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ADC8497 Schematic ( PDF Datasheet ) - Analog Devices

Teilenummer ADC8497
Beschreibung (ADC8494 - ADC8497) Precision Thermocouple Amplifiers
Hersteller Analog Devices
Logo Analog Devices Logo 




Gesamt 16 Seiten
ADC8497 Datasheet, Funktion
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
1M
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.
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ADC8497 Datasheet, Funktion
AD8494/AD8495/AD8496/AD8497
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
–IN
REF
–VS
NC
AD849x
18
–+
27
36
45
TOP VIEW
(Not to Scale)
NC = NO CONNECT
+IN
+VS
OUT
SENSE
Figure 2. Pin Configuration
Table 5. Pin Function Descriptions
Pin No.
Mnemonic
Description
1 −IN Negative Input.
2 REF Reference. This pin must be driven by low impedance.
3 −VS Negative Supply.
4 NC No Connect.
5
SENSE
Sense Pin. In measurement mode, connect to output; in setpoint mode, connect to setpoint voltage.
6 OUT Output.
7 +VS Positive Supply.
8 +IN Positive Input.
Rev. C | Page 6 of 16
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ADC8497 pdf, datenblatt
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 junc-
tion 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).
1M
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.
MAXIMUM ERROR CALCULATION
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.
Table 7. AD849x ±2°C Accuracy Temperature Ranges
Thermo-
Ambient
Measurement
couple Max Temperature Temperature
Part Type
Error Range
Range
AD8494 J
±2°C 0°C to 50°C −35°C to +95°C
AD8495 K
±2°C 0°C to 50°C −25°C to +400°C
AD8496 J
±2°C 25°C to 100°C +55°C to +565°C
AD8497 K
±2°C 25°C to 100°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 AN-1087 Application Note for
additional details.
Rev. C | Page 12 of 16
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