Data Sheet
AD5755-1
Rev. E | Page 49 of 52
INDUSTRIAL HART CAPABLE ANALOG OUTPUT
APPLICATION—SHARED VOUT_X AND IOUT_X PIN
Many industrial control applications have requirements for
accurately controlled current output signals, and the AD5755-1
is ideal for such applications.
Figure 89 shows the AD5755-1 in
a circuit design for a HART-enabled output module, specifically
for use in an industrial control application in which both the
voltage output and current output are available—one at a time—on
one pin, thus reducing the number of screw connections required.
There is no conflict with tying the two output pins together
because only the voltage output or the current output can be
enabled at any one time.
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 AD5755-1. 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
suppressor (TVS) is placed on the IOUT/VOUT connection. For
added protection, clamping diodes are connected from the
IOUT_x/VOUT_x pin to the AVDD and AVSS 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.
HART_OUT
GND
TXD
RXD
RTS
CD
VCC
AD5700/AD5700-1
0.1F
ADC_IP
REF
GND
REFIN
RESET
AVDD
SW (×4)
VBOOST (×4)
CHART A
AD5755-1
DVDD
+15V
+5V
10k
0.1F
10F
2.7V TO 5.5V
1.2M
150k
300pF
150pF
1F
MCU
D1
D2
RL
500
RP
VOUTA
+VSENSE_A
D3
AVSS
UART
INTERFACE
AVCC
IOUT B, C, D
IOUT A
CHART B, C, D
VOUT B, C, D
REFOUT
DGND
LDAC
SDO
SDIN
SCLK
SYNC
CLEAR
FAULT
ALERT
47nF
C2
22nF
C1
5k
4mA TO 20mA
CURRENT LOOP
09226-
089
Figure 89. AD5755-1 in HART Configuration