CN-0172: High Accuracy Multichannel Thermocouple Measurement Solution

Circuit Note
CN-0172
Devices Connected/Referenced
Circuits from the Lab™ reference circuits are engineered and
tested for quick and easy system integration to help solve today’s
analog, mixed-signal, and RF design challenges. For more
information and/or support, visit www.analog.com/CN0172.
ADT7320
±0.25°C Accurate, 16-Bit Digital SPI
Temperature Sensor
AD7793
3-Channel, Low Noise, Low Power, 24-Bit
Σ-Δ ADC with On-Chip In-Amp and
Reference
3-Channel Thermocouple Temperature Measurement System with 0.25°C Accuracy
EVALUATION AND DESIGN SUPPORT
Circuit evaluation boards
CN-0172 circuit evaluation board (EVAL-CN0172-SDPZ)
System Demonstration Platform (EVAL-SDP-CB1Z)
CN0172 breakout board (included with EVAL-CN0172SDPZ board)
Design and integration files
Schematics, Layout Files, Bill of Materials, Software
This type of application is popular where a cost-effective, accurate
temperature measurement is required over the wide temperature
ranges offered by thermocouples.
CIRCUIT DESCRIPTION
The circuit in Figure 1 is designed to measure the temperature
of three K type thermocouples at the same time using the AD7793
24-bit Σ-Δ ADC. The reference junction temperature is measured
using the ADT7320, a ±0.25°C accurate, 16-bit digital SPI
temperature sensor.
CIRCUIT FUNCTION AND BENEFITS
Thermocouple Voltage Measurement
The function of the circuit shown in Figure 1 is to provide a high
accuracy multichannel thermocouple measurement solution.
Achieving a precision thermocouple measurement requires a
signal chain of precision components that amplifies the small
thermocouple voltage, reduces noise, corrects nonlinearity, and
provides accurate reference junction compensation (commonly
referred to as cold junction compensation). This circuit addresses
all these challenges for measuring thermocouple temperature
with better than ±0.25°C accuracy.
The interface between the thermocouple and the AD7793 ADC
is a thermocouple connector and filter. Each connector (J1, J2,
and J3) connects directly to a set of differential ADC inputs.
The filter on the inputs to the AD7793 reduces any noise pickup
in the thermocouple leads before the signal reaches the AIN(+)
and AIN(−) inputs of the ADC. The AD7793 has an on-chip
multiplexer, buffer, and instrumentation amplifier to amplify the
small voltage from the thermocouple measurement junction.
The circuit shown in Figure 1 shows how three K type thermocouples are connected to the AD7793 precision 24-bit sigmadelta (Σ-Δ) analog-to-digital converter (ADC) to measure the
thermocouple voltage. Because the thermocouple is a differential
device rather than an absolute temperature measurement device,
the reference junction temperature must be known to get an
accurate absolute temperature reading. This process is known
as reference junction compensation, commonly referred to as
cold junction compensation. In this circuit, the ADT7320
precision 16-bit digital temperature sensor is used for the cold
junction reference measurement and provides the required
accuracy.
The ADT7320 precision 16-bit digital temperature sensor is used
to measure the reference (cold) junction temperature with an
accuracy of ±0.25°C over the −20°C to +105°C temperature
range. The ADT7320 is fully calibrated at the factory, and no user
calibration is required. The ADT7320 contains an internal band
gap reference, a temperature sensor, and a 16-bit Σ-Δ ADC to
measure and digitize the temperature to 0.0078°C resolution.
Cold Junction Measurement
Both the AD7793 and ADT7320 are controlled by an SPI interface
using the System Demonstration Platform (EVAL-SDP-CB1Z).
Both AD7793 and ADT7320 can also be controlled by a microcontroller.
Rev. A
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engineers. Standard engineering practices have been employed in the design and construction of
each circuit, and their function and performance have been tested and verified in a lab environment at
room temperature. However, you are solely responsible for testing the circuit and determining its
suitability and applicability for your use and application. Accordingly, in no event shall Analog Devices
be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause
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CN-0172
Circuit Note
FILTERING
5
AD7793
R20
1kΩ
R11
1kΩ
SPI_DOUT
4
6
5
4 IOUT 1
CLK
11 IOUT 2
CS
J6
CS1
3
CS3
4
8
9
GND
CS4
7
VDD
6
CS3
CS2
CS1
SPIDIN
SPIDOUT
SPICLK
5
NC
NC
NC
NC
8
VDD
GND
ADT7320 EPAD
CT
U4
3 DIN
4 CS
5
NC
6
4
INT 9
16 15 14 13
1 SCLK
2 DOUT
CS4
3
7
C1
0.1µF
REFERENCE (COLD) JUNCTION
COMPENSATION FOR CHANNEL 3 (J3)
CS1
3
1
2
SPI_CLK
6
12
11
17
10
2
VDD VDD = 3.3V
CS2
2
VDD
GND
ADT7320 EPAD
CT
U3
3 DIN
4 CS
5
SPI_CLK
10 AIN3(–)/REFIN(–) SCLK 1
SPI_DIN
1
NC
NC
3
SPI_DIN
NC
C7
0.01µF
DIN
1 SCLK
2 DOUT
5
CS2
CS3
NC
R9
1kΩ
C11
0.01µF
8
16 15 14 13
1
2
16
C1
0.1µF
INT 9
6
7
12
11
17
10
C1
0.1µF
INT 9
NC
VBIAS
–
THERMOCOUPLE
REFERENCE
JUNCTION
C13
0.1µF
9 AIN3(+)/REFIN(+)
J5
NC
DOUT/RDY
R8
1kΩ
J3
+
SPI_DOUT
12
11
17
10
REFERENCE (COLD) JUNCTION
COMPENSATION FOR CHANNEL 2 (J2)
AIN2(–)
15
7
NC
8
C6
0.01µF
6
NC
R7
1kΩ
5
12
GND
C8
0.01µF
NC
VBIAS
–
THERMOCOUPLE
REFERENCE
JUNCTION
7 AIN2(+)
C10
0.1µF
NC
+
6
NC
R3
1kΩ
3 DIN
4 CS
NC
4
VDD
GND
ADT7320 EPAD
CT
U2
NC
3
C5
C4
+ 10µF 0.1µF
U1
1 SCLK
2 DOUT
NC
13
AVDD
16 15 14 13
1
2
NC
CHANNEL 3
THERMOCOUPLE
MEASUREMENT
JUNCTION
14
DVDD
6 AIN1(–)
C2
0.01µF
J2
CHANNEL 2
THERMOCOUPLE
MEASUREMENT
JUNCTION
AIN1(+)
C1
0.01µF
J4
NC
–
THERMOCOUPLE
REFERENCE
JUNCTION
C3
0.1µF
R2
1kΩ
VDD
NC
VBIAS
5
NC
J1
+
NC
CHANNEL 1
THERMOCOUPLE
MEASUREMENT
JUNCTION
REFERENCE (COLD) JUNCTION
COMPENSATION FOR CHANNEL 1 (J1)
R1
1kΩ
8
CS4
SPI_DIN
SPI_DOUT
SPI_CLK
J7
VDD
1
2
3
4
5 6
1
J9
2
3
4
J10
5 6
1
2
3
4
5 6
J11
09240-001
NOTES:
1. EPAD = EXPOSED PADDLE OF THE LFCSP PACKAGE (ADT7320)
2. THE REFERENCE (COLD) JUNCTION COMPENSATION SENSOR BOARDS CONNECTIONS TO THE REST OF THE CIRCUIT:
A) CHANNEL 1 (J1): J4 TO J9
B) CHANNEL 2 (J2): J5 TO J10
C) CHANNEL 3 (J3): J6 TO J11
09240-002
Figure 1. Multichannel Thermocouple Measurement System (Simplified Schematic: All Connections and Decoupling Not Shown)
Figure 2. EVAL-CN0172-SDPZ Circuit Evaluation Board
Rev. A | Page 2 of 6
Circuit Note
CN-0172
Figure 2 shows the EVAL-CN0172-SDPZ circuit evaluation board
with three K type thermocouple connectors, the AD7793 ADC,
and the ADT7320 temperature sensor mounted on a separate
flexible printed circuit board (PCB) between two copper contacts
for the reference temperature measurement.
Figure 3 shows a side view of the ADT7320 mounted on a separate
flexible PCB and inserted between the two copper contacts of
the thermocouple connector. The flexible PCB shown in Figure 3
has advantages over small FR4-type PCBs because it is thinner
and more flexible. This allows the ADT7320 to be mounted neatly
in between the copper contacts of the thermocouple connector
to minimize temperature gradients between the reference
junction and ADT7320.
The advantages of the exposed tip are that it provides the best
heat transfer, has the quickest response time, and is low cost and
light weight. The disadvantages of the exposed tip are that it is
susceptible to mechanical damage and corrosion. As a result, it
is not suitable for harsh environments. However in cases where
quick response time is needed, the exposed tip is the best choice. If
an exposed tip is required in an industrial environment, electrical
isolation may be required in the signal chain. This can be addressed
by using digital isolators (see www.analog.com/icoupler).
Unlike a traditional thermistor or resistance temperature detector
(RTD), the ADT7320 is a fully plug-and-play solution that does
not require multipoint calibration after board assembly or
consume processor and memory resources with calibration
coefficients and linearization routines. The ADT7320 avoids
self-heating issues that undermine the accuracy of traditional
resistive sensor solutions because it only dissipates 700 µW
(typical) of power at 3.3 V.
Guidelines for Accurate Temperature Measurement
09240-003
The following guidelines ensure that the ADT7320 accurately
measures the temperature of the reference junction.
Figure 3. Side View of ADT7320 Mounted on a Flexible PCB
The small size and thinness of the flexible PCB also allows the
ADT7320 to quickly respond to temperature changes at the
reference junction. Figure 4 shows the typical thermal response
time of the ADT7320.
140
125°C
120
DUT TEMPERATURE (°C)
105°C
100
85°C
IT TAKES LESS THAN 2 SECONDS
FOR THE DUT TO REACH 63.2%
OF ITS FINAL TEMPERATURE SPAN
40
0
5
10
15
20
TIME (s)
25
09240-004
20
0
Decoupling: The ADT7320 must have a decoupling capacitor
mounted as close as possible to VDD to ensure accurate
temperature measurement. A decoupling capacitor, such as a
0.1 µF high frequency ceramic type is recommended. In addition,
use a low frequency decoupling capacitor in parallel with the
high frequency ceramic, such as a 10 µF to 50 µF tantalum
capacitor.
Maximizing Thermal Conduction: The primary thermal path
from the reference junction to the ADT7320 is through the
plastic package and the backside exposed paddle (GND). Because
the copper contacts are connected to the ADC inputs, the backside
paddle cannot be connected in this application because it affects
the biasing to the ADC inputs.
80
60
Power Supply: If the ADT7320 is powered from a switching
regulator, noise generated above 50 kHz may affect the temperature
accuracy specification. To prevent this, use an RC filter between
the power supply and VDD. Carefully choose the value of
components used to ensure that the peak value of the supply
noise is less than 1 mV.
Figure 4. ADT7320 Typical Thermal Response Time
This solution is flexible and allows other types of thermocouples
to be used, such as J type or T type thermocouples. The K type
was selected for this circuit note because of its popularity. The
actual thermocouple chosen has an exposed tip. The measurement
junction is outside the probe wall and is exposed to the target
medium.
Rev. A | Page 3 of 6
CN-0172
Circuit Note
Guidelines for Accurate Voltage Measurement
COMMON VARIATIONS
The following guidelines ensure that the AD7793 accurately
measures the voltage at the thermocouple measurement junction.
For applications that require less accuracy and precision, the
AD7792 16-bit Σ-Δ ADC can be used instead of the AD7793
24-bit Σ-Δ ADC. For the reference temperature measurement,
the ±0.5°C accurate ADT7310 digital temperature sensor can be
used instead of the ±0.25°C accurate ADT7320. Both the AD7792
and ADT7310 are available with an SPI interface.
Decoupling: The AD7793 must have decoupling capacitors
mounted as close as possible to both AVDD and DVDD to ensure
accurate voltage measurement. Decouple AVDD with a 10 µF
tantalum capacitor in parallel with a 0.1 µF ceramic capacitor to
GND. In addition, decouple DVDD with a 10 µF tantalum capacitor
in parallel with a 0.1 µF ceramic capacitor to GND. Refer to
Tutorial MT-031 and Tutorial MT-101 for more discussion on
grounding, layout techniques, and decoupling techniques.
Filtering: The differential inputs of the AD7793 act to remove
most of the common-mode noise on the thermocouple lines.
Differential low-pass filters comprising R1, R2, and C3, for
example, placed at the front end of the AD7793 reduce noise
pickup that can be present in the thermocouple leads. The C1
and C2 capacitors provide additional common-mode filtering.
Because the AIN(+) and AIN(−) analog inputs to the ADC are
differential, most of the voltages in the analog modulator are
common-mode voltages. The excellent common-mode rejection
(100 dB minimum) of the AD7793 further removes commonmode noise on these inputs.
Other Challenges Resolved with this Solution
The following summarizes how other challenges mentioned
earlier with thermocouples are resolved with this solution.
Thermocouple Voltage Amplification: The output voltage of the
thermocouple changes by only a few µV per degree. In this case,
the popular K type thermocouple changes 41 µV/°C. This small
signal requires a large gain stage before the ADC conversion.
The AD7793 internal programmable gain amplifier (PGA)
provides a gain up to 128. For this solution, a gain of 16 was
used to allow the AD7793 to run its own internal full-scale
calibration using its internal reference.
Correction for Thermocouple Nonlinearity: The AD7793 provides
excellent linearity across a wide temperature range (−40°C to
+105°C), requiring no correction or calibration by the user.
To determine the actual thermocouple temperature, the reference
temperature measurement is first converted into an equivalent
thermoelectric voltage using equations provided by the National
Institute of Standards and Technology (NIST). This voltage is
added to the thermocouple voltage measured by the AD7793,
and the summation is then translated back into a thermocouple
temperature, again using NIST equations. An alternative approach
involves using look-up tables. However, to get the same accuracy,
the size of the lookup table could be substantial which would
require additional memory resources from the host controller. All
the processing is done in the software using the EVAL-SDP-CB1Z.
CIRCUIT EVALUATION AND TEST
The system described uses the EVAL-CN0172-SDPZ and the
EVAL-SDP-CB1Z. The CN0172 Breakout Board is included
with the EVAL-CN0172-SDPZ board.
Equipment Needed
The following equipment is needed:
•
•
•
•
•
•
•
•
•
•
An Oil bath
The EVAL-CN0172-SDPZ circuit evaluation board
The CN0172 Breakout Board (Included with the EVALCN0172-SDPZ board)
The EVAL-SDP-CB1Z circuit evaluation board
The CN0172 evaluation board software
A Datron 4808 calibrator
A Hart Scientific 1590 super thermometer
A Hart Scientific precision probe
GPIB cables (3)
A PC and Windows XP or later running LabVIEW with a
GPIB card and an USB 2.0 port
Setup and Test
The test setup shown in Figure 5 was used to evaluate the
performance of the multichannel thermocouple solution. A
Datron calibrator was used to provide a precise voltage source
for the three thermocouple inputs. The temperature of the oil
baths was measured with the super thermometer and controlled
via the GPIB bus.
LabVIEW software for the CN0172 controls the EVALCN0172-SDPZ via the USB port, the EVAL-SDP-CB1Z, Breakout
Board, and the SPI bus. The power for the EVAL-SDP-CB1Z is
obtained from the USB bus, and the 3.3 V output of the EVALSDP-CB1Z supplies power for the EVAL-CN0172-SDPZ.
If oil bath measurements are not required, the EVAL-CN0172SDPZ can be used to measure the three thermocouple
temperatures by using the USB interface with a PC and the
software provided on the CD.
Information and details regarding test setup and calibration, and
how to use the evaluation software for data capture can be found in
the CN0172 User Guide found at: www.analog.com/CN0172UserGuide.
Complete schematics and layouts for the EVAL-CN0172-SDPZ
can be found in the CN-0172 Design Support Package:
www.analog.com/CN0172-DesignSupport.
Rev. A | Page 4 of 6
Circuit Note
CN-0172
HART SCIENTIFIC
PRECISION PROBE
OIL BATH FOR REFERENCE (COLD JUNCTION)
DATRON 4808 CALIBRATOR
(VOLTAGE SOURCE)
CH1
CH2
VOUT
CH3
EVAL-CN0172-SDPZ
GPIB BUS
SPI BUS AND
+3.3V POWER SUPPLY
9-PIN
CONNECTOR
120-PIN
CONNECTOR
USB
PC
SDP BOARD
CN0172
BREAKOUT
BOARD
09240-005
HART SCIENTIFIC
1590 SUPER THERMOMETER
Figure 5. Test Setup Functional Block Diagram
Test Results
Figure 6 shows a plot of the thermocouple solution error over
various thermocouple temperatures using various fixed values of
cold junction (CJ) temperatures. Total solution error ≤ ±0.25°C is
achieved over a wide temperature range. Note that the accuracy
of the solution can be further improved by performing a system
calibration of the AD7793 ADC.
Figure 7 shows a plot of the thermocouple solution error over
various CJ temperatures using various fixed values of thermocouple
temperatures. Total solution error ≤ ±0.25°C is achieved over a
wide temperature range.
0.25
0.20
0.15
0.25
0.10
0.15
ERROR (°C)
0.10
0.05
0
–0.05
0.05
–0.10
0
–0.15
–0.05
–0.20
–0.10
–0.20
–0.25
–270
CJ = –20°C
CJ = +20°C
CJ = +60°C
CJ = +100°C
–70
130
–0.25
–20
CJ = 0°C
CJ = +40°C
CJ = +80°C
CJ = +105°C
330
530
0
20
40
60
TC = –70°C
TC = +480°C
TC = +880°C
80
COLD JUNCTION TEMPERATURE (°C)
730
THERMOCOUPLE TEMPERATURE (°C)
930
1130
09240-006
–0.15
TC = –270°C
TC = +280°C
TC = +680°C
TC = +1080°C
Figure 6. Error vs. Thermocouple Temperature for Fixed Cold Junction (CJ)
Temperature
Rev. A | Page 5 of 6
100
09240-007
ERROR (°C)
0.20
Figure 7. Error vs. Cold Junction Temperature for Fixed Thermocouple
Temperatures
CN-0172
Circuit Note
LEARN MORE
Data Sheets and Evaluation Boards
CN-0172 Design Support Package:
http://www.analog.com/CN0172-DesignSupport
CN-0172 Circuit Evaluation Board (EVAL-CN0172-EB1Z)
Standard Development Platform Board (EVAL-SDP-CB1Z)
Duff, Matthew, and Joe Towey, Two Ways to Measure
Temperature Feature Simplicity, Accuracy and Flexibility ,
Analog Dialogue,Vol 44, October 2010.
ADT7320 Data Sheet and Evaluation Board
ADT7310 Data Sheet and Evaluation Board
AD7793 Data Sheet and Evaluation Board
Thermocouple 101: What is a Thermocouple? ADI Video.
AD7792 Data Sheet and Evaluation Board
McNamara, Donal, Temperature Measurement Theory and
Practical Techniques, AN-892 Application Note, Analog
Devices.
REVISION HISTORY
AD779x Instrumentation Converters, Frequently Asked
Questions.
Changes to Title .................................................................................1
8/13—Rev. 0 to Rev. A
ADT7320/ADT7420 Digital Temperature Sensors, Frequently
Asked Questions.
12/12—Revision 0: Initial Version
Kester, Walt. 1999. Sensor Signal Conditioning. Section 7. Analog
Devices.
MT-004 Tutorial, The Good, the Bad, and the Ugly Aspects of
ADC Input Noise—Is No Noise Good Noise? Analog Devices.
MT-022 Tutorial, ADC Architectures III: Σ-Δ ADC Basics,
Analog Devices.
MT-023 Tutorial, ADC Architectures IV: Σ-Δ ADC Advanced
Concepts and Applications, Analog Devices.
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of "AGND" and "DGND", Analog Devices.
MT-101 Tutorial, Decoupling Techniques, Analog Devices.
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CN09240-0-8/13(A)
Rev. A | Page 6 of 6
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