Circuit Note CN-0300

Circuit Note CN-0300
Circuit Note
CN-0300
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/CN0300.
ADuCM360
Cortex-M3 Based Microcontroller with
Dual 24-Bit Σ-Δ ADCs
ADP1720-3.3
Low Dropout Linear Regulator
12-Bit, 4-20mA Loop-Powered Thermocouple Measurement
System Using ARM Cortex-M3
EVALUATION AND DESIGN SUPPORT
a 12-bit digital-to-analog converter (DAC), and a 1.2 V internal
reference, as well as an ARM Cortex-M3 core, 126 kB flash, 8 kB
SRAM, and various digital peripherals such as UART, timers,
SPIs, and I2C interfaces.
Circuit Evaluation Board
CN-0300 Evaluation Board (EVAL-CN0300-EB1Z) includes
Analog Devices J-Link OB emulator (USB-SWD/UARTEMUZ)
Design and Integration Files
Schematics, Layout Files, Bill of Materials, source code for
ADuCM360
In the circuit, the ADuCM360 is connected to a Type T
thermocouple and a 100 Ω platinum resistance temperature
detector (RTD). The RTD is used for cold junction compensation.
The low power Cortex-M3 core converts the ADC readings to a
real temperature value. The Type T temperature range supported is
−200°C to +350°C, and this temperature range is converted to
an output current range of 4 mA to 20 mA.
CIRCUIT FUNCTION AND BENEFITS
This circuit uses the ADuCM360 precision analog microcontroller
in an accurate thermocouple temperature monitoring application
and controls the 4 mA to 20 mA output current accordingly. The
ADuCM360 integrates dual 24-bit sigma-delta (Σ-Δ) analog-todigital converters (ADCs), dual programmable current sources,
The circuit provides a complete solution for thermocouple
measurements with a minimum requirement for external
components, and it is loop powered for loop voltages up to 28 V.
3.3V
ADP1720-3.3
VLOOP
IN
10µF
1.6Ω
OUT
GND
10µF
FERRITE BEAD
600Ω AT 100MHz
MURATA
BLM31AJ601SN1L
INTERFACE
BOARD
CONNECTOR
RESET
AVDD
SWDIO
DAC
ADC0
10Ω
NC
AIN9
AIN8
VREF+
5.6kΩ
0.1%
VREF–
RESET
AIN2
P2.2/BM
10nF
RESET
SD
AIN3
10nF
0.47µF
AGND
10955-001
AIN7/VBIAS
P0.2/SOUT
DVDD_REG
P0.1/SIN
10kΩ
THERMOCOUPLE
JUNCTION
100kΩ
ADuCM360
IOVDD
10kΩ
VLOOP–
100kΩ
ADC1
0.01µF
RREF
NPN
BC548
RLOOP
47Ω
0.01µF
SWCLK
VLOOP+
IOVDD
10Ω
100Ω
PtRTD
SIN
IOVDD
CURRENT
METER
0.1µF
IEXC
SOUT
SWCLK
0.1µF
RESET
GND
SWDIO
10µF
Figure 1. ADuCM360 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 ©2012–2013 Analog Devices, Inc. All rights reserved.
CN-0300
Circuit Note
•
The following features of the ADuCM360 are used in this
application:
•
•
•
•
•
•
•
•
The 12-bit DAC output with its flexible on-chip output
buffer is used to control an external NPN transistor, BC548.
By controlling the VBE voltage of this transistor, the current
passing through a 47 Ω load resistor can be set to the
desired value. When NPN mode is selected, the buffered
on chip 1.2 V reference voltage is present on AIN8.
The DAC is 12-bit monotonic; however, the accuracy of
the DAC output is typically around 3 LSBs. In addition, the
bi-polar transistor introduces linearity errors. To improve
the accuracy of the DAC output and to eliminate offset and
gain end-point errors, ADC0 measures, on AIN9, a
feedback voltage reflecting the voltage across the load
resistor (RLOAD). Based on this ADC0 reading, the DAC
output is corrected by the source code. This provides
±0.5°C accuracy on the 4 mA to 20 mA output.
The 24-bit Σ-Δ ADC with a PGA set for a gain of 32 in the
software for the thermocouple and the RTD. ADC1 switches
continuously between sampling the thermocouple and the
RTD voltages.
Programmable excitation current sources force a controlled
current through the RTD. The dual current sources are
configurable in steps from 0 µA to 2 mA. For this example,
a 200 µA setting is used to minimize the error introduced
by the RTD self-heating.
An internal 1.2 V reference is provided for the ADC in the
ADuCM360. When measuring the thermocouple voltage,
the internal voltage reference is used due to its precision.
An external voltage reference for the ADC in the ADuCM360.
When measuring the RTD resistance, a ratiometric setup
was used where an external reference resistor (RREF) was
connected across the external VREF+ and VREF− pins. The
on-chip reference input buffer is enabled because the reference
source in this circuit is high impedance. The on-chip reference
buffer means no external buffer is required to minimize input
leakage effects.
A bias voltage generator (VBIAS). The VBIAS function is
used to set the thermocouple common-mode voltage to
AVDD_Reg/2 (900 mV). Again, this removes the need for
external resistors to set the thermocouple common-mode
voltage.
The ARM Cortex-M3 core. The powerful 32-bit ARM core
with integrated 126 kB flash and 8 kB SRAM memory runs
the user code that configures and controls the ADCs and
converts the ADC conversions from the thermocouple and
RTD inputs to a final temperature value. It also controls the
DAC output and continuously monitors this DAC output
using the closed-loop feedback from the voltage level on
AIN9. For extra debug purposes, it also controls the
communications over the UART/USB interface.
•
•
The UART is used as the communication interface to the host
PC. This is used to program the on-chip flash. It is also used as
a debug port and for calibrating the DAC and ADC.
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 enters boot mode instead of normal user
mode. In boot mode, the internal flash can be reprogrammed
through the UART interface.
The J1 connector, an 8-pin dual-in-line connector, connects to
the Analog Devices J-Link OB emulator that is provided with
the CN-0300 support hardware. This allows programming
and debugging of this application board. See Figure 3.
Both the thermocouple and the RTD generate very small signals;
therefore, a programmable gain amplifier (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.
Note that the reference resistor, RREF, must be a precision 5.6 kΩ
(±0.1%).
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.
Rev. C | Page 2 of 7
10955-002
CIRCUIT DESCRIPTION
Figure 2. EVAL-CN0300-EB1Z Board Used for this Circuit
CN-0300
10955-003
10955-104
Circuit Note
Figure 3. EVAL-CN0300-EB1Z Board Connected to the Analog Devices J-Link
OB Emulator
The Analog Devices J-Link OB emulator (USB-SWD/UARTEMUZ) supports the following:
•
•
For downloading and debugging, LK1, LK2, LK4, and LK6 must
be inserted. LK3 and LK5 are required to communicate via
UART. Required software for the J-Link OB is included in the
software installation.
Note that the J-Link OB emulator replaces the J-Link Lite and
related interface boards previously shipped with the ADuCM360
development system.
For more details, see UG-457, ADuCM360 Development
Systems Getting Started Tutorial.
The link to the source code used to test the circuit can be found
in the CN-0300 Design Support Package at
http://www.analog.com/CN0300-DesignSupport
The source code uses the function libraries provided with the
example code. Figure 6 shows the list of source files used in the
project when viewed with the Keil µVision4 tools.
10955-004
•
When plugged into a PC USB port, it can also be used
to connect to a COM port (virtual serial port) on the
PC. This is required for running the calibration routines.
Provides SW (Serial Wire) debugging and
programming for the ADuCM360.
This USB port can be used to program the part using
the UART-based downloader. Code Description
Figure 4 shows a top view of the emulator board. J2
connector plugs into the EVAL-CN0300-EB1Z board.
The J2 connector pinout is shown in Figure 5
Figure 6. Source Files Viewed in µVision4
Calibration Section of Code
10955-103
•
Figure 5. J2 Connector
Figure 4. Analog Devices J-Link OB Emulator Top View
The compiler #define values, calibrateADC1 and calibrateDAC,
can be adjusted to enable or disable calibration routines for the
ADC and the DAC.
To calibrate either the ADC or the DAC, the Analog Devices J-Link
OB emulator (USB-SWD/UART-EMUZ) must be connected to J1
and to the USB port on a PC. A COM port viewer program, such
as HyperTerminal, can be used to view the calibration menus
and step through the calibration routines.
When calibrating the ADC, the source code prompts the user to
connect zero-scale and full-scale voltages to AIN2 and AIN3. Note
that AIN2 is the positive input. On completion of the calibration
routine, the new calibration values for the ADC1INTGN and
ADC1OF registers are stored to the internal flash.
Rev. C | Page 3 of 7
Circuit Note
CN-0300
When calibrating the DAC, connect the VLOOP+ output through
an accurate current meter. The first part of the DAC calibration
routine calibrates the DAC to set a 4 mA output, and the second
part of the DAC calibration routine calibrates the DAC to set a
20 mA output. The DAC code used to set a 4 mA and 20 mA
output is stored to flash. The voltage measured at AIN9 for the
final 4 mA and 20 mA settings is also recorded and saved to flash.
Because the voltage at AIN9 is linearly related to the current
flowing across RLOOP, these values are used to calculate the
adjustment factor for the DAC. This closed-loop scheme means
any linearity errors on the DAC and transistor based circuit are
fine-tuned out using the on-chip 24-bit Σ-Δ ADC.
For the thermocouple, temperatures for a fixed number of voltages
are stored in an array. Temperature values in between are calculated
using a linear interpolation between the adjacent points.
Figure 8 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
less than 1°C.
0.5
0.4
0.3
0.2
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
HyperTerminal, can be used to view the results sent by the
program to the UART, as shown in Figure 7.
0.1
0
–0.1
–0.2
–0.3
To enter the characters required by the calibration routines,
type the required character in the viewing terminal and this
character will be received by the ADuCM360 UART port.
–0.5
–210
–140
–70
0
70
140
210
280
350
TEMPERATURE (°C)
10955-006
–0.4
Figure 8. 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.
10955-005
Temperature-to-Current Output Section of Code
Figure 7. Output of HyperTerminal when Calibrating the DAC
Temperature Measurement Section of Code
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.
Once the final temperature has been measured, set the DAC output
voltage to the appropriate value that gives the required current
across RLOOP. The input temperature range is expected to be −200°C
to +350°C. The code sets the output current to 4 mA for −200°C
and 20 mA for +350°C. The code implements a closed-loop scheme,
as shown in Figure 9, where the feedback voltage on AIN9 is
measured by ADC0, and this value is used to compensate the
DAC output setting. The FineTuneDAC(void) function performs
this correction.
For best results, calibrate the DAC before beginning performance
testing of this circuit.
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 4 of 7
Circuit Note
CN-0300
circuit performance when the initial calibration is performed
and when using the closed-loop control of the VDAC output
results in temperature values of 0.5°C being reported by the DAC
output circuit. Nonlinearity errors from the DAC and the external
transistor circuit are adjusted out thanks to the 24-bit ADC.
Because temperature is a slow changing input parameter, this
closed scheme is ideal for this application. Figure 11 shows the
ideal DAC output in blue and the real DAC output with no closedloop control (ADC0 is not used to compensate the DAC output).
The error can be >10°C without closed-loop control.
VLOOP+
DAC
NPN
BC548
RLOOP
47Ω
VLOOP–
AIN9
100kΩ
AIN8
(BUFFERED VREF )
10955-007
100kΩ
ADC0
25
Figure 9. Closed-Loop Control 4 mA to 20 mA DAC Output
20
CURRENT OUTPUT (mA)
For debug purposes, the following strings are sent to the UART
during normal operation (see Figure 10).
15
ACTUAL CURRENT
10
IDEAL CURRENT
0
–200 –150 –100
–50
0
50
100
150
200
250
350
TEMPERATURE (°C)
10955-009
5
10955-008
Figure 11. Temperature in °C vs. Current Out in mA (Blue = Ideal Value, Open
Loop Operation: DAC Output Uncompensated)
Figure 10. UART Strings Used for Debug
Figure 12 shows the same information when the closed-loop
control is used as is recommended. The error is tiny, less than
0.5°C from the ideal value.
COMMON VARIATIONS
25
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.
CURRENT OUTPUT (mA)
20
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 via the I2C interface.
15
ACTUAL CURRENT
IDEAL CURRENT
10
For more details on cold junction compensation, refer to Sensor
Signal Conditioning, Analog Devices, Chapter 7, “Temperature
Sensors.”
0
–200
–100
0
100
200
TEMPERATURE (°C)
300
400
10955-010
5
Figure 12.Temperature in °C vs. Current out in mA (Blue = Ideal Value,
Closed-Loop Operation: DAC Output Compensated by ADC0 Measurement
If isolation between the USB connector and this circuit is required,
the ADuM3160/ADuM4160 isolation devices must be added.
Thermocouple Measurement Test
CIRCUIT EVALUATION AND TEST
The basic test setup is shown in Figure 13. The thermocouple is
connected to J2.
Current Output Measurements
The DAC and external-voltage-to-current-convertor circuit
performance tests were all completed together.
A current meter was placed in series with the VLOOP+ connection,
as shown in Figure 1. The meter used was a HP 34401A. The
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.
Rev. C | Page 5 of 7
Circuit Note
CN-0300
A Wavetek 4808 multifunction calibrator was used to fully evaluate
the error, as shown in Figure 13. In this mode, the thermocouple
was replaced with the calibrator as the voltage source. 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). Figure 8 shows the test results.
0
–0.01
–0.02
ERROR (°C)
–0.03
When doing performance checks and using the CN-0300 circuit
for normal operation, please ensure the J-Link OB emulator is
unplugged from the EVAL-CN0300-EB1Z board—only use the
J-Link OB when programming, calibrating and debugging the
EVAL-CN0300-EB1Z board.
J2
–0.06
–0.07
–0.08
–0.10
–25
–5
15
35
55
75
95
115
TEMPERATURE (°C)
10955-013
–0.09
Current Measurement Tests
When operating normally, the entire circuit consumes 2.25 mA
typically. When held in a reset state, the entire circuit consumes
less than 600 µA.
AIN7/VBIAS
SEE TEXT
USB
CABLE
WAVETEK 4808
MULTIFUNCTION
CALIBRATOR
10955-011
PC
Figure 13. Test Setup Used to Calibrate and Test the Circuit Over Full
Thermocouple Output Voltage Range
When doing performance checks and using the CN-0300 circuit
for normal operation, please ensure the J-Link OB emulator is
unplugged from the EVAL-CN0300-EB1Z board—only use the
J-Link OB when programming, calibrating and debugging the
EVAL-CN0300-EB1Z board.
For more details on the current consumption figures for the
ADuCM360, see Application Note AN-1111.
RTD Measurement Test
To evaluate the RTD circuit and linearization source code, the
RTD on the board was replaced 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.
The test setup circuit for measuring the RTD is shown in Figure 14,
and the error results for the RTD tests are shown in Figure 15.
AVDD
IOVDD
0.1µF
0.1µF
AVDD
IOVDD
AIN5/IEXC
10Ω
AIN0
0.01µF
0.01µF
AIN1
ADuCM360
VREF+
VREF–
10955-012
10Ω
RREF
5.6kΩ
0.1%
–0.05
Figure 15. Error in °C of RTD Measurement Using Piecewise Linearization
Code and ADC0 Measurements
EVAL-CN0300-EB1Z
THERMOCOUPLE
JUNCTION
1433-Z
DECADE
RESISTOR
–0.04
Figure 14. Test Setup for Measuring RTD Error
Rev. C | Page 6 of 7
Circuit Note
CN-0300
LEARN MORE
Data Sheets and Evaluation Boards
CN0300 Design Support Package:
http://www.analog.com/CN0300-DesignSupport
ADuCM360/ADuCM361 Data Sheet
ADIsimPower Design Tool.
ADM3202 UART to RS232 Transceiver Data Sheet
Kester, Walt. 1999. Sensor Signal Conditioning. Analog Devices.
Chapter 7, "Temperature Sensors."
ADP1720 Data Sheet
Kester, Walt. 1999. Sensor Signal Conditioning. Analog Devices.
Chapter 8, "ADCs for Signal Conditioning."
12/13—Rev. B to Rev. C
ADuCM360/ADuCM361 Evaluation Kit
REVISION HISTORY
Changes to Circuit Description Section......................................... 3
Looney, Mike. RTD Interfacing and Linearization Using an
ADuC706x Microcontroller. AN-0970 Application Note.
Analog Devices.
8/13—Rev. A to Rev. B
MT-022 Tutorial, ADC Architectures III: Sigma-Delta ADC
Basics. Analog Devices.
5/13—Rev. 0 to Rev. A
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.
ITS-90 Table for Type T Thermocouple.
Changes to Title ................................................................................. 1
Changed USB-SWD/UART and SEGGER J-Link Lite Board to
J-Link OB Emulator ............................................................Universal
Changes to Circuit Description Section......................................... 2
Changes to Figure 3 and Calibration Section of Code Section;
Added Figure 4 and Figure 5, Renumbered Sequentially ............ 3
Changes to Figure 9 .......................................................................... 4
Changes to Thermocouple Measurement Test Section and
Current Measurement Tests Section............................................... 6
Change to Data Sheets and Evaluation Boards Section ............... 7
10/12—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.
©2012–2013 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
CN10955-0-12/13(C)
Rev. C | Page 7 of 7
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