Datasheet

Data Sheet AD5757
Rev. D | Page 43 of 44
INDUSTRIAL HART CAPABLE ANALOG OUTPUT
APPLICATION
Many industrial control applications have requirements for
accurately controlled current output signals, and the AD5757 is
ideal for such applications. Figure 67 shows the AD5757 in a
circuit design for a HART-enabled output module, specifically
for use in an industrial control application.
The design provides for a HART-enabled current output, with
the HART capability provided by the AD5700/AD5700-1 HART
modem, the industry’s lowest power and smallest footprint HART-
compliant IC modem. For additional space-savings, the AD5700-1
offers a 0.5% precision internal oscillator. The HART_OUT signal
from the AD5700 is attenuated and ac-coupled into the CHARTx
pin of the AD5757. Such a configuration results in the AD5700
HART modem output modulating the 4 mA to 20 mA analog
current without affecting the dc level of the current. This circuit
adheres to the HART physical layer specifications as defined by
the HART Communication Foundation.
For transient overvoltage protection, a 24 V transient voltage sup-
pressor (TVS) is placed on the I
OUT
/V
OUT
connection. For added
protection, clamping diodes are connected from the I
OUT_x
/V
OUT_x
pin to the AV
DD
and GND power supply pins. A 5 kΩ current
limiting resistor is also placed in series with the +VSENSE_X
input. This is to limit the current to an acceptable level during a
transient event. The recommended external band-pass filter for
the AD5700 HART modem includes a 150 kΩ resistor, which
limits current to a sufficiently low level to adhere to intrinsic safety
requirements. In this case, the input has higher transient voltage
protection and should, therefore, not require additional protection
circuitry, even in the most demanding of industrial environments.
AD5757
09225-065
MCU
UART
INTERFACE
10µF
10kΩ
10µF
0.1µF
0.1µF
0.1µF
0.1µF
2.7V TO 5.5V
DV
DD
AV
DD
RESET
ALERT
FAULT
CLEAR
SYNC
LDAC
SCLK
SDIN
SDO
DGND
AD5700/AD5700-1
TXD
V
CC
RXD
RTS
CD
HART_OUT
15V
AV
CC
5V
SW(X4)
V
BOOST
(X4)
IOUT B,C,D
CHART B,C,D
IOUTA
D2
D1
D3
R
P
R
L
REFOUT REFIN CHART A GND
22nF
C1
47nF
C2
4.20mA
CURRENTLOOP
ADC_IP
REF
GND
1µF
150kΩ
150pF
300pF
1.2MQ
1.2MQ
Figure 66. AD5757 in HART Configuration