
Rating
±18 V
+VS – 25 V
–VS + 25 V
±VS
Indefinite
−65°C to +150°C
−40°C to +125°C
140°C
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.
AD8494/AD8495/AD8496/AD8497
THERMAL RESISTANCE
θJA is specified for a device on a 4-layer JEDEC PCB in free air.
Table 4.
Package
8-Lead MSOP (RM-8)
θJA Unit
135 °C/W
ESD CAUTION
Rev. C | Page 5 of 16
Free Datasheet http://www.datasheet4u.com/
5 Page 
THEORY OF OPERATION
THERMOCOUPLES
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.
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.
MEASUREMENT
JUNCTION
REFERENCE
JUNCTION
PCB
TRACES
AD849x
THERMOCOUPLE WIRES
Figure 26. Thermocouple Junctions
To derive the temperature at the measurement junction (TMJ),
the user must know the differential voltage created by the thermo-
couple. 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.
AD8494/AD8495/AD8496/AD8497
Table 6. J Type Thermocouple Voltages and AD8494 Readings
Measurement
Junction
Temperature
(TMJ)
Reference
Junction
Temperature
(TRJ)
Thermocouple
Voltage
AD8494
Reading
50°C
0°C
+2.585 mV
250 mV
50°C 50°C 0 mV 250 mV
0°C 0°C 0 mV 0 mV
0°C
50°C
−2.585 mV
0 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.
REF
+IN ESD AND
OVP
AD8494/AD8495/
AD8496/AD8497
A2
THERMO-
COUPLE
COLD JUNCTION
COMPENSATION
A3
OUT
–IN
1MΩ
ESD AND
OVP
A1
Figure 27. Block Diagram
SENSE
The AD849x output is a voltage that is proportional to the tem-
perature 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 accu-
rately maintained for a given temperature measurement range.
• 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.
Rev. C | Page 11 of 16
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11 Page | ||
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