Design Trade-Offs for Loop-Powered Transmitters PDF

TECHNICAL ARTICLE
DESIGN TRADE-OFFS
FOR LOOP-POWERED
TRANSMITTERS
Derrick Hartmann
Applications Engineer,
Analog Devices, Inc.
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Loop-powered transmitters have progressed from purely analog signal
conditioners to highly flexible smart transmitters, but the chosen design
approach still depends on a system’s performance, functionality, and cost
requirements. This article presents three different bench tested transmitter
designs.
In loop-powered designs, the 4 mA to 20 mA loop provides both power
and data, so the system must operate on less than a 4 mA loop current.
In fact, 3.6 mA or lower is a more typical target, as this represents a low
alarm current on the loop. Other key considerations for a design are the
target performance, functionality, size, and cost.
The first circuit we’ll discuss (Figure 1) uses a purely analog signal chain.
Pressure
Sensor
Resistive
Bridge
+
VREF
R2
126.25 k
In-Amp
AD8226
R1
31.56 k
R3
1 k
5V
ADR02
4 mA to 20 mA
Op Amp
ADA4091-2
Q1
R4
10 k
EVAL-CN0289-EB1Z
−
Figure 1. Analog 4 mA to 20 mA loop-powered transmitter (reference to CN0289).
This circuit measures a resistive bridge pressure sensor, which is excited
by a 5 V reference. An instrumentation amplifier gains up the sensor
signal. Its voltage output is converted to a current by R1 and is summed
with an offset current generated through R2. This current flows through
R3 and is amplified via the op amp configuration, then through R4 to
form the 4 mA to 20 mA output. As the current, consumed by the entire
transmitter, returns through R4, it is included in the regulated 4 mA to
20 mA current, making the circuit loop powered.
Using 0.1% resistors, this circuit can achieve better than 1% max
accuracy at 25°C. Calibration would greatly increase the accuracy,
and allowing adjustments to R2 and R1 would cater for offset and
gain calibration respectively. However, the accuracy is still limited by
sensor performance and component drift over temperature, as the
circuit does not easily allow calibration over temperature or sensor
linearization.
This circuit consumes less than 1.9 mA (excluding sensor excitation),
which is well below the 4 mA target.
In summary, this purely analog transmitter allows for a simple, low cost
solution. However, the sensor cannot be linearized, it does not offer
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calibration over temperature, and it provides no diagnostics. Any changes
to the sensor or output range would also require hardware changes.
A number of drawbacks in the purely analog circuit can be resolved by
adding digital processing capability, as shown in Figure 2.
VDD
IEXC
Temperature
Sensor
Pt100
R1
5.62 k
VREF
ADC 1
VREF+
VREF−
MicroComputer
SRAM
Flash
Clock
Reset
Watchdog
2.5 V
+
VREF
R2
100 k
1.2 V
ADR280
Op Amp
OP193
COM
ADuC7061
ADP1720
VREG
R3
100 k
4 mA to 20 mA
+HART
R4
47.5 
CN0145
−
Figure 2. 4 mA to 20 mA loop-powered transmitter (reference to CN0145).
This circuit measures an RTD temperature sensor, which is excited by a
current source. A ratiometric measurement is taken between the RTD
and precision resistor R1. The RTD signal is conditioned using a PGA,
whose output is converted to digital by a 24-bit Σ-∆ ADC. This data can
be manipulated using the ARM7 microcontroller, which can be used to
calibrate and linearize the temperature sensor and the 4 mA to
20 mA output.
The 4 mA to 20 mA output is controlled via a PWM signal, which is able
to achieve 12-bit resolution. Though similar to the previous architecture,
the output uses an op amp’s noninverting terminal as the control voltage
for the 4 mA to 20 mA loop. A 1.2 V reference, along with R2, generates a
current equivalent to 24 mA on the loop. This means a 0 V control voltage
from the PWM results in a 24 mA output. As the control voltage on the
PWM increases, the output current decreases. For a 4 mA current output,
the PWM should be programmed to 500 mV. The advantage of this
technique is that the PWM does not need to be buffered, reducing both
current consumption and cost.
The current consumption of the entire RTD temperature transmitter was
measured as 2.73 mA at 25°C and 3.13 mA at 85°C (excluding sensor
excitation). This circuit meets the power consumption requirements, but
there is little current left for any additional diagnostics or features once
the sensor excitation current is added.
Though slightly higher in cost than the purely analog transmitter, the
ability to fully calibrate and linearize the sensor and output offers
significantly improved accuracy. It also has greater flexibility to allow for
diagnostics, and changes in sensor type can easily be accounted for
in software.
2
Design Trade-Offs for Loop-Powered Transmitters
3.3 V
Pressure
Sensor
Resistive
Bridge
ADuCM360
ADC 0
Temperature
Sensor
Pt100
IEXC
VREF
ADC 1
AD5421
VDD REGOUT
MicroComputer
SRAM
Flash
Clock
Reset
Watchdog
3.3 V
REGIN
V-Regulator
VREF
COM
Watchdog
Timer
3.3 V
4mA to 20 mA
+HART
Temp Sensor
DAC
COM
+
VOUT
ADC
SPI
UHART
50 
Cin
Loop (−)
−
VDD
AD5700
HART Modem
DEMO–AD5700D2Z
Figure 3. 4 mA to 20 mA loop-powered smart transmitter (reference to CN0267).
There are still some limitations though: the 4 mA to 20 mA loop can only
transmit the primary variable, in this case temperature, and no other
information. Additional diagnostics and system functionality may not be
possible while staying within the power budget, and with higher input
performance, the 4 mA to 20 mA output driver may become a significant
source of system error. A circuit that overcomes these limitations is shown
in Figure 3.
1.72 mA by the ADuCM360, and the remaining current by other circuitry
such as an on-board LED. The ADuCM360 is running with both 24-bit Σ-∆
ADCs and PGAs active and the following peripherals enabled: on-chip
reference, clock generator, watchdog timer, SPI, UART, timers, flash, SRAM,
and the core running at 2 MHz. This extremely low power consumption,
along with HART communication, means that additional system diagnostics
and functionality can easily be added to this system.
This circuit is truly a smart transmitter. As well as providing exceptional
performance, it allows bidirectional communication over the 4 mA to
20 mA loop via the highway addressable remote transducer (HART®)
protocol. The HART protocol operates over a traditional, low frequency loop
by modulating a higher frequency 1.2 kHz, 2.2 kHz frequency shift keyed
(FSK) digital signal over the standard 4 mA to 20 mA analog signals. HART
communication enables, among other things, remote configuration
transmission of diagnostic information, and device parameters, and
additional measurement information.
One aspect not discussed in any of the above circuits is isolation. Isolation
is especially useful in thermocouple transmitter applications where the
exposed sensor may be bonded directly to a metal surface. Optocouplers
are one solution, though they typically require a relatively large bias current
to ensure reliable behavior. New devices that overcome such challenges
are the ADuM124x and ADuM144x 2-channel/4-channel micropower isolators.
Per Figure 3, a pressure sensor and RTD are measured independently on
the ADuCM360 via dual precision, 24-bit Σ-∆ ADCs with on-board PGA.
The low power Cortex®-M3 core calibrates and linearizes the pressure
sensor input, and the RTD is used for temperature compensation. The
microcontroller also runs the stack for the HART protocol and communicates
via UART with the AD5700 HART physical layer modem. Lastly, the
microcontroller communicates with the AD5421 loop-powered DAC via
SPI to control the 4 mA to 20 mA loop. The AD5421 is a fully integrated,
loop-powered 4 mA to 20 mA DAC; it includes the loop driver, 16-bit
DAC, loop regulator, and diagnostic features.
PLC/DCS
Input/Output Cards
I/O to Device
Field Instruments
Analog 4 mA to 20 mA
These devices consume a mere 0.3 µA quiescent current per channel and
148 µA/Mbps dynamic current per channel. They enable isolation in
systems where it previously was not an option due to power constraints.
In conclusion, loop-powered transmitter designs can vary significantly in
performance, functionality, and cost. The three solutions discussed provide
different design trade-offs, starting from the simplest analog transmitter
to a feature rich smart transmitter. New low power products enable a level
of performance, functionality, and integration not previously achievable in
smart transmitter designs.
About The Author
Derrick Hartmann is a systems application engineer for ADI’s
industrial and instrumentation segment. His area of expertise is in
process control applications, with a background in industrial DACs.
He earned his bachelor’s degree in electronic engineering from the
University of Limerick, Ireland. He can be reached at
[email protected]
1.2 kHz and 2.2 kHz
HART Enabled
I/O
HART Digital Data
Intelligent
HART Device
Figure 4. HART communication.
With the ADC running at 50 SPS, the pressure sensor input was able to
achieve 18.5 bits of effective resolution. On the output side, the AD5421
provides a guaranteed 16-bit resolution and INL of 2.3 LSB maximum.
The whole circuit consumes 2.24 mA typical (excluding sensor excitation),
with 225 µA consumed by the AD5421, 157 µA by the AD5700,
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