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AN-1065
APPLICATION NOTE
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Configuring the AD5420 for HART Communication Compliance
by Maurice Egan
Rev. A | Page 1 of 4
INTRODUCTION
For many years, 4 mA to 20 mA communications have been used
in process control instrumentation. The 4 mA to 20 mA communi-
cations method is reliable and robust, and offers high immunity
to environmental interference over long communication distances.
The disadvantage of 4 mA to 20 mA communication is that it
is a one-way communication and can only transmit one process
variable, which in modern industrial control systems is a limitation.
The development of the highway addressable remote transducer
(HART
®) standard opened up new possibilities for 4 mA to 20 mA
communication. HART is a digital two-way communication that
is compatible with 4 mA to 20 mA current loops. A 1 mA peak-
to-peak frequency shift keyed (FSK) signal is modulated on top
of the 4 mA to 20 mA analog current signal. The two frequencies
used are 1200 Hz representing Logic 1 and 2200 Hz representing
Logic 0, based on the BELL 202 communications standard.
IMPLEMENTATION
The AD5420 is a 16-bit, digital to 4 mA to 20 mA converter
with inputs to accommodate the output of a HART modem.
The output of the AD5700 HART modem is attenuated and ac
coupled at the CAP2 pin of the AD5420; this results in the
modem output being modulated on top of the 4 mA to 20 mA
analog current without affecting the dc level of the current.
The circuit in Figure 1 shows how the AD5420 can be inter-
faced with a microcontroller and the AD5700 HART modem to
construct a HART capable 4 mA to 20 mA current output,
typical of PLC and DCS systems.
08875-001
DV
CC
CAP2
AV
DD
CAP1 AV
DD
DV
DD
GND
FAULT
CLEAR
0.1µF
REFIN
I
OUT
REFOUT
LATCH
SCLK
SDIN
SDO
TXD
RXD
RTS
HART_OUT
V
CC
REF
ADC_IP
CD
XTAL1
XTAL2
AGND
DGND
AD5420
0.1µF
10µF
UART
INTERFACE
0.1µF
10µF
0.1µF
10µF
10.8V TO 60V
AV
DD
C3
C2
C1
HOST CONTROLLER
SUCH AS
ADuC7060
2.7V TO 5.5V
10kΩ
AD5700
4mA TO 20mA
CURRENT LOOP
R
L
500Ω
8.2pF8.2pF
3.6864MHz
150kΩ
1.2MΩ
1.2MΩ
300pF
1µF
0.1µF
150pF
Figure 1. AD5420 in HART-Enabled Circuit
