Texas Instruments | How to Isolate Two-Wire Loop-Powered Field Transmitters (Rev. B) | Application notes | Texas Instruments How to Isolate Two-Wire Loop-Powered Field Transmitters (Rev. B) Application notes

Texas Instruments How to Isolate Two-Wire Loop-Powered Field Transmitters (Rev. B) Application notes
How to Isolate Two-Wire Loop-Powered Field Transmitters
Tony Calabria, Systems Manager, Isolation, Interface Group
Improving process, quality, and consistency starts with
automation in today’s expanding industrial world.
Growth in system monitoring equipment, such as field
transmitters, has made manufacturing in factories
more efficient and reliable. Loop-powered field
transmitters are a sensor variant which use the twowire 4 mA to 20 mA bus for both communication and
power for the equipment. This removes the need for
additional wiring and components to separately power
the system, saving the factory cost on both materials
and installation.
A field transmitter consists of a sensor, a transmitter,
and, in certain circumstances, an isolator. Isolation is
needed when the sensor has the ability to be
electrically connected to a voltage potential that is
different than the loop supply. Since the entire system
is powered from 4 mA–20 mA, the zero scale value
sets the maximum budget for the system, which can
be on the order of 3.3 mA. Power savings from any of
the components, including the isolator, can be
distributed among the system allowing for more end
product differentiation. Figure 1 shows a typical block
diagram including the current flow used for the
communication bus and power for the system.
However, advancements in digital isolation technology
have brought power consumption down an order of
magnitude and increased isolator channel density,
giving designers more design flexibility in sensor
design systems.
Historically, the only available low power isolation
options were either pulse transformers or optocouplers. These isolation solutions can consume
anywhere from 500 µA up to 1 mA per channel, forcing
designers to minimize the number of communication
lines brought across the isolation barrier or slowing
data transfer speeds. Additional components, such as
a second micro controller (MCU), could be used to
decrease the communication lines across the isolation
barrier, further consuming more of the constrained
power budget. In these solutions, communication
speed and, in some cases, data transfer rate can be
limited to just a single direction with so few isolation
lines. A typical implementation, shown in Figure 2,
uses a pulse transformer as a unidirectional solution
for implementing isolation in a field transmitter and an
additional MCU to convert the isolated data interface
to a single line.
LDO
PLC Input Module
2-Wire Sensor Transmitter
3.3 V
+
±
Thermocouple
Sensor
GND
Transmitter
ISO
GND
Half
Bridge
4.5 V
4.7 V
LDO
Supply Output
Power
Supply
Current
Flow
LDO
3.3 V
Thermocouple
Sense
Resistor
ADC
ADC
MCU
MCU
Buffer
DAC
Protections
4-20mA
Isolation
Supply Input
4mA ± 20mA
9V ± 33V
Single Wire
Pulse Transformer
Figure 1. Loop-Powered Field Transmitter with a
PLC Input Module
Figure 2. Field Transmitter - Single Channel Pulse
Transformer for Data Isolation
Implementing isolation within 4 mA to 20 mA looppowered field transmitters can be challenging for
process automation end equipment designers. Since
these systems operate off a strict power budget,
decisions are made to trade off performance with
features. Designers may fear that integrating isolation
uses board space they may not have, while taking
significant power away from the other critical blocks in
the system, potentially limiting equipment capability.
Texas Instruments' ISO70xx digital isolator family
integrates two to four ultra-low-power digital isolator
channels in a small 8-pin D and 16-pin DBQ package.
This device offers up to 3.0 kVrms isolation rating per
UL1577 and 400 Vrms of isolation working voltage per
VDE. In loop-powered field transmitters, the ISO70xx
family can be used to replace opto-couplers or a pulse
transformer at a fraction of the power and footprint
size. Figure 3 shows an example of the four channel
ISO7041 used in a field transmitter.
SLLA434B – December 2018 – Revised October 2019
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How to Isolate Two-Wire Loop-Powered Field Transmitters Tony Calabria, Systems
Copyright © 2018–2019, Texas Instruments Incorporated
Manager, Isolation, Interface Group
1
www.ti.com
LDO
Half
Bridge
4.5 V
4.7 V
LDO
3.3 V
Isolation
LDO
Protections
3.3 V
Thermocouple
ADC
ISO7041
MCU
DAC
4mA ± 20mA
9V ± 33V
•
Figure 3. Field Transmitter - ISO7041 for Multichannel and Bi-directional Data Isolation
Benefits using digital isolators in field transmitters:
• Power scalability: The ISO7041 power scales with
the data transfer rate as shown in Figure 4. At 10
kbps, all four channels consume under 20 µA
combined. The power savings can be distributed to
the sensor to improve accuracy or precision of the
system.
200
Current per Channel (PA)
100
10
4
1
10
100
Data Rate (Kbps)
1000 2000
D001
Figure 4. ISO7041 Individual Channel Power
Versus Data Rate
•
•
•
2
Increased data throughput: Four digital isolator
channels can operate at up to 2 Mbps while
consuming very low power for easy two way
communication. This flexibility allows designers to
rethink and re-architect how they transfer data
between the sensor and transmitter.
Extended temperature range: The ISO7021 and
ISO7041 are rated from -55°C to 125°C while most
opto-couplers are rated up to a maximum of 85°C.
There are some opto-couplers available in the
market rated for 105°C that come at a higher cost.
Smaller solution size: A single channel pulse
transformer for data isolation can require a circuit
board surface area as large as 50 mm2 with an
additional height requirement of 5 mm. Optocoupler designs can occupy similar board area or
more depending on the amount of data lines. The
ISO7041 in the 16-pin DBQ package offers four
channels of data isolation across 17.5 mm2 of area
with a height requirement of 2.5 mm. Overall, a
digital isolator solution is a more compact design,
including lower package height.
Stability and reliability: ISO70xx uses TI’s
capacitive isolation technology where the high
voltage capacitors made from SiO2 dielectric
provide high level of isolation. The high voltage
capacitors are constructed in a well controlled
semiconductor process and offer very low part-topart variation. Also, the isolation barrier lifetime is
well defined by the time dependent dielectric
breakdown (TDDB) technique. Opto-couplers have
a lot of manufacturing variability and no defined
technique in the standard to calculate lifetime of the
device.
For years, a single pulse transformer was the only
power efficient way to isolate data between the sensor
and transmitter in a 4 mA–20 mA loop-powered field
transmitter. Later, low speed opto-coupler solutions
were able to replace pulse transformers at adequate
power budgets. Now, ultra-low-power digital isolators
are available to provide data isolation between the
sensor and transmitter at extremely low power levels.
The ISO70xx is the latest release in low power
technologies for galvanic isolation. For a deeper
explanation of TI's isolation technology, refer to the
Enabling High Voltage Signal Isolation Quality and
Reliability White Paper. The size, scalability,
technology robustness, and ultra-low-power
consumption of the ISO70xx digital isolators make
them an optimal choice for power constrained designs.
Table 1. Related Technical Documents
Literature
Number
Document Title
TIDU813B
Uniquely Efficient Isolated DC/DC Converter for UltraLow Power and Low Power Apps Reference Design
SLAA426
Considerations for Selecting Digital Isolators
SLYY063
High-voltage reinforced isolation: Definitions and test
methodologies
SLLA284A
Digital Isolator Design Guide
How to Isolate Two-Wire Loop-Powered Field Transmitters Tony Calabria, Systems
Manager, Isolation, Interface Group
SLLA434B – December 2018 – Revised October 2019
Submit Documentation Feedback
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