CN-0145: 4 mA-to-20 mA Loop-Powered Temperature Monitor Using the...

Circuit Note
CN-0145
Devices Connected/Referenced
Circuit Designs Using Analog Devices Products
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ADuC7060/
ADuC7061
ARM7-Based Microcontroller with Dual
24-Bit Σ-Δ ADCs, VDAC, and PWM
ADP1720
2.5 V Output Linear Regulator
OP193/OP293
Low Power Op Amp
ADR280
Low Power 1.2 V Reference Source
4 mA-to-20 mA Loop-Powered Temperature Monitor Using the
ADuC7060/ADuC7061 Precision Analog Microcontroller
microcontrollers that have integrated analog circuits. The
circuit presented here represents the most cost-effective, lowest
power, lowest board area solution compared to using discrete
ADCs and a separate microcontroller. Because of its high level
of integration and low power, the ADuC7060/ADuC7061 can
operate directly from the loop power in the 4 mA-to-20 mA
application. With the ARM7 core running at 640 kHz, the
primary ADC active and measuring the external RTD temperature sensor, and the PWM controlling the 4 mA-to-20 mA
feedback circuit, the entire circuit typically consumes 3.15 mA.
Further details on the power consumption are provided in the
Circuit Description section.
CIRCUIT FUNCTION AND BENEFITS
This circuit provides a simple highly integrated temperature
monitor solution that can interface to a 4 mA-to-20 mA host
controller. This circuit provides a very low cost solution to
temperature monitoring because so much of the circuit
functionality is integrated on the ADuC7060/ADuC7061
precision analog microcontroller, including dual 24-bit Σ-Δ
ADCs, the ARM7 processor core, and the DAC/PWM features
for controlling the 4 mA-to-20 mA feedback circuit.
The performance of the ADCs and other analog circuits
within the ADuC7060/ADuC7061 is superior to competitive
AVDD
0.1µF
RTD
10Ω
0.01µF
10Ω
0.01µF
RREF
5.62kΩ
(0.1%)
PWR
0.1µF
AVDD
IEXC0
LOW POWER OPTION:
AVDD USE ADR280
INSTEAD OF DAC
AVDD
ADR280
DVDD
R3
57.6kΩ
GND REFOUT
BEAD
R4
60.4kΩ
2.5V
10Ω
4.7µF
DAC
DVDD
ADJ
ADP1720
OUT
IN
4.7µF
VREF = 1.2V
ADC0
BEAD: TAYIO YUDEN
BK2125HS 102-T
1kΩ @ 100MHz
R1
100kΩ
ADuC7061
4V TO 28V
ADC1
VREF +
PWM
VREF –
0.1µF
ADC2
ADC3
AGND
47kΩ
DVDD
47kΩ
0.1µF
VIN
VR12
OP193
DGND
DGND
LOOP+
10kΩ
VRLOOP
MEASURE VR12 VIA ADC2.
OPTIONAL, USED FOR
FEEDBACK CONTROL OF
LOOP CURRENT
R2
100kΩ
BC548
RLOOP
47.5Ω
LOOP–
08836-001
PT RTD: ENERCORP
PCS-11503.1, 100Ω
DVDD
Figure 1. ADuC7061 Controlling 4 mA-to-20 mA Loop-Based Temperature Monitor Circuit. (Simplified Schematic: All Connections and Decoupling Not Shown)
Rev. 0
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CN-0145
Circuit Note
During the intervals between temperature measurements, the
ADC and the RTD excitation current sources can be turned off
to further reduce power.
Full-scale current will flow when VIN = 0, at which point
VRLOOP = VREF. Hence, full-scale current is VREF/RLOOP, or ≈24 mA.
When VIN = VREF/2, no current will flow.
The 100 Ω Pt RTD is an Enercorp #PCS11503.1. The complete
circuit meets a better than ±1°C specification. The temperature
range of this RTD is −50°C to +130°C. The RTD is available in a
1206 size SMD package and has a temperature coefficient of
0.385 Ω/°C.
The OP193 amplifier impedance at VIN is high and will not load
the PWM filtered output. The amplifier output varies only a
small amount around 0.7 V.
The primary ADC within the ADuC7060/ADuC7061 has a
peak-to-peak noise-free code resolution of greater than 18 bits.
The PWM based DAC output provides 12 effective bits of
resolution. The performance of the overall circuit is discussed
in the Circuit Description section.
Due to the small form factor of the ADuC7061, a 5 mm × 5 mm
32-LFCSP package, the entire circuit will fit on an extremely
small PCB thus further reducing cost.
The focus here is primarily the 4 mA-to-20 mA interface
aspect of the circuit. For details on the RTD sensor interface
to the ADC and on linearization techniques of the RTD
measurements, see Application Note AN-0970 and Circuit
Note CN-0075.
CIRCUIT DESCRIPTION
The circuit is powered via the ADP1720 linear regulator
(adjustable version), which regulates the Loop+ supply to 2.5 V
for the ADuC7060/ADuC706161, the OP193 op amp, and the
optional ADR280 reference.
The 4 mA-20 mA feedback circuit is primarily controlled by the
ADuC7060’s on-chip 16-bit PWM (pulse width modulator).
The duty cycle of the PWM is configured in software to control
the voltage across the 47.5 Ω RLOOP resistor, which in turn sets
the loop current. Note the top of RLOOP connects to the
ADuC7060 ground. The bottom of RLOOP connects to the loop
ground. Because of this, the current due to the ADuC7060/
ADuC7061, ADP1720, ADR280, and OP193, plus the current
set by the filtered PWM output, flows across RLOOP.
VREF is supplied by the ADR280, a 1.2 V precision reference.
Alternatively, the on-chip DAC of the ADuC7060/ADuC7061
could be configured to provide the 1.2 V reference, but this
would consume extra power by enabling the internal DAC.
The voltage at the junction of R1 and R2 can be expressed:
VR12 = (VRLOOP + VREF) × R2/(R1 + R2) – VRLOOP
After the loop settles:
VIN = VR12
Since R1 = R2:
Performance at the range extremes, 0 mA to 4 mA, and 20 mA
to 24 mA, is not critical; therefore, the op amp does not require
good performance at the supply rails.
Absolute values of R1 and R2 are not critical. However, note
that matching of R1 and R2 is important.
Also note the possibility of measuring the voltage at the
point marked VR12 by using an input channel of ADC0 on
the ADuC7060/ADuC7061. This ADC measurement can be
used as feedback for the PWM control software to adjust the
4 mA-to-20 mA current setting.
The primary ADC of the ADuC7060/ADuC7061 measures the
voltage across the RTD. The RTD is excited by the on-chip
excitation current source, IEXC0. It is recommended that the
excitation current is configured for 200 µA to conserve power,
and it should be switched off between measurements. The
internal PGA on the front end of the primary ADC is configured
for either a gain of 16 or 32. The reference source for the RTD
measurement may be either the internal reference or the external 5.62 kΩ reference resistor. Selecting the external resistor will
conserve further power. For further details on the RTD to ADC
interface and on linearization techniques for the ADC results
see Application Note AN-0970 and Circuit Note CN-0075.
Power requirements for the circuit depend on whether the
module is powered directly from the 4 mA-to-20 mA loop
supply or actively from a 4-wire loop where the module is
powered separately. For this document, we will assume the
temperature monitor module is loop powered, therefore the
total current consumption of the module should not exceed
approximately 3.6 mA.
To facilitate low power operation, the ADuC7060/ADuC7061
core operating speed can be reduced by programming the
internal POWCON0 register. This allows the top frequency of
10.28 MHz to be divided in binary multiples of 2 to 128. During
the tests, a clock divide value of 16 was used, giving a core speed
of 640 kHz. The primary ADC was enabled with a gain of 32.
The PWM was also enabled. All other peripherals were
disabled.
For our circuit and test setup, Table 1 gives a breakdown of the
IDD current consumption, and Table 2 gives the currents for the
various peripherals.
VIN = (VRLOOP + VREF)/2 – VRLOOP = VREF/2 – VRLOOP /2
VRLOOP = VREF – 2VIN
Rev. 0 | Page 2 of 4
Circuit Note
CN-0145
Table 1. Typical IDD Values for Components of Temperature
Monitor Circuit
Component
IDD Value
at 25°C
IDD Value
at 85°C
ADuC7060/ADuC7061
2.45 mA
2.74 mA
Note: Add excitation current value to
this figure. Typical value is 200 µA.
0.2 mA
0.2 mA
ADR280, 1.2 V Reference
12 µA
15 µA
ADP1720, 2.5 V Output Linear Regulator
200 µA
300 µA
OP193, Low Power Op Amp
15 µA
25 µA
Remaining Circuitry
50 µA
50 µA
Total Current Less Excitation Current
2.73 mA
3.13 mA
ADC0 On, Gain = 32, FADC = 100 Hz
CPU speed = 640 kHz
(POWCON0 = 0x7C)
PWM On. PWMCON1 = 0x100
External reference selected by ADC0.
All other peripherals off.
The DNL graph in Figure 2 shows a better than 0.6 LSB
typical performance in the critical 4 mA-to-20 mA range.
These tests were conducted with a second order filter on
the PWM output. Two 47 kΩ resistors and two 100 nF
capacitors were used as shown in Figure 1.
Table 2. Typical IDD Values for Different Peripherals on the
ADuC7060/ADuC7061
Peripheral of ADuC7060/61
IDD Value, Typical, 25°C
ARM7 Core @ 10.24 MHz
5.12 MHz
2.56 MHz
1.28 MHz
640 kHz
320 kHz
160 kHz
80 kHz
5.22 mA
4.04 mA
2.7 mA
2 mA
1.674 mA
1.5 mA
1.42 mA
1.38 mA
Primary ADC, G = 1
G=4
G ≥ 128
30 µA
440 µA
630 µA
Auxiliary ADC
350 µA
DAC
330 µA
PWM
340 µA
SPI
40 µA
UART
200 µA
COMMON VARIATIONS
Instead of using the PWM, the on-chip DAC could be used for
the same function. The advantage would be quicker response
time and a cheaper solution. The ADR280 reference would not
be required in this case.
The OP90 can be used instead of the OP193.
DNL (LSBs)
LEARN MORE
ADIsimPower Design Tool, Analog Devices.
LOOP CURRENT (µA)
08836-002
CN-0075 Circuit Note, USB Based Temperature Monitor Using
the ADuC7061 Precision Analog Microcontroller and an
External RTD. Analog Devices.
Figure 2. Typical DNL performance of circuit
Kester, Walt. Sensor Signal Conditioning, Analog Devices, 1999,
Chapter 7, Temperature Sensors.
Kester, Walt. Sensor Signal Conditioning, Analog Devices, 1999,
Chapter 8, ADCs for Signal Conditioning.
The performance of the PWM based output could be
enhanced by using the ADC to measure the voltage point
VR12 and other points on the circuit. This method of
feedback could be used to calibrate the PWM output for
greater precision.
Looney, Mike. AN-0970 Application Note, RTD Interfacing and
Linearization Using an ADuC706x Microcontroller, Analog
Devices.
Note that the PWM circuit was only used to set the output
voltage between 0 V and 600 mV, hence the reduced number of
codes. Code 0 above is not important as it represents a value
greater than 24 mA.
For ADC measurement performance, see AN-970 and CN-0075
and the ADuC7060/ADuC7061 data sheet.
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.
Rev. 0 | Page 3 of 4
CN-0145
Circuit Note
Data Sheets and Evaluation Boards
ADP1720 Data Sheet
ADR280 Data Sheet
ADuC7060 Data Sheet
ADuC7060 Evaluation Board and Software
ADuC7061 Data Sheet
ADuC7061 Evaluation Board and Software
OP193 Data Sheet
OP293 Data Sheet
OP90 Data Sheet
REVISION HISTORY
2/10—Revision 0: Initial Version
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CN08836-0-2/10(0)
Rev. 0 | Page 4 of 4
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