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Circuit Note
CN-0354
Devices Connected/Referenced
Precision Thermocouple Amplifier with
AD8495
Cold Junction Compensation
Circuits from the Lab® reference designs are engineered and
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AD7787
Low Power, 24-Bit Σ-Δ ADC
REF194
Precision Micropower, Low Dropout
Voltage Reference
ADG1609
4.5 Ω RON, 4-Channel ±5 V, +12 V, +5 V, and
+3.3 V Multiplexer
ADR3412
Micropower, High Accuracy Voltage Reference
ADM8829
Switched Capacitor Voltage Inverter
Low Power Multichannel Thermocouple Measurement System with
Cold Junction Compensation
EVALUATION AND DESIGN SUPPORT
cold junction compensation (0°C to 50°C) and converts the
thermocouple output to a voltage with a precise scale factor of
5 mV/°C. The error is less than 2°C, over a measurement range
of −25°C to +400°C, and is primarily due to the thermocouple
nonlinearity. A nonlinearity correction algorithim reduces the
error to less than 0.5°C over a 900°C measurement range. Noise
free resolution is less than 0.1°C.
Circuit Evaluation Boards
CN-0354 Evaluation Board (EVAL-CN0354-PMDZ)
System Demonstration Platform (EVAL-SDP-CB1Z)
SDP-I-PMOD Interposer Board (PMD-SDP-IB1Z)
Design and Integration Files
Schematics, Layout Files, Bill of Materials
The signal is then digitized by a 24-bit Σ-Δ ADC, and the digital
value is provided on an SPI serial interface. With the PMOD
form factor for rapid prototyping, the design requires minimal
PC board area and is ideal for applications that require precise
thermocouple temperature measurements.
CIRCUIT FUNCTION AND BENEFITS
The circuit shown in Figure 1 is a flexible, 4-channel, low power
thermocouple measurement circuit with an overall power
consumption of less than 8 mW. The circuit has a multiplexed
front end, followed by an instrumentation amplifier that performs
ALUMEL
REFERENCE
JUNCTION
CHROMEL
5V
P2
0.01µF
+ V1 –
ALUMEL
CHROMEL
ISOTHERMAL
BLOCK
SENSE
100Ω
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Cu
A0 A1 EN
1.1kΩ AIN1(–)
SERIAL
INTERFACE
AND
CONTROL
LOGIC
Σ-Δ
ADC
DOUT/RDY
SCLK
DOUT/RDY
CS
AIN2
CLOCK
GND
GND
OUT
AD8495
0.01µF
1.2V
P1
1
−5V
BUF
MUX
IN-AMP
REF
ADG1609
AIN1(+)
1nF
1µF
100Ω
1MΩ
AD7787
VDD
COLD
JUNCTION
COMPENSATION
5V
REFIN
5V
REF194
IN
EN
OUT
ADR3412
4.5V
1.2V
IN
OUT
EN GND
2 3
ADM8829
ISOTHERMAL REGION
IN
OUT
CAP+ CAP− GND
−5V
4.7µF
4.7µF
12641-001
+ V2 – Cu
Cu
4.5V
5V
VDD
Figure 1. Multichannel K Type Thermocouple Measurement System (Simplified Schematic: All Connections and Decoupling Not Shown)
Rev. A
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CN-0354
Circuit Note
The thermocouple voltage generated at the output of the
isothermal block is proportional to the difference between the
temperature at the measurement thermocouple and the
temperature of the isothermal block (cold junction).
The signal is amplified by the AD8495, a precision instrumentation
amplifier that is laser trimmed to provide a precise 5 mV/°C
output for a K type thermocouple. The AD8495 also provides
cold junction compensation over a range of 0°C to 50°C.
The fifth thermocouple, shown in Figure 1, is added to cancel
the voltage generated by any temperature differential that exists
between the isothermal block and the AD8495 cold junction
compensation circuit. With the multiplexer enabled, the constantan
(alumel) copper junction, formed by the thermocouple connection
at the isothermal block, is in series with a copper constantan
(alumel) junction, formed by the reference thermocouple
connection. This series combination contributes equal but
opposite voltages because the block is isothermal. In this
condition, the AD8495 internal cold junction compensation
compensates for the reference junction at the IC, which must
remain between 0°C and 50°C. Therefore, the fifth thermocouple
connection eliminates the requirement of mounting the AD8495
directly on the isothermal block, as is normally the case.
Thermocouple Signal Conditioning
Thermocouples are used in temperature measurements
requiring high temperature ranges. They are preferred over
resistance temperature detectors (RTDs) for this reason, as well
as for their lower cost. However, thermocouples are nonlinear,
which means that the voltage generated by a thermocouple
changes at different rates and at different temperatures. For
example, a J type thermocouple changes by 52 µV/°C at 25°C
and by 55 µV/°C at 150°C. K type thermocouples tend to be
more linear, at approximately 41 µV/°C, when temperatures
are above 0°C. The voltage response of a thermocouple to a
temperature gradient is generally described by a polynomial of
greater than the sixth-order.
Figure 2 shows the Seebeck coefficient of the different types of
thermocouples over their operating temperature ranges. Figure 2
shows that the K type thermocouple has the widest temperature
range and can measure temperatures up to 1250°C.
100
The output of the AD8495 is filtered by the 1.1 kΩ/1 nF singlepole filter that has a −3 dB cutoff frequency of 145 kHz. This
filter minimizes broadband noise at the AD7787 ADC input.
The AD7787 is a 24-bit, low noise, low power Σ-Δ ADC for low
frequency measurement applications, such as thermocouple
measurement systems. Its internal clock eliminates the need for
an external clock and makes the output data rate user configurable,
which can reduce current consumption because it functions at a
lower internal clock frequency. It contains a Σ-Δ ADC with one
differential input and one single-ended input, either of which can
be buffered or unbuffered after passing through the multiplexer.
The AD7787 operates from an internal clock. Therefore, the
user does not have to supply a clock source to the device. The
output data rate from the device is software programmable and
can be varied from 9.5 Hz to 120 Hz, with the rms noise equal
to 1.1 µV at the lower update rate. The AD7787 operates at an
update rate of 9.5 Hz in this circuit. The internal clock frequency
can be divided by a factor of 2, 4, or 8, which leads to a reduction in
the current consumption. The update rate, cutoff frequency,
and settling time all scale with the clock frequency.
80
E
J
T
60
40
K
20
0
–400
–200
0
200
400
600
800
1000
1200
1400
TEMPERATURE (°C)
12641-002
The four thermocouple inputs to the circuit are terminated at
the isothermal block, P2. The ADG1609 complementary metal
oxide semiconductor (CMOS) analog multiplexer switches the
four thermocouple channels to a single signal conditioning
block to handle the four thermocouple inputs. The switches
exhibit a break-before-make switching action and an inherent
low charge injection to minimize transients when switching
between channels.
The AD7787 operates with a power supply from 2.5 V to 5.25 V.
When operating from the 3 V supply, the power dissipation for
the device is 225 µW maximum. It is packaged in a 10-lead MSOP.
SEEBECK COEFFICIENT (µV/°C)
CIRCUIT DESCRIPTION
Figure 2. Seebeck Coefficient of Thermocouple vs. Temperature
Due to the nonlinear characteristic of thermocouples, complex
signal processing and signal conditioning are required to obtain
a precise temperature reading, and the AD8495 provides an
ideal solution.
The AD8495 is trimmed to provide cold junction compensation
over a reference junction range of 0°C to 50°C and provides a
linear 5 mV/°C transfer function. Over a measurement range of
−25°C to +400°C, the maximum output error is ±2°C. To
calculate the output voltage, use the following equation:
TMJ =
VOUT − VREF
5 mV/°C
The AD8495 operates on a single 5 V supply in the circuit.
Because of the PNP transistor input structure, the input voltage
can be as low as −200 mV, thereby allowing the measurement of
negative temperatures. However, the output voltage must be
offset to process negative temperatures, which is accomplished
using the reference voltage input pin (REF).
Rev. A | Page 2 of 5
Circuit Note
CN-0354
With the REF pin grounded using the P1 jumper, the minimum
temperature that the system can measure is 5°C. The P1 jumper
also allows the REF pin bias voltage to connect to a 1.2 V ADR3412
reference, thereby allowing measurement temperatures as low as
−235°C. The corresponding temperature range for either
condition is specified in Table 1. In both cases, the temperature
span is 875°C. It is important to drive the REF pin with a low
impedance source, such as a voltage reference or a buffer
amplifier, to prevent errors.
Table 1. Measurement Ranges for REF = 0 V and 1.2 V with
Single 5 V Supply
Temperature Range
5°C to 880°C
−235°C to +640°C
Open Thermocouple Detection
The AD8495 can detect an open or broken thermocouple. The
inputs of the AD8495 are the bases of PNP transistors; therefore,
the bias current always flows out of the inputs. If either input is
open, the output goes to one of the supply rails. Connecting the
negative input to ground through a 1 MΩ resistor causes the
AD8495 output to rail high in an open thermocouple condition.
In cases where any of the four channels are not used, short the
inputs to prevent the AD8495 from railing to the positive
supply if that channel is connected. The 1 MΩ resistor also
provides a bias current return path to ground.
The AD8495 has a high common-mode rejection, thereby
minimizing the common-mode noise picked up by long
thermocouple leads. The high impedance inputs of the
amplifier make it easy to add extra filtering for additional
electromagnetic interference/radio frequency interference
(EMI/RFI) protection.
Table 2. Circuit Current Consumption
Part Number
AD8495
AD7787
ADG1609
REF194
ADR3412
ADM8829
Maximum Current Consumption
250 µA
160 µA
1 µA
45 µA
100 µA
1000 µA
Note that the total current is 1556 µA.
Test Results
The AD8495 has a maximum temperature error due to the
thermocouple nonlinearity of ±2°C from −25°C to +400°C, for a
reference junction temperature of 0°C to 50°C. Wider temperature
ranges or better accuracy requires a linearity correction algorithm
that can be implemented in the software. Nonlinearity correction is
discussed in the AN-1087 Application Note, Thermocouple
Linearization When Using the AD8494/AD8495/AD8496/AD8497.
Figure 3 shows the linearity error of the circuit, with and
without the correction algorithm implemented.
2.0
1.5
1.0
The ADM8829 switched capacitor voltage inverter provides the
−5 V required by the ADG1609 multiplexer to accommodate
negative temperatures.
Rev. A | Page 3 of 5
–0.5
AD8495
CN-0354
CN-0354 WITH
NONLINEARITY
CORRECTION
–1.0
–1.5
900
800
12641-003
TEMPERATURE (°C)
700
600
500
400
300
–2.0
200
The REF194 supplies the 4.5 V reference for the AD7787. The
ADR3412 provides the 1.2 V optional offset voltage for the REF
input of the AD8495. Jumper P1 allows the REF pin to be
connected either to 1.2 V or to ground.
0
100
The circuit in Figure 1 is powered from a single 5 V supply that
powers the ADG1609, the AD8495, the VDD pin of the
AD7787, the REF194, the ADR3412, and the ADM8829.
0.5
0
Power Considerations
ERROR (°C)
REF Pin Voltage
0 V or Grounded
1.2 V
Table 2 shows the current consumed by each device in the
circuit, based on the data sheet specifications. The design
consumes a maximum of 1.56 mA. Note that the maximum
current consumption is due to the ADM8829 switched capacitor
voltage inverter. This inverter can be eliminated if a negative
supply is available to drive the VSS pin of the ADG1609,
thereby reducing the total current to approximately 556 µA.
Figure 3. Linearity Error with and Without the Correction Algorithm
Implemented
CN-0354
Circuit Note
The noise of the system was tested in a controlled temperature
environment to check the noise free code resolution of the
system. Figure 4 shows the noise distribution, which spans
approximately 1098 codes. For a 24-bit resolution and a span
of 900°C, 1 LSB = 900°C/224 = 0.07°C resolution.
Equipment Required
The following equipment is needed:
200
•
•
•
•
•
•
175
Software Installation
 16,777,216 

Noise Free Resolution = log 2 
 Code Spread 


225
NUMBER OF OCCURENCES
CIRCUIT EVALUATION AND TEST
The CN-0354 evaluation kit includes self installing software on
a CD. The software is compatible with Windows® XP (SP2),
Windows Vista (32-bit and 64-bit), and Windows 7 (32-bit and
64-bit). If the setup file does not run automatically, run the
setup.exe file from the CD. Install the evaluation software
before connecting the evaluation board and the SDP board to
the USB port of the PC to ensure that the evaluation system is
correctly recognized when connected to the PC. The software
allows full configuration of the serial interface. It is important
that the configurations of the masters and the slaves match for
proper operation.
150
125
100
75
50
25
12641-004
0709C3
0709EC
070A15
070A3E
070A67
070A90
070AB9
070AE2
070B0B
070B34
070B5D
070B86
070BAF
070BD8
070C01
070C2A
070C53
070C7C
070CA5
070CCE
070CF7
070D20
070D49
070D72
070D9B
070DC4
070DED
0
ADC CODE IN HEX
The EVAL-CN0354-PMDZ evaluation board
The EVAL-SDP-CB1Z system demonstration platform
The PMD-SDP-IB1Z, SDP-I-PMOD, interposer board
The CN0354 Evaluation Software
6 V wall wart power supply
PC (Windows 32-bit or 64-bit)
Figure 4. Noise Distribution Histogram of Circuit with the AD7787 Input
Buffer Disabled, Output Data Rate = 9.5 Hz
Software operation is described in the CN-0354 Software User
Guide.
Power Supply Requirements
Calculate the noise free code resolution as follows:
The EVAL-CN0354-PMDZ evaluation board must be powered
by a 5 V power supply. It is recommended that the supply provide
at least 2 mA. If powered directly from the PMD-SDP-IB1Z
interposer board, ensure that the supply is well filtered and free
of digital noise.
16,777,216 
Noise Free Resolution = log 2 
 = 13.9 Bits
 1098 
COMMON VARIATIONS
The CN-0354 circuit was proven to work with good stability
and accuracy with the devices selected for the design.
This design can also be implemented using the AD594/AD595,
which are also single chip thermocouple, signal conditioning
amplifiers, with an output of 10 mV/°C.
The AD8495 has other variants, such as the AD8494, the
AD8496, and the AD8497, for different thermocouple types,
different ambient temperature ranges, and different
measurement temperature ranges, as shown in Table 3.
Table 3. AD849x with ±2°C Accuracy Temperature Ranges
Device
AD8494
AD8495
AD8496
AD8497
Thermocouple Type
J
K
J
K
Maximum 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
Rev. A | Page 4 of 5
Measurement Temperature Range
−35°C to +95°C
−25°C to +400°C
55°C to 565°C
−25°C to +295°C
Circuit Note
CN-0354
Test Setup Functional Diagram
A functional diagram of the test setup is shown in Figure 5, and
a photo of the EVAL-CN0354-PMDZ board is shown in Figure 6.
EVAL-CFTL-6V-PWRZ
6V WALL WART
PC
Marcin, Joe. AN-369. Thermocouple Signal Conditioning Using
the AD594/AD595. Analog Devices.
120
PINS EVAL-SDP-CB1Z
SDP BOARD
MT-035 Tutorial. Op Amp Inputs, Outputs, Single-Supply, and
Rail-to-Rail Issues. Analog Devices.
12641-005
12 SDP TO PMOD
PINS INTERPOSER
BOARD
SDP
CONNECTOR
PMOD
P2
USB
J1
MT-023 Tutorial. ADC Architectures IV: Sigma-Delta ADC
Advanced Concepts and Applications. Analog Devices.
Malik, Reem. AN-1087. Thermocouple Linearization When
Using the AD8494/AD8495/AD8496/AD8497. Analog Devices.
USB CABLE
EVAL-CN0354-PMDZ
BOARD
MT-022 Tutorial. ADC Architectures III: Sigma-Delta ADC
Basics. Analog Devices.
PMD-SDP-IB1Z
MT-031 Tutorial. Grounding Data Converters and Solving the
Mystery of AGND and DGND. Analog Devices.
Figure 5. Test Setup Functional Diagram
MT-101 Tutorial. Decoupling Techniques. Analog Devices.
12641-006
NIST (National Institute of Standards and Technology) Table
for Type K Thermocouple.
Figure 6. EVAL-CN0354-PMDZ Evaluation Board
Data Sheets and Evaluation Boards
AD8495 Data Sheet
AD7787 Data Sheet
LEARN MORE
ADG1609 Data Sheet
CN0354 Design Support Package:
http://www.analog.com/CN0354-DesignSupport
REF194 Data Sheet
Corrigan, Theresa. AN-1024. How to Calculate Settling Time
and Sampling Rate of a Multiplexer. Analog Devices.
ADM8829 Data Sheet
ADR3412 Data Sheet
Duff, Matthew and Towey, Joseph. Two Ways to Measure
Temperature Using Thermocouples Feature Simplicity, Accuracy,
and Flexibility. Analog Dialogue, 44-10, October 2010.
Kester, Walt, et. al. “Temperature Sensors”, Chapter 7 in Sensor
Signal Conditioning. Analog Devices, 1999.
REVISION HISTORY
2/15—Rev. 0 to Rev. A
Change to Circuit Function and Benefits Section ........................ 1
Change to Table 1 .............................................................................. 3
9/14—Revision 0: Initial Version
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CN12641-0-2/15(A)
Rev. A | Page 5 of 5