CN-0206: Thermocouple Temperature Measurement System with Less Than 500 μA...

Circuit Note
CN-0206
Devices Connected/Referenced
Circuits from the Lab™ reference circuits are engineered and
tested for quick and easy system integration to help solve today’s
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information and/or support, visit www.analog.com/CN0206.
3-Channel, Low Noise, Low Power, 24-Bit, Σ-Δ
ADC with On-Chip In-Amp and Reference
AD7793
Thermocouple Temperature Measurement System with Less Than 500 μA
Current Drain
The AD7793 consumes only 500 μA maximum, making it suitable
for any low power application, such as smart transmitters where
the complete transmitter must consume less than 4 mA. The
AD7793 has a power-down option. In this mode, the complete
ADC, along with its auxiliary functions, is powered down so
that the part consumes 1 μA maximum.
EVALUATION AND DESIGN SUPPORT
Circuit Evaluation Boards
CN-0206 Circuit Evaluation Board (EVAL-CN0206-SDPZ)
System Demonstration Platform (EVAL-SDP-CB1Z)
Design and Integration Files
Schematics, Layout Files, Bill of Materials
Because the AD7793 provides an integrated solution for thermocouple design, it interfaces directly to the thermocouple. For the
cold junction compensation, a thermistor along with a precision
resistor is used. These are the only external components required
for the cold junction measurement other than some simple RC
filters for EMC considerations.
CIRCUIT FUNCTION AND BENEFITS
The circuit shown in Figure 1 is a complete thermocouple system
based on the AD7793 24-bit sigma-delta (Σ-Δ) analog-to-digital
converter (ADC). The AD7793 is a low power, low noise, complete
analog front end for high precision measurement applications.
The device includes a programmable gain amplifier (PGA), an
internal reference, an internal clock, and excitation currents,
thereby greatly simplifying the thermocouple system design.
+5V
+
0.1µF
THERMOCOUPLE
CONNECTOR
1kΩ
+
DVDD
1kΩ
0.1µF
AD7793
AIN1(–)
0.01µF
SCLK
IOUT2
DIN
IOUT1
COLD JUNCTION
THERMISTOR
KTY81-110
1kΩ AT 25°C
2kΩ
0.1%
10ppm
DOUT/RDY
AIN2(+)
CS
AIN2(–)
REFIN(+)
REFIN(–)
CLK
GND
09776-001
–
AVDD
AIN1(+)
0.01µF
THERMOCOUPLE
10µF
Figure 1. Thermocouple Measurement System with Cold Junction Compensation (Simplified Schematic: All Connections and Decoupling Not Shown)
Rev. B
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CN-0206
Circuit Note
CIRCUIT DESCRIPTION
A Type T thermocouple is used in the circuit. This thermocouple
(made from copper and constantan) measures temperature from
−200°C to +400°C. It generates a typical temperature dependent
voltage of 40 µV/°C.
The thermocouple response is approximately linear over a small
portion of its entire temperature range, from 0°C to 60°C (see
Figure 2). To allow accurate measurements over the entire
temperature range, the CN0206-SDP-0 evaluation software
implements a linearization routine.
20
THERMOCOUPLE EMF (mV)
15
10
5
0
0
100
200
300
TEMPERATURE (°C)
400
09776-002
–5
–100
ADC Channel 2 Configuration, Thermistor
The thermistor value varies from 0°C (815 Ω) to 30°C (1040 Ω),
producing a voltage signal range of 815 mV to 1040 mV. The
precision resistor produces 2.0 V for use as an external reference.
With a gain of 1, the analog input range is ±2 V (±VREF/Gain).
This architecture gives a ratiometric configuration. Any change in
the value of the excitation current does not alter the accuracy of
the system.
APPROXIMATELY
LINEAR REGION
–200
Because the AD7793 operates from a single power supply, the
signal generated by the thermocouple must be biased above
ground so that it is within the acceptable range of the ADC. The
bias voltage generator on-board the AD7793 biases the thermocouple signal so that it has a common-mode voltage of AVDD/2.
The second channel of the ADC monitors the voltage generated
across a thermistor being driven by one of the current output
pins of the AD7793. A 1mA excitation current drives the series
pair of thermistor and precision resistor (2 kΩ, 0.1%), as shown
in Figure 1.
TYPE T THERMOCOUPLE
–10
–300
When reading the thermocouple voltage, the ADC uses the
external 2 V reference and is configured for a gain of 64. This
defines the analog input voltage range as ±31.25 mV (±VREF/Gain).
For a gain of 64, the absolute voltage on the analog inputs must
be between GND + 300 mV and AVDD − 1.1 V.
Assuming a linear transfer function between 0°C and 30°C, the
relationship between the cold junction temperature and the
thermistor resistance, R, is
Figure 2. Thermocouple EMF vs. Temperature
Cold Junction Compensation
Thermocouples measure the temperature difference between
two points, not an absolute temperature. To measure a single
temperature, maintain one of the junctions (normally the cold
junction) at a known reference temperature, and the other
junction at the temperature to be sensed.
Having a junction of known temperature is not convenient for
most applications; therefore, a thermally sensitive device is placed
on the printed circuit board (PCB). This thermistor is used to
measure the temperature of the thermocouple input connection.
The thermistor fits inside the metallic tab found on the thermocouple connection, minimizing any temperature gradients that
may exist.
Because the voltage from a known cold junction is simulated, the
appropriate correction can be applied. See the Thermocouple
Linearization section for more detailed information on processing
and manipulating the thermocouple and thermistor voltages to
produce an accurate temperature reading.
ADC Channel 1 Configuration, Thermocouple
The temperature range for the thermocouple is −200°C to +400°C.
The typical temperature dependent voltage generated by the
thermocouple is 40 µV/°C. This voltage generates a thermocouple
voltage range of −8 mV to +16 mV.
Cold Junction Temperature = 30 × (R − 815)/(1040 − 815)
One other consideration is the output compliance of the IOUT1
pin of the AD7793. When the 1 mA excitation current is used,
the output compliance equals AVDD − 1.1 V. This specification is
met because the maximum voltage at IOUT1 equals the voltage
across the precision resistor plus the voltage across the thermistor,
which equals 2 V + 1.04 V = 3.04 V.
Output Coding
The output code for an input voltage on either channel is
Code = 2N − 1 × [(AIN × Gain/VREF) +1]
where:
AIN is the analog input voltage.
GAIN is the in-amp setting
N = 24.
The EVAL-SDP-CB1Z analog microcontroller processes the
conversions from the AD7793.
Thermocouple Linearization
As mentioned in the Circuit Description section, the thermocouple is only approximately linear over a small temperature
range. In fact, the thermocouple is highly nonlinear over the
rest of the temperature range. For this reason, a linearization
procedure is implemented in the CN0206-SDP-0 LabVIEW
software.
Rev. B | Page 2 of 5
Circuit Note
CN-0206
1.2
0.8
300
0.6
200
0.4
0.2
100
0
0
–0.2
–100
–0.4
–200
OUTPUT TEMPERATURE ERROR (°C)
1.0
400
–0.6
09776-003
400
350
300
250
200
150
50
100
0
–50
–0.8
–100
–300
–150
The evaluation software then searches the modified look-up table
for the thermocouple input voltage, as sampled by the AD7793.
When the CN0206-SDP-0 evaluation software finds what two
points this thermoelectric voltage lies between, a linear interpolation
is performed to precisely calculate the thermocouple temperature.
500
–200
To implement the cold junction compensation previously
mentioned, the CN0206-SDP-0 evaluation software takes the
Type T thermocouple look-up table and searches for the
thermoelectric voltage associated with the cold junction
temperature. It then subtracts this thermoelectric voltage
from every element in the look-up table to produce a cold
junction compensated thermoelectric voltage look-up table.
According to Figure 3, the error over the entire temperature
range is less than 1°C. However, over most of the range, the
error is less than 0.5°C
OUTPUT TEMPERATURE READING (°C)
The National Institute of Standards and Technology provides
ITS-90 look-up tables for thermocouples. Each table contains a
list of thermoelectric voltages (mV) and their corresponding
temperatures.
TEMPERATURE (°C)
System Calibration
In addition to linearizing the thermocouple temperature, the
evaluation software performs a two-point calibration. The user
must input the lowest and highest temperature and use the
CN0206-SDP-0 evaluation software to produce the corresponding
temperature readings.
The software takes these readings and calculates the gain and offset,
and then applies these values to any future thermocouple
temperature calculations.
Figure 3. Linearized and Calibrated Output Temperature (with Associated
Error Plot) vs. Input Temperature
The peak-to-peak noise of the AD7793 was determined by
shorting the input pins of the ADC together and acquiring 1000
samples. As seen in the histogram found in Figure 4, the code
spread is approximately 220 codes, which translates to a
temperature spread of 0.02°C peak-to-peak.
35
System Noise Considerations
30
For an output data rate of 16.7 Hz and a gain of 64, the rms noise of
the AD7793 equals 0.086 µV (noise is referred to input). The peakto-peak noise is
COUNTS
25
6.6 × RMS Noise = 6.6 × 0.086 µV = 0.5676 µV
If the thermocouple has a sensitivity of precisely 40 µV/°C, the
thermocouple should measure the temperature to a resolution of
20
15
10
0.5676 µV ÷ 40 µV = 0.014°C
5
Test Data and Results
By sweeping the CL540ZA input source from −200°C to +400°C
in +5°C increments, the CN0206-SDP-0 evaluation software
was able to capture, linearize, and calibrate the system according to
the two user-defined calibration points.
7FFF40
7FFF60
7FFF80 7FFFA0 7FFFC0 7FFFE0
ADC CODE
800000
800020
800040
09776-004
0
All data capture was performed using the CN0206-SDP-0
LabVIEW evaluation software. A CL540ZA source (and
appropriate Type T cable) was used to simulate the thermocouple input. The CL540ZA is capable of simulating several
different types of thermocouples (J, T, E, K, R, S, B, N, and so on).
Figure 4. Histogram Showing Output Code Spread with AD7793 Input Pins
Shorted Together
Test data was taken using the board shown in Figure 5. Complete
documentation for the system can be found in the CN-0206
Design Support package.
Rev. B | Page 3 of 5
Circuit Note
09776-005
CN-0206
Figure 5. Photo of EVAL-CN0206-SDPZ Board
COMMON VARIATIONS
Functional Block Diagram
The AD7793 is a low noise, low power ADC. Other suitable ADCs
are the AD7792 and AD7785. Both parts have the same feature
set as the AD7793. However, the AD7792 is a 16-bit ADC while
the AD7785 is a 20-bit ADC.
See Figure 1 of this circuit note for the circuit block diagram,
and the EVAL-CN0206-SDPZ-SCH-RevA.pdf file for the
circuit schematics. This file is contained in the CN0206 Design
Support Package.
CIRCUIT EVALUATION AND TEST
Setup
This circuit uses the EVAL-CN0206-SDPZ circuit board and the
EVAL-SDP-CB1Z System Demonstration Platform (SDP)
evaluation board. The two boards have 120-pin mating
connectors, allowing for the quick setup and evaluation of the
circuit’s performance.
Connect the 120-pin connector on the EVAL-CN0206-SDPZ
circuit board to the CON A connector on the EVAL-SDP-CB1Z
evaluation (SDP) board. Use nylon hardware to firmly secure the
two boards, using the holes provided at the ends of the 120-pin
connectors. With power to the supply off, connect a 6 V power
supply to the +6 V and GND pins on the board. If available, a 6 V
wall wart can be connected to the barrel connector on the board
and used in place of the 6 V power supply. Plug the thermocouple
connector into J1. Note: Do not turn on the thermocouple
source at this time.
The EVAL-CN0206-SDPZ board contains the circuit to be
evaluated, as described in this note, and the SDP evaluation
board is used with the CN0206 Evaluation Software to capture
the data from the EVAL-CN0206-SDPZ circuit board.
Equipment Needed
Connect the USB cable supplied with the SDP board to the USB
port on the PC. Note: Do not connect the USB cable to the mini
USB connector on the SDP board at this time.
The following equipment is needed:
• A PC with a USB port and Windows® XP and Windows
Vista® (32-bit), or Windows® 7 (32-bit)
Test
• The EVAL-CN0206-SDPZ circuit evaluation board
Apply power to the 6 V supply (or wall wart) connected to
EVAL-CN0206-SDPZ circuit board. Turn on the CL540ZA
thermocouple source, connect the USB cable from the PC to the
mini USB connector on the SDP board and launch the evaluation
software.
• The EVAL-SDP-CB1Z SDP evaluation board
• The CN0206 Evaluation Software
• A power supply: 6 V or 6 V wall wart
• The CL540ZA thermocouple source or alternate
Getting Started
Load the evaluation software by placing the CN0206 Evaluation
Software CD in the PC. Using My Computer, locate the drive
that contains the evaluation software CD and open the Readme
file. Follow the instructions contained in the Readme file for
installing and using the evaluation software.
When USB communications are established, the SDP board can
be used to send, receive, and capture serial data from the EVALCN0206-SDPZ board.
Information and details regarding test setup and calibration,
and how to use the evaluation software for data capture can be
found in the CN-0206 User Guide.
Information regarding the SDP board can be found in the SDP
User Guide.
Rev. B | Page 4 of 5
Circuit Note
CN-0206
LEARN MORE
REVISION HISTORY
CN-0206 Design Support Package:
www.analog.com/CN0206-DesignSupport
8/13—Rev. A to Rev. B
Changes to Title ................................................................................. 1
Changes to Test Section.................................................................... 4
Kester, Walt. 1999. Temperature Sensors. 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: Sigma-Delta ADC
Basics, Analog Devices.
MT-023 Tutorial, ADC Architectures IV: Sigma-Delta 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.
Data Sheets and Evaluation Boards
AD7793 Data Sheet
AD7793 Evaluation Board
8/12—Rev. 0 to Rev. A
Added Circuit Evaluation Boards Section ..................................... 1
Changes to Circuit Note Title, Circuit Function and Benefits
Section and Figure 1 .................................................................................. 1
Added Cold Junction Compensation Section, ADC Channel 1
Configuration, Thermocouple Section, ADC Channel 2
Configuration, Thermistor Section, Output Coding Section,
and Thermocouple Linearization Section ..................................... 2
Deleted Figure 3 and Table 1; Renumbered Sequentially ............ 3
Added System Calibration Section, System Noise
Considerations Section, Test Data and Results Section, Figure 3,
and Figure 4 ....................................................................................... 3
Added Figure 5, Equipment Needed Section, Getting Started
Section, Functional Block Diagram Section, Setup Section, and Test
Section.......................................................................................................... 4
Changes to Circuit Evaluation and Test Section ........................... 4
Changes to Data Sheets and Evaluation Boards Section ............. 5
10/11—Revision 0: Initial Version
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CN09776-0-8/13(B)
Rev. B | Page 5 of 5
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