2-Wire Loop-Powered Transmitter Solution PDF

Project Code: APM-TwoWire-Transmitter-2013
Analog Devices’ 2-Wire Loop-Powered Transmitter Solution
Industry Loop-Powered Transmitter System Theory and Trends
Modern field instruments, otherwise known as smart transmitters, are intelligent microprocessor-based field instruments that monitor process control
variables (e.g., temperature, mass flow rate, and pressure). The incorporation of extra intelligence, functionality, and diagnostic capabilities, while developing a
system which can operate effectively within the limited power available from 4 mA to 20 mA loop, is the immediate challenge facing system designers. Strict safety
standards, space constraints, HART® communication, diagnostics, and predictive maintenance will be key trends in the future.
PLC/DCS
INPUT/OUTPUT CARDS
I/O TO DEVICE
FIELD INSTRUMENTS
ANALOG 4mA TO 20mA
1.2kHz AND 2.2kHz
HART DIGITAL DATA
HART-ENABLED
INTELLIGENT
I/O
HART DEVICE
Figure 1. Transmitter communication with HART.
Industry 2-Wire Loop-Powered Transmitter System Design Considerations and Major Challenges
To have an appropriate 2-wire loop-powered transmitter system design, designers must consider many different system requirements including accuracy,
power budget, diagnostics, and silence noise restriction with HART.
• Minimum loop current is 4 mA; the maximum allowable total system power budget available for board design is 3.5 mA.
• Loop voltage range supported from 12 V to 48 V.
• Measurement accuracy can have readings as high as 0.1%; ADC resolution needs 16 bits to 24 bits, and DAC resolution needs 14 bits to 16 bits.
• Low noise precision instrumentation amplifier is typically required for sensor output.
• Space constraints within a transmitter and a high level of on-chip integration is required to maximize functionality.
• HART (Highway Addressable Remote Transducer) communication has an essential role to play in retrieving the field instruments process and diagnostics
information; FSK frequency of 1200 Hz and 2200 Hz with an amplitude of 1 mA p-p.
Smart transmitters are often located in hazardous or remote areas. Low temperature drift coefficient and low power consumption are very important for looppowered transmitters to endure a wide working temperature range. ADI offers a perfect portfolio like precision amplifiers, precision references, precision
analog-to-digital converters, and Cortex™-M3 core microprocessors.
instrumentation.analog.com
VOLTAGE
REGULATOR
REF
μC
—
+
SENSOR
ADC 0
—
+
SENSOR
CURRENT
LOOP
FLASH MEMORY
SRAM
CURRENT
DRIVER
DAC
POR
CLOCK GENERATION
WATCHDOG TIMER
ADC 1
HART
MODEM
Figure 2. General smart transmitter signal chain.
Figure 2 shows a typical 2-wire smart transmitter’s architecture, which combines sensors, signal conditioning, MCU, current driver, reference, HART modem,
and so on. Again, smaller size, higher integration, and intelligent diagnostics will be the key trends for field instrument.
Total Solutions from ADI (High End Market)
For the high end market, a total solution provided from ADI with sample developed and registered with the HART Communication Foundation focuses on the 2-wire
loop-powered transmitter with high performance. Figure 3 shows that only three chips were used to build the demo, which represents the leading integration level in
the market.
Main Signal Chain
PRESSURE 3.3V
SENSOR
RESISTIVE
BRIDGE
ADC 0
µC
ADC 1
SRAM
FLASH
CLOCK
RESET
WATCHDOG
LEXC
+
_
AD5421
VDD REGOUT
REGIN
V-REGULATOR
3.3V
VLOOP
ADC
DAC
COM
+
_
WATCHDOG
TIMER
3.3V
4mA to 20mA
+HART
TEMP SENSOR
SPI
UART
TEMPERATURE
SENSOR PT100
ADuCM360
+
_
COM
50Ω
CIN
LOOP(—)
VDD
AD5700
HART MODEM
HART_OUT
REF
ADC_IN
COM
Figure 3. HART enabled smart transmitter demo block diagram.
2 | Analog Devices’ 2-Wire Loop-Powered Transmitter Solution
Auxiliary Cost Effective Solutions From ADI (Mid/Low End Market)
For the mid/low market, customers used to be concerned with keeping costs down; ADI provides two additional cost effective solutions with the ADuCM360 and the
AD693, as shown below. The AD5700 HART modem could also be a common variation for these circuits.
As the ADuCM360 contains a high resolution 16-bit PWM, it can easily implement a 4 mA to 20 mA output with simple RC filters, amplifiers, and transistors. This is a
very typical method used in low end smart transmitter customers.
AVDD
OPTIONAL FOR
LOWER POWER
DVDD
DVDD
10K
PWR
0.1μF
REFERENCE
0.1μF
GND
VREF=1.2V
10Ω
0.01μF
ADJ
REGULATOR
OUT
4.7μF
IN
4.7μF
DAC
IEXC0
RTD
11K
BEAD
10Ω
REFOUT
DVDD
AVDD
DVDD
AVDD
ADC0
R1
10Ω
100K
ADuCM360
0.01μF
47K
PWM
VREF+
0.1μF
RREF 5.6K(0.1%)
OPAMP
DVDD
47K
10K
LOOP+
BC548
VIN
0.1μF
DGND
VREF—
100K
VRLOOP
RLOOP=47Ω
LOOP—
R2
AGND
DGND
Figure 4. Cost effective solution with the ADuCM360 PWM.
Besides a smart transmitter solution, ADI also provides a pure analog loop-powered transmitter solution—the AD693, a sensor transmitter, which is a very standard
single-chip solution to implement various signal conditioning, like thermocouple, RTD, and pressure signal inputs.
P1 P2
6.2V
14
SIGNAL
AMPLIFIER
4mA
15 16
−SIG
IX
17
600Ω
VOLT
REF
AUXILIARY AMPLIFIER
2.25kΩ
2.25
kΩ
20
1
−AUX
3.75Ω
8
VOLTAGE
REFERENCE
& OFFSET
DIVIDER
2
AUX
4
100Ω
5
3
6
100Ω
150mV
COM
75
mV
IIN
3.75Ω
G2
2.25kΩ
VX
100Ω
RTD
AD693
BOOST
ILOOP
A
G2
60
kΩ
19
ZERO
12
1.2kΩ
A
G1
9
600Ω
800Ω
G2
18
VIN
11
10
800Ω
+SIG
12mA
13
POWER
SUPPLY
CURRENT
LIMITEA
VOLTAGE TO CURRENT
CONVERTER
3kΩ
7
ILOOP
IOUT
RL
CC = 0.1μF to 0.01μF
CC
Figure 5. Cost effective solution with the AD693 for analog transmitter.
instrumentation.analog.com 
| 3
Main Product Introduction
Device
Description
Key Features
Benefits
AFE + MCU
ADuCM360/
ADuCM361
Analog microcontroller
(ARM Cortex-M3 core)
Dual 24-bit Σ-∆ ADC (ADuCM360)
Single 24-bit Σ-∆ ADC (ADuCM361)
Programmable ADC output rate (3.5 kHz to 3.906 kHz), 12-bit DAC
Programmable current sources, temperature sensor
Internal reference 4 ppm/°C
16 MHz Cortex, 128 kB Flash, 8 kB RAM
Core consumes 290 μA/MHz, power-down mode: 4 kA (wake-up timer active)
Low power HART modem
HART—compliant fully integrated FSK modem, 1200 Hz and 2200 Hz sinusoidal
Lowest power, smallest footprint, on-chip
shift frequencies, 115 μA max in receive mode. Integrated receive band-pass filter,
oscillator
optional clocking configurations, buffered HART output, 2 V to 5.5 V power supply
Ultralow power, high precision, leading
integration chip solution
HART Modem
AD5700/
AD5700-1
4 mA to 20 mA Loop Driver
AD5421
Loop-powered DAC
16-bit resolution and monotonicity, pin selectable NAMUR-compliant ranges.
Downscale alarm 3.2 mA, upscale alarm 22.8 mA/24 mA, TUE: 0.05% max,
output TC: 3 ppm/°C typ. Quiescent current: 300 μA max
Very low power, leading integration with
best performance, compatible with HART
connection
AD693
Loop-powered sensor
transmitter
Loop-powered operation, instrumentation amplifier front end.
Precalibrated 30 mV or 60 mV input spans, precalibrated PT100 interface.
Precalibrated output spans 4 mA to 20 mA unipolar, 12 mA to 8 mA bipolar
A complete monolithic low level V-I loop
signal conditioner
Zero-drift micropower OP
Very low supply current: 13 μA typ. Vos 15 μV max
Offset drift: 20 nV/°C, PSRR 110 dB min, CMRR 110 dB min. Rail-to-rail input/
output
High cost performance
Linear regulator
Supply range: 4 V to 28 V, low light load current: 28 μA at 0 μA load, low dropout:
275 mV @ 50 mA, initial accuracy: ±0.5%, logic controlled enable, current and
thermal overload protection
Low power consumption
Low power reference
1.2 V precision output, supply current: 100 μA max, low noise (<10 Hz): 8 μV p-p
typ. Supply range: 2.3 V to 5.5 V, long term stability: 30 ppm/1000 hrs @ 50°C
Low power, small package
Amplifier
ADA4051-1
Regulator
ADP1720
REF
ADR3412
In conclusion, those circuits outlined above demonstrate some possible solutions to the multifaceted design challenges, that is, the selection of components
for a smart/analog transmitter signal chain with sufficiently low power consumption, to ensure adherence to the limited power budget while still retaining the
necessary performance specifications, functionality, and diagnostics capability to meet the ever increasing market demands. The generic transmitter signal
chain discussion, along with the solution offered by Analog Devices, addresses these challenges directly, balancing the allocation of power required by each
component, resulting in a comprehensive signal chain addressing the power, performance, size, and diagnostic requirements of a modern, multifeature marketleading smart transmitter design.
CFTL Link
• Complete 4 mA to 20 mA loop-powered field instrument with HART interface (CN0267)—www.analog.com/CN0267
• Complete 4 mA to 20 mA HART solution with additional voltage output capability (CN0278)—www.analog.com/CN0278
• Complete 4 mA to 20 mA HART solution (CN0270)—www.analog.com/CN0270
• 4 mA to 20 mA loop-powered temperature monitor using the ADuC7060/ADuC7061 precision analog microcontroller (CN0145)—www.analog.com/CN0145
• Complete closed-loop precision analog microcontroller thermocouple measurement system with 4 mA to 20 mA output (CN0300)—www.analog.com/CN0300
For more information about the electromagnetic flow meter, please contact ADI.
Customer Interaction Center
Technical Hotline 1-800-419-0108 (India)
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82-2-2155-4200 (Korea)
Email [email protected]
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Free Sample www.analog.com/sample
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BR127051-0-2/13
What ADI Can Provide to Customers
• ADC ADIsimADC—www.analog.com/zh/ADIsimADC
Σ-Δ ADC register configuration assistant—
www.analog.com/SigmaDeltaRegisterConfigurationAssistant
• DAC ADIsimDAC—www.analog.com/ADIsimDAC
• AMP ADIsimOpAmp—www.analog.com/ADIsimOpAmp
ADIsimDiffAmp—www.analog.com/ADIsimDiffAmp
• Power EVB ADIsimPower—www.analog.com/ADIsimPower
• Processor EVB emulation tools and some software
instrumentation.analog.com
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