Circuit Note CN-0221

Circuit Note CN-0221
Circuit Note
CN-0221
Devices Connected/Referenced
Circuits from the Lab® reference designs 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/CN0221.
ADuCM360/
ADuCM361
Cortex-M3 Based Microcontroller with
Dual 24-Bit Σ-Δ ADCs
ADP1720-3.3
Low Dropout Linear Regulator
USB-Based, Thermocouple Temperature Measurement System Using ARM Cortex-M3
EVALUATION AND DESIGN SUPPORT
converter (DAC), and a 1.2 V internal reference, as well as an ARM
Cortex-M3 core, 128 kB flash, 8 kB SRAM, and various digital
peripherals such as UART, timers, SPIs, and I2C interfaces.
Circuit Evaluation Board
CN-0221 Evaluation Board (EVAL-ADuCM360TCZ)
Design and Integration Files
Schematics, Layout Files, Bill of Materials, source code for
ADuCM360
In the circuit, the ADuCM360/ADuCM361 is connected to a
thermocouple and a 100 Ω platinum resistance temperature
detector (RTD). The RTD is used for cold junction compensation.
In the source code, an ADC sampling rate of 4 Hz is chosen. When
the ADC input programmable gain amplifier (PGA) is configured
for a gain of 32, the noise-free code resolution of the ADuCM360/
ADuCM361 is greater than 18 bits.
CIRCUIT FUNCTION AND BENEFITS
This circuit uses the ADuCM360/ADuCM361 precision analog
microcontroller in an accurate thermocouple temperature
monitoring application. The ADuCM360/ADuCM361 integrates
dual 24-bit sigma-delta (Σ-Δ) analog-to-digital converters (ADCs),
dual programmable current sources, a 12-bit digital-to-analog
3.3V
USB HEADER
ADP1720-3.3
BEAD
IN
5V
BEAD
10Ω
OUT
GND
4.7µF
4.7µF
10µF
0.1µF
FT232R
D–
D+
RxD
AVDD
AIN5/IEXC
GND
SHIELD
0.1µF
TxD
0.1µF
BEAD
IOVDD
RESET
10Ω
100Ω
PtRTD
RESET
P2.2/BM
AIN0
SD
0.01µF
10Ω
AIN1
0.01µF
FERRITE BEADS:
1kΩ @ 100MHz
TAIYO YUDEN
BK2125HS102-T
ADuCM360
VREF +
RREF
5.6kΩ
0.1%
VREF –
AIN7/VBIAS
TxD
RxD
AGND
09985-001
J1
P0.1/SIN
AIN3
THERMOCOUPLE
JUNCTION
P0.2/SOUT
AIN2
Figure 1. ADuCM360/ADuCM361 as a Temperature Monitor Controller with a Thermocouple Interface (Simplified Schematic, All Connections Not Shown)
Rev. C
Circuits from the Lab® reference designs from Analog Devices have been designed and built by Analog
Devices 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
toanycausewhatsoeverconnectedtotheuseofanyCircuitsfromtheLabcircuits. (Continuedonlastpage)
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 ©2012–2014 Analog Devices, Inc. All rights reserved.
CN-0221
Circuit Note
The following features of the ADuCM360/ADuCM361 are used
in this application:








A 24-bit Σ-Δ ADC with a PGA set for a gain of 32 in the
software for the thermocouple and RTD. The ADC1 was
switched continuously between sampling the thermocouple
and the RTD voltages.
Programmable excitation current sources for forcing a
controlled current through the RTD. The dual current
sources are configurable in from 0 μA to 2 mA. For this
example, a 200 μA setting was used to minimize the error
introduced by the RTD self-heating.
An internal 1.2 V reference for the ADC in the ADuCM360/
ADuCM361. It measures the thermocouple voltage; the
internal voltage reference was used due to its precision.
An external voltage reference for the ADC in the
ADuCM360/ADuCM361. It measures the RTD resistance;
a ratiometric setup was used where an external reference
resistor (RREF) was connected across the external VREF+
and VREF− pins.
A bias voltage generator (VBIAS). The VBIAS function was
used to set the thermocouple common-mode voltage to
AVDD/2.
The ARM Cortex-M3 core. The powerful 32-bit ARM core
with integrated 128 kB flash and 8 kB SRAM memory runs
the user code that configures and controls the ADC, processes
the ADC conversions from the RTD, and controls the
communications over the UART/USB interface.
The UART was used as the communication interface to the
host PC.
Two external switches are used to force the part into its
flash boot mode. By holding SD low and toggling the RESET
button, the ADuCM360/ADuCM361 enters boot mode
instead of normal user mode. In boot mode, the internal
flash can be reprogrammed through the UART interface.
Note that the reference resistor, RREF, should be a precision
5.6 kΩ (±0.1%).
The USB interface to the ADuCM360/ADuCM361 is
implemented with an FT232R UART to USB transceiver, which
converts USB signals directly to the UART.
In addition to the decoupling shown Figure 1, the USB cable itself
must have a ferrite bead for added EMI/RFI protection. The ferrite
beads used in the circuit were Taiyo Yuden, #BK2125HS102-T,
which have an impedance of 1000 Ω at 100 MHz.
Construct the circuit on a multilayer printed circuit board (PCB)
with a large area ground plane. Use proper layout, grounding,
and decoupling techniques to achieve optimum performance (see
Tutorial MT-031, Grounding Data Converters and Solving the
Mystery of "AGND" and "DGND," Tutorial MT-101, Decoupling
Techniques, and the ADuCM360TCZ Evaluation Board layout).
The PCB used for evaluating this circuit is shown in Figure 2.
Both the thermocouple and the RTD generate very small signals;
therefore, a PGA is required to amplify those signals.
The thermocouple used in this application is a Type T (copperconstantan) that has a temperature range of −200°C to +350°C.
Its sensitivity is approximately 40 μV/°C, which means that the
ADC in bipolar mode, with a PGA gain of 32, can cover the
entire temperature range of the thermocouple.
The RTD was used for cold junction compensation. The
particular one used in this circuit was a platinum 100 Ω RTD,
Enercorp PCS 1.1503.1. It is available in a 0805, surface-mount
package. This RTD has a temperature variation of 0.385 Ω/°C.
Rev. C | Page 2 of 5
09985-002
CIRCUIT DESCRIPTION
Figure 2. EVAL-ADuCM360TCZ Board Used for this Circuit
Circuit Note
CN-0221
Code Description
20
The link to the source code used to test the circuit can be found in
the CN0221 Design Support Package at
http://www.analog.com/CN0221-DesignSupport
0
–20
ERROR (°C)
The UART is configured for a baud rate of 9600, 8 data bits, no
parity, and no flow control. If the circuit is connected directly to
a PC, a communication port viewing application, such as a
HyperTerminal, can be used to view the results sent by the
program to the UART, as shown in Figure 3.
–40
–60
–100
–210
–140
–70
0
70
140
210
280
350
TEMPERATURE (°C)
09985-004
–80
Initially, this was done using a simple linear assumption that the
voltage on the thermocouple was 40 µV/°C. It can be seen from
Figure 4 that this gives an acceptable error only for a small range,
around 0°C. A better way of calculating the thermocouple
temperatures is to use a six-order polynomial for the positive
temperatures and a seventh-order polynomial for the negative
temperatures. This requires mathematical operations that add
to computational time and code size. A suitable compromise is to
calculate the respective temperatures for a fixed number of
voltages. These temperatures are stored in an array, and values in
between are calculated using a linear interpolation between the
adjacent points. It can be seen from Figure 5 that the error is
drastically reduced using this method. Figure 5 gives the algorithm
error using ideal thermocouple voltages.
0.30
Figure 3. Output of HyperTerminal Communication Port Viewing Application
First, the voltage measured between the two wires of the
thermocouple (V1). The RTD voltage is measured, converted to
a temperature via a look-up table, and then, this temperature is
converted to its equivalent thermocouple voltage (V2). V1 and
V2 are then added to give the overall thermocouple voltage, and
this is then converted to the final temperature measurement.
Rev. C | Page 3 of 5
0.20
ERROR (°C)
To get a temperature reading, measure the temperature of the
thermocouple and the RTD. The RTD temperature is converted
to its equivalent thermocouple voltage via a look-up table (see the
ISE, Inc., ITS-90 Table for Type T Thermocouple). These two
voltages are added together to give the absolute value at the
thermocouple.
0.25
0.15
0.10
0.05
0
–0.05
–210
–140
–70
0
70
140
210
280
350
TEMPERATURE (°C)
Figure 5. Error When Using Piecewise Linear Approximation Using
52 Calibration Points and Ideal Measurements
09985-005
09985-003
Figure 4. Error When Using Simple Linear Approximation
CN-0221
Circuit Note
Figure 6 shows the error obtained when using ADC1 on the
ADuCM360 to measure 52 thermocouple voltages over the full
thermocouple operating range. The overall worst-case error
is <1°C.
CIRCUIT EVALUATION AND TEST
To test and evaluate the circuit, the thermocouple measurements
and the RTD measurements were evaluated separately.
Thermocouple Measurement Test
0.5
The basic test setup is shown in Figure 7. The thermocouple is
connected to J5, and Jumper J1 must be installed to allow the
AIN7/VBIAS pin to set the thermocouple common-mode
voltage. The circuit board receives its power from the USB
connection to the PC.
0.4
0.3
0.1
0
Two methods were used to evaluate the performance of the
circuit. Initially, the circuit was tested with the thermocouple
attached to the board and it was used to measure the temperature
of an ice bucket. Then, it was used to measure the temperature
of boiling water.
–0.1
–0.2
–0.3
–0.5
–210
–140
–70
0
70
140
210
280
350
TEMPERATURE (°C)
09985-006
–0.4
Figure 6. Error When Using Piecewise Linear Approximation Using
52 Calibration Points Measured by ADuCM360/ADuCM361
The RTD temperature is calculated using lookup tables and is
implemented for the RTD the same way as for the thermocouple.
Note that the RTD has a different polynomial describing its
temperatures as a function of resistance.
For details on linearization and maximizing the performance of
the RTD, refer to Application Note AN-0970, RTD Interfacing
and Linearization Using an ADuC706x Microcontroller.
COMMON VARIATIONS
Note that the ADuCM360/ADuCM361 can be programmed or
debugged via a standard serial wire interface.
A Wavetek 4808 Multifunction Calibrator was used to fully
evaluate the error, as shown in Figure 4 and Figure 6. In this
mode, the thermocouple was replaced with the calibrator as the
voltage source, as shown in Figure 7. To evaluate the entire range
of a Type T thermocouple, the calibrator was used to set the
equivalent thermocouple voltage at 52 points between −200°C
to +350°C for the negative and positive ranges of the T-type
thermocouple (see the ISE, Inc., ITS-90 Table for Type T
Thermocouple).
To evaluate the accuracy of the lookup algorithm, 551 voltage
readings, equivalent to temperatures in the range of −200°C to
+350°C spaced at +1°C, were passed onto the temperature
calculation functions. Errors were calculated for the linear
method and the piecewise linear approximation method as is
shown in Figure 4 and Figure 5.
For a standard UART to RS-232 interface, the FT232R transceiver
can be replaced with a device such as the ADM3202, which
requires a 3 V power supply. For a wider temperature range, a
different thermocouple can be used, such as a Type J. To minimize
the cold junction compensation error, a thermistor can be placed in
contact with the actual cold junction instead of on the PCB.
EVAL-ADuCM360TCZ
THERMOCOUPLE
JUNCTION
J5
J1
SEE TEXT
AIN7/VBIAS
USB
CABLE
Instead of using the RTD and external reference resistor for
measuring the cold junction temperature, an external digital
temperature sensor can be used. For example, the ADT7410 can
connect to the ADuCM360/ADuCM361 via the I2C interface.
WAVETEK 4808
MULTIFUNCTION
CALIBRATOR
PC
For more details on cold junction compensation, refer to Sensor
Signal Conditioning, Analog Devices, Chapter 7, “Temperature
Sensors.”
If isolation between the USB connector and this circuit is required,
the ADuM3160/ADuM4160 isolation devices must be added.
Rev. C | Page 4 of 5
09985-007
ERROR (°C)
0.2
Figure 7. Test Setup Used to Calibrate and Test the Circuit Over Full
Thermocouple Output Voltage Range
Circuit Note
CN-0221
RTD Measurement Test
LEARN MORE
To evaluate the RTD circuit and linearization source code, the
RTD on the board was replacement with an accurate, adjustable
resistance source. The instrument used was the 1433-Z Decade
Resistor. The RTD values are from 90 Ω to 140 Ω, which represents
an RTD temperature range of −25°C to +114°C.
CN0221 Design Support Package:
http://www.analog.com/CN0221-DesignSupport
The test setup circuit is shown in Figure 8, and the error results
for the RTD tests are shown in Figure 9.
AVDD
0.1µF
IOVDD
AIN0
0.01µF
MT-023 Tutorial, ADC Architectures IV: Sigma-Delta ADC
Advanced Concepts and Applications. Analog Devices.
AIN1
10Ω
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of "AGND" and "DGND." Analog Devices.
ADuCM360
0.01µF
VREF +
RREF
5.6kΩ
0.1%
MT-101 Tutorial, Decoupling Techniques. Analog Devices.
ITS-90 Table for Type T Thermocouple.
09985-008
VREF –
Data Sheets and Evaluation Boards
Figure 8. Test Setup for Measuring RTD Error
ADuCM360/ADuCM361 Data Sheet
0
ERROR (°C)
Kester, Walt. 1999. Sensor Signal Conditioning. Analog Devices.
Chapter 8, "ADCs for Signal Conditioning."
MT-022 Tutorial, ADC Architectures III: Sigma-Delta ADC
Basics. Analog Devices.
AIN5/IEXC
10Ω
Kester, Walt. 1999. Sensor Signal Conditioning. Analog Devices.
Chapter 7, "Temperature Sensors."
Looney, Mike. RTD Interfacing and Linearization Using an
ADuC706x Microcontroller. AN-0970 Application Note.
Analog Devices.
0.1µF
AVDD
1433-Z
DECADE
RESISTOR
IOVDD
ADIsimPower Design Tool.
–0.01
ADuCM360/ADuCM361 Evaluation Kit
–0.02
ADM3202 UART to RS232 Transceiver Data Sheet
–0.03
ADP120 Data Sheet
–0.04
ADP1720 Data Sheet
–0.05
REVISION HISTORY
–0.06
4/14—Rev. B to Rev. C
–0.07
Change to Circuit Function and Benefits Section ........................ 1
Change to Circuit Description Section .......................................... 2
Change to Common Variations Section ........................................ 4
09985-009
–0.08
–0.09
–0.10
–25
–5
15
35
55
75
95
12/13—Rev. A to Rev. B
115
TEMPERATURE (°C)
Figure 9. Error in °C of RTD Measurement Using Piecewise Linearization Code
and ADC0 Measurements
Changes to Code Description Section ........................................... 3
8/13—Rev. 0 to Rev. A
Changes to Title ................................................................................. 1
5/12—Revision 0: Initial Version
(Continued from first page) Circuits from the Lab reference designs are intended only for use with Analog Devices products and are the intellectual property of Analog Devices or its licensors.
While you may use the Circuits from the Lab reference designs in the design of your product, no other license is granted by implication or otherwise under any patents or other intellectual
property by application or use of the Circuits from the Lab reference designs. Information furnished by Analog Devices is believed to be accurate and reliable. However, Circuits from the
Lab reference designs are supplied "as is" and without warranties of any kind, express, implied, or statutory including, but not limited to, any implied warranty of merchantability,
noninfringement or fitness for a particular purpose and no responsibility is assumed by Analog Devices for their use, nor for any infringements of patents or other rights of third parties
that may result from their use. Analog Devices reserves the right to change any Circuits from the Lab reference designs at any time without notice but is under no obligation to do so.
©2012–2014 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
CN09985-0-4/14(C)
Rev. C | Page 5 of 5
Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement