AN-534
APPLICATION NOTE
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Adding HART® Capability to the AD421, Loop-Powered 4 mA to 20 mA DAC Using the
AD5700 HART Modem
by Albert O’Grady
INTRODUCTION
The highway addressable remote transducer (HART) protocol
is widely accepted in the industry as the standard for digitally
enhanced 4 mA to 20 mA communication with smart field
instruments. This protocol makes use of the Bell 202 frequency
shift keying (FSK) standard to superimpose digital signals at a
low level on the 4 mA to 20 mA analog signal. This allows twoway communication to take place and makes it possible for
additional information, beyond just the normal process
variable, to be communicated to/from a smart field instrument.
The AD5700 is designed to allow the user to implement a
HART-compliant physical layer. This device implements
the necessary modulation, demodulation, carrier detection,
waveshaping, and band-pass filtering to implement the protocol
on-chip. The HART protocol communicates without interrupting the 4 mA to 20 mA signal and allows a host application
(master) to get two or more digital updates per second from a
field device. As the digital FSK signal is phase continuous, there
is no interference with the 4 mA to 20 mA signal. Two different
frequencies, 1200 Hz and 2200 Hz, respectively, are used to
represent binary 1 and 0 as shown in Figure 1.
APPROX
–0.5mA
1200Hz
“1”
2200Hz
“1”
Figure 1. Digital Transmission Using the HART Protocol
11179-001
APPROX
+0.5mA
These tones are superimposed on the dc signal at a low level
with the average value of the sine wave signal being zero.
Therefore, no dc component is added to the existing 4 mA to
20 mA signal regardless of the digital data being sent over the
line. Consequently, existing analog instruments continue to
work in systems that apply HART as the low-pass filtering
present removes the digital signal. A single-pole, 10 Hz, lowpass filter effectively reduces the communication signal to a
ripple of about ±0.01% of the full-scale signal. Figure 2 shows
a block diagram of a field instrument that uses the HART
protocol.
MEMORY
SENSORS
ADC
MICROPROCESSOR
4mA TO 20mA
DAC
MEASUREMENT
CIRCUIT
COMMUNICATION
SYSTEM
11179-002
The AD421 is a complete, loop-powered, digital to 4 mA to
20 mA converter, designed to meet the needs of smart transmitter manufacturers in the industrial control industry. It
provides a high precision, fully integrated, low cost solution
in a compact, 16-lead package and is ideal for extending the
resolution of smart 4 mA to 20 mA transmitters at very
low cost.
Figure 2. Smart and Intelligent Field Instrument
This application note deals primarily with the interface between
the AD421 loop-powered 4 mA to 20 mA DAC and the
AD5700 HART modem, and the necessary steps required to
implement a HART-compliant instrument.
AD421 CURRENT CONTROL CIRCUITRY
The AD421 contains a 16-bit sigma-delta DAC to convert the
digital information loaded to the input latch into a current. The
sigma-delta architecture is particularly useful for the relatively
low bandwidth requirements of the industrial control environment because of its inherent monotonicity at high resolution.
The AD421 guarantees monotonicity to the 16-bit level.
The sigma-delta DAC on the part consists of a second-order
modulator followed by a continuous time filter. The single bit
stream from the modulator controls a switched current source.
This current source is then filtered by three resistor capacitor
filter sections. The resistors for each of the filter sections are
on-chip while the capacitors should be connected to the C1,
C2, and C3 pins.
Rev. B | Page 1 of 4
AN-534
Application Note
TABLE OF CONTENTS
Introduction ...................................................................................... 1
Selecting Coupling Capacitor CC.....................................................3
AD421 Current Control Circuitry ................................................. 1
AD421/AD5700 Interface ................................................................3
Revision History ............................................................................... 2
Learn More .........................................................................................3
REVISION HISTORY
12/12—Rev. A to Rev. B
Updated Format .................................................................. Universal
Changed SYM20C15 HART to AD5700 ......................... Universal
Changes to Figure 3 .......................................................................... 3
Changes to Figure 4 .......................................................................... 4
Rev. B | Page 2 of 4
Application Note
AN-534
To meet the specified HART physical layer low pass characteristic of a double pole with cutoff frequency of 25 Hz, capacitor
C1 should be 0.01 µF, C2 = 0.5 µF, and C3 = 0.16 µF. The HART
signal is coupled into the loop through capacitor CC at node C3.
Capacitors CC and C3 ensure a flat response at the HART
frequencies of 1200 Hz and 2200 Hz. Figure 3 shows a block
diagram of the current control circuitry within the AD421.
AD421
BOOST
Σ-Δ
DAC
12.5kΩ
40kΩ
80kΩ
C2
C1
40kΩ
LOOP
RTN
CC
C1
0.01µF
C2
0.5µF
LOOP(–)
C3
C3
0.16µF
FROM
AD5700
11179-003
4kΩ
Figure 3. AD421 Current Control Circuitry
The DAC output current drives the second section, an operational amplifier and NPN transistor, which acts as a current
amplifier to set the current flowing from the LOOP RTN pin.
The base drive to the NPN transistor servos the voltage across
the 40 Ω sense resistor to equal the voltage across the 80 kΩ
resistor connected between the DAC output and loop return.
SELECTING COUPLING CAPACITOR CC
The HART output signal on the loop is 1 mA p-p and is
generated in the AD421 by applying a suitable voltage signal to
the C3 pin. HART applications accommodate both digital and
analog communications; therefore, it is very important that the
digital HART signal injected onto the loop not interfere with
the mean value of the analog loop current. The HART signal is
ac-coupled to avoid introducing errors. Capacitors CC and C3
attenuate the HART voltage signal from the AD5700 to ensure
that a HART signal of 1 mA p-p gets onto the loop during
HART transmissions. The signal appearing at the C3 pin is
applied across a 40 kΩ resistor and twice this signal is forced
across the 40 Ω loop current sense resistor. Therefore, a 20 mV
signal is required at the C3 pin to ensure a 1 mA current on the
loop. The following attenuation factor is required on the HART
input signal VH from the AD5700:
Attenuation Factor: 20 mV/VH = CC/(CC + C3)
CC = (20E–3 × C3)/(VH − 20E−3)
Filter Time Constant = (CC + C3) × (40E + 3)
As an example, if the HART input signal applied to CC is
500 mV p-p, and the filter time constant is to correspond to 25
Hz, the HART signal band requirement, then CC should be
approximately 6.2 nF.
AD421/AD5700 INTERFACE
Figure 4 shows an evaluation circuit for the AD421/AD5700
interface, which shows the required components to implement
the transmit interface. The external transistor Q1 (DN25D
depletion mode MOSFET), in association with the internal
op amp and voltage reference on the AD421, form a voltage
regulator that regulates the supply voltage for the AD421 from
the loop voltage. LK1, LK2 and LK3 allow this regulated voltage
to be programmed to either 5 V, 3.3 V or 3 V. The external pass
transistor (Q2) is added to reduce the power loading on the
depletion mode MOSFET Q1.
The MOSFET supplies the quiescent current for the AD421.
The BOOST pin on the AD421 sinks the necessary current
through the pass transistor Q2 from the loop so that the current
flowing into BOOST, plus the quiescent current flowing into
COM, equals the programmed loop current.
U3 shows the AD5700 HART modem and its associated
circuitry required to implement the HART interface. The
coupling capacitor C15 couples the HART signal onto the
analog 4 mA to 20 mA loop at the C3 pin of the AD421.
LEARN MORE
While the AD421 is described in this application note, a more
recent solution has been developed by Analog Devices, Inc.,
that uses the AD5421, the ADuCM360 microcontroller and the
AD5700 HART modem IC. This circuit has been compliance
tested, verified, and registered as an approved HART solution
by the HART Communication Foundation. The AD5421 is a
complete, loop-powered, digital to 4 mA-to-20 mA converter
designed to meet the needs of smart transmitter manufacturers
in the industrial control industry. It has a maximum quiescent
current of only 300 µA and provides a high precision, fully
integrated, low cost solution in compact TSSOP and LFCSP
packages.
The AD5421 provides two regulated voltage outputs, which are
used to power itself and other devices in the transmitter. The
DVDD regulator provides a regulated 3.3 V output and the
REGOUT regulator provides a user-selectable 1.8 V, 2.5 V, 3.0 V,
3.3 V, 5 V, 9 V, or 12 V output. The AD5421 also contains a
precision 2.5 V reference, thus eliminating the need for a
discrete regulator and voltage reference.
Rev. B | Page 3 of 4
AN-534
Application Note
L1
VCC
LOOP PLUS
J2-1
28F0195-100
Q1
D
S
C11
0.1µF
G
C10
0.01µF
1 REFOUT1
+ C5
4.7µF
C
Q2
BC548
B
VCC 16
E
2 REFOUT2 BOOST 15
C6
0.01µF
+ C4
4.7µF
LK3
LK2
3 REFIN
COMP 14
4 LV
DRIVE 13
C7
2.2µF
LK1
U1
C1 12
C2
0.5µF
6 CLOCK
C2 11
ADuCM360
C8
0.1µF
C1
0.01µF
AD421
5 LATCH
+
R1
1kΩ
C9
1nF
C3
0.16µF
7 DATA
C3 10
8 LOOP_RTN
COM 9
L2
R1
470Ω
28F0195-100
LOOP MINUS
J2-2
C15
6.2nF
1µF
VCC
AD5700/AD5700-1*
TXD
RXD
RTS
CD
HART_OUT
REF
1µF
AGND
DGND
1.2MΩ
300pF
150kΩ
150pF
11179-004
ADC_IP
1.2MΩ
*NOT ALL PINS ARE SHOWN, FOR THE PURPOSES OF CLARITY.
Figure 4. AD421/AD5700 Interface Circuit Diagram
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registered trademarks are the property of their respective owners.
AN11179-0-12/12(B)
Rev. B | Page 4 of 4