Texas Instruments | Implementing HART in 2-Wire Field Sensor-Transmitters | Application notes | Texas Instruments Implementing HART in 2-Wire Field Sensor-Transmitters Application notes

Texas Instruments Implementing HART in 2-Wire Field Sensor-Transmitters Application notes
Implementing HART in 2-Wire Field Sensor-Transmitters
Garrett Satterfield
Introduction
Modern 4-20mA loop current drivers in loop powered
field transmitters utilize the HART protocol to achieve
digital bidirectional communication on top of the
traditional 4-20mA current loop. The 4-20mA loop
represents a single primary variable based on the
value of the DC loop current. The addition of HART
communication superimposes an AC coupled FSK
waveform on the loop as digital data, often used for
calibration and diagnostics. A key challenge for field
transmitter designers is implementing HART on 2-wire
analog output modules while achieving HART physical
layer compliance and minimizing overall quiescent
current. The DAC874xH family of devices are
standalone modems supporting HART, Foundation
Fieldbus, and Profibus PA. These HART certified
modems feature SPI/UART interface options, internal
filtering, and low quiescent current of 180µA (Max,
-40°C to 85°C), making DAC874xH ideal for
implementing HART certified 2-wire analog outputs for
field transmitter applications.
of the devices flows up from this ground (IQ), across
R4 and out of Loop-. All of the current flowing into
Loop+ must flow out of Loop- as the power supply and
signaling current are combined. The current flowing
through the transistor can be viewed as the additional
current required reach the current set point. A large
ratio of R3/R4 is typically used so that most of the
current flows through the BJT.
VREG
R2
R1
DAC
In the 2-wire topology the loop power supply,
transmitter, and analog input module are connected in
a series loop. The transmitter regulates the loop
current to indicate the sensor value (primary variable).
The analog input module then calculates and digitizes
the DC loop current for PLC processing.
Figure 1 shows the discrete implementation of the 2wire analog output circuit and Equation 1 shows the
DC transfer function. Resistors R1 and R2 set the
span and zero-scale current respectively. The current
flowing through R3 is multiplied by a gain of (1+R3/R4)
setting the total loop current. R4 is a current sense
resistor that provides feedback for regulation and R5 is
a small degeneration resistance for stability. As shown
in Figure 1 the ground point for this circuit represents
the local transmitter ground and the quiescent current
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+
A1
IBJT
R5
IQ
R3
R4
Loop-
Figure 1. DC Loop Powered Transmitter Circuit
2-Wire Analog Output Circuit
In low power field transmitters, the loop powered 2wire analog output circuit is the ideal configuration as it
eliminates the need for a separate power supply cable.
In the loop powered 2-wire 4-20mA analog output, the
analog signaling current and supply current are
combined. The output NPN transistor is responsible for
regulating the loop current to a minimum of 4mA. The
regulator, op amp, DAC, modem, and sensing circuitry
must therefore require less than 4mA of supply
current.
Loop+
REG
IOUT
§ VDAC
¨
© R1
VREG · § R3
¸u¨
R2 ¹ © R 4
·
1¸
¹
(1)
HART Implementation in Loop Powered
Transmitters
One drawback with the traditional 4-20mA field
transmitter is that only one variable can be
represented. The addition of HART provides an
additional channel of communication between the field
transmitter and analog input module through an FSK
waveform representing digital data. Both the field
transmitter and PLC can transmit and receive data
over HART. The PLC acts as the master and requests
data from the field transmitter to ensure that both
devices are never transmitting simultaneously. HART
is commonly used to transmit device information,
diagnostic information, and can also be used to
represent another process/control variable.
Figure 2 shows how the HART signal is coupled to the
2-wire transmitter circuit. As seen previously, R1-R5
are selected based on DC loop current requirements
and the HART signal is injected as a current waveform
through R6 and is multiplied (1+R3/R4) yielding the
1mAp-p AC waveform in the loop current . Equation 2
shows the required value of R6 to set the amplitude of
the HART signal in the current loop.
Implementing HART in 2-Wire Field Sensor-Transmitters Garrett Satterfield
Copyright © 2018, Texas Instruments Incorporated
1
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VREG
Loop+
REG
R2
R1
DAC
+
A1
IHART
MODOUT
DAC874xH
MODIN
R5
R6
Figure 3 shows the voltage across a 250ohm load
resistor connected to a two-wire transmitter. The DC
current value is set to approximately 5mA which is
represented by the DC voltage of 1.26V across the
load resistor. The HART waveform can be seen
superimposed on top of this DC voltage as a 250mVpp
signal which corresponds to a 1mAp-p current
waveform.
C1
C2 Loop+
R3
HART Modem
R4
Loop-
Figure 2. HART Coupling in Loop Powered
Transmitter
R6
VHARTpk
IOUTpk
pk
pk
§ R3 ·
¨1
¸
© R4 ¹
(2)
As shown in Figure 2 MODIN is coupled to Loop+
where the HART signal from the input module is
received as a voltage waveform across the sense
resistor. In both the 2-wire and 3-wire transmitter
configurations, the analog output module transmits
HART in the current domain since it is responsible for
regulating the current. The input module (master)
transmits HART in the voltage domain across the
sense resistor since it has no control over the loop
current.
The additional considerations for HART
implementations in the 2-wire transmitter are modem
quiescent current and the related overall current
consumed by the sensor, transmitter, and supporting
circuitry or devices. The modem should have as low
quiescent current to leave room for the other sensortransmitter elements. DAC874xH devices have 180µA
maximum quiescent current over temperature (-40°C
to 85°C) with the modulator active, leaving significant
headroom for other circuitry.
The overall current consumption must also be
considered to avoid clipping of the HART signal. The
nominal zero-scale current is 4mA but if HART
signaling is required at zero-scale then the sensortransmitter must consume less than 3.5mA as the
HART signal is 1mAp-p. This can be further
complicated if under current values are used for error
signaling as described by the NAMUR specification.
An error or alarm may be represented by setting the
current to 3.6mA for instance. During the error HART
communication may still be required to transmit
diagnostic information. To avoid HART signal clipping
the sensor-transmitter must consume less than 3.1mA
of total quiescent current.
2
Figure 3. 2-Wire Transmitter DC Value with HART
HART Physical Layer Compliance
Achieving HART physical layer certification in 2-wire
transmitter designs can be especially challenging due
to power supply and analog signals sharing the same
transmitter connections. The input capacitance must
be minimized to achieve high input impedance which
can limit the selection of LDOs and transient protection
components. The HART certification process includes
tests to assess waveshape, noise, analog rate of
change, and input impedance to ensure the transmitter
complies with the physical layer specification. The
DAC874xH family of modems have been tested and
HART registered in a 2-wire transmitter reference
design.
Conclusion
An increasing number field transmitter designs are
including HART to take advantage of the additional
communication it adds to the traditional 4-20mA loop.
The incorporation of HART creates additional design
challenges, particularly for the 2-wire field transmitter,
to ensure HART physical layer compliance.
DAC874xH HART modems are HART physical layer
certified enabling designers to meet system level
HART compliance. The devices need minimal
supporting components, offer multiple interfaces with
extended SPI features, and require minimal input
current to meet the requirements of field transmitter
design.
Implementing HART in 2-Wire Field Sensor-Transmitters Garrett Satterfield
Copyright © 2018, Texas Instruments Incorporated
SBAA306 – October 2018
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