Circuit Note
CN-0075
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/CN0075.
ADuC7061
ARM7 Based Microcontroller with Dual
24-Bit Σ-Δ ADCs
ADP3333-2.5
2.5 V Low Dropout Linear Regulator
ADP7102-2.5
2.5 V Low Dropout Linear Regulator
USB Based Temperature Monitor Using the ADuC7061 Precision Analog
Microcontroller and an External RTD
CIRCUIT FUNCTION AND BENEFITS
the noise free code resolution of the ADuC7061 is greater
than 18 bits.
This circuit shows how the ADuC7061 precision analog
microcontroller can be used in an accurate RTD temperature
monitoring application. The ADuC7061 integrates dual 24-bit
Σ-Δ ADCs, dual programmable current sources, a 14-bit DAC,
and a 1.2 V internal reference as well as an ARM7 core, 32 kB
flash, 4 kB SRAM, and various digital peripherals such as
UART, timers, SPI, and I2C interfaces. The ADuC7061 is
connected to a 100 Ω RTD.
CIRCUIT DESCRIPTION
The circuit shown in Figure 1 is powered entirely from the USB
interface. The 5 V supply from the USB is regulated to 2.5 V
using the ADP3333 2.5 V low dropout linear regulator. The
regulated 2.5 V supplies the DVDD voltage to the ADuC7061.
The AVDD supply to the ADuC7061 has additional filtering as
shown. A filter is also placed on the USB supply at the input of
the linear regulator.
In the source code, an ADC sampling rate of 100 Hz is chosen.
When the ADC input PGA is configured for a gain of 32,
2.5V
USB HEADER
ADP3333-2.5
BEAD
5V
IN
BEAD
10Ω
OUT
4.7µF
4.7µF
4.7µF
0.1µF
FT232R
D–
D+
0.1µF
TxD
RxD
DVDD
AVDD
IEXC0
GND
100Ω
P tRTD
0.1µF
BEAD
10Ω
0.01µF
10Ω
nTRST/BM
ADC0
1kΩ
S1
1kΩ
ADC1
0.01µF
FERRITE BEADS:
1kΩ @ 100MHz
TAIYO YUDEN
BK2125HS102-T
S2
/RST
ADuC7061
VREF +
RREF
5.62kΩ
0.1%
P1.1/SOUT
P1.0/SIN
TxD
08283-001
VREF –
RxD
Figure 1. ADuC7061 As a Temperature Monitor Controller with an RTD Interface
(Simplified Schematic, All Connections Not Shown)
Rev. A
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CN-0075
Circuit Note
The following features of the ADuC7061 are used in this
application:
Note that the reference resistor, RREF, should be a precision
5.62 kΩ (±0.1%).
•
Primary 24-bit Σ-Δ ADC with programmable gain
amplifier (PGA): The PGA is set for a gain of 32 in the
software for this application.
The USB interface to the ADuC7061 is implemented with an
FT232R UART to USB transceiver, which converts USB signals
directly to the UART.
•
Programmable excitation current sources for forcing a
controlled current through the RTD: The dual current
sources are configurable in 200 µA steps from 0 µA to
2 mA. For this example, a 200 µA setting was used.
•
External voltage reference for the ADC in the ADuC7061:
For this application, we used a ratiometric setup where an
external reference resistor (RREF) is connected across the
external VREF+ and VREF− pins. Alternatively, an internal
1.2 V reference is provided in the ADuC7061.
In addition to the decoupling shown in Figure 1, the USB
cable itself should have a ferrite 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.
•
ARM7TDMI® core: The powerful 16-/32-bit ARM7 core
with integrated 32 kB flash and 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.
•
•
UART: The UART was used as the communication
interface to the host PC.
The circuit must be constructed on a multilayer PC board with
a large area ground plane. Proper layout, grounding, and
decoupling techniques must be used to achieve optimum
performance (see Tutorial MT-031, Grounding Data Converters
and Solving the Mystery of "AGND" and "DGND," Tutorial MT101, Decoupling Techniques, and the ADuC7061 Evaluation
Board layout).
CODE DESCRIPTION
The source code used to test the attached circuit can be
downloaded as a zip file at www.analog.com/cn0075_source.
The UART is configured for a baud rate of 9600, 8 data bits, no
parity, 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. See Figure 2. The source code is
commented to make it easier to understand and manipulate.
Two external switches are used to force the part into its
flash boot mode: By holding S1 low and toggling S2, the
ADuC7061 will enter boot mode instead of normal user
mode. In boot mode, the internal flash may be
reprogrammed through the UART interface.
For details on linearization and maximizing the performance of
the circuit, refer to Application Note AN-0970, RTD Interfacing
and Linearization Using an ADuC706x Microcontroller.
08283-002
The RTD used in the circuit is a platinum 100 Ω RTD, model
number Enercorp PCS 1.1503.1. It is available in a 0805 surfacemount package. This RTD has a temperature variation of
0.385 Ω/°C.
Figure 2. Output of HyperTerminal Communication Port Viewing Application
Rev. A | Page 2 of 3
Circuit Note
CN-0075
COMMON VARIATIONS
The ADP7102 regulator can be used as a newer alternate
to the ADP3333. If more GPIO pins are required on the
microcontroller, the ADuC7060, which comes in a 48-LFCSP or
48-LQFP package, is another option. Note that the
ADuC7060/ADu7061 can be programmed or debugged via a
standard JTAG interface. For a standard UART to RS-232
interface, the FT232R transceiver could be replaced with a
device such as the ADM3202, which requires a 3 V volt power
supply.
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
ADuC7061 Data Sheet.
ADuC7061 Evaluation Kit.
LEARN MORE
ADM3202 UART to RS232 Tranceiver Data Sheet.
ADIsimPower Design Tool.
ADP7102 Data Sheet.
CN-0075 Circuit Test Source Code File..
Kester, Walt. 1999. Sensor Signal Conditioning. Analog Devices.
Chapter 7, "Temperature Sensors."
Kester,Walt. 1999. Sensor Signal Conditioning. Analog Devices.
Chapter 8, "ADCs for Signal Conditioning."
Looney, Mike. RTD Interfacing and Linearization Using an
ADuC706x Microcontroller. AN-0970 Application Note.
Analog Devices.
MT-022 Tutorial, ADC Architectures III: Sigma-Delta ADC
Basics. Analog Devices.
ADP3333 Data Sheet.
REVISION HISTORY
7/13—Rev. 0 to Rev. A
Changes to Devices Connected/Referenced Section.................... 1
Changes to Common Variations Section and Data Sheets and
Evaluation Board ............................................................................... 3
7/09—Revision 0: Initial Version
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CN08283-0-7/13(A)
Rev. A | Page 3 of 3