CN-0278 Circuit Note
Figure 10 shows the results of changing the slew rate control
settings to SR clock = 5 and SR step = 2, while leaving the C1
capacitor value unchanged at 4.7 nF. This results in a transition
time of approximately 240 ms. The peak amplitude at the output
of the filter can be reduced further by increasing the value of
C1, configuring a slower slew rate, or a combination of both.
CH1
5.00V CH2 50.0mV M 50.0ms CH1 6.20V
<10Hz
2
1
CH1
p-p
8.00V
CH1
FREQ
?
CH2
p-p
88.0mV
CH2
MAX
42.0mV
CH2
MIN
–46.0mV
MEASURE
10803-010
Figure 10. AD5422 Output (CH1) and HART Filter Output (CH2), SR Clock = 5,
SR Step = 2, C1 = 4.7 nF, C2 = NC
Transient Voltage Protection
The AD5422 contains ESD protection diodes that prevent damage
from normal handling. The industrial control environment can,
however, subject I/O circuits to much higher transients. To protect
the AD5422 from excessively high voltage transients, external
power diodes and a surge current limiting resistor may be required,
as shown in Figure 1. The constraint on the resistor value, shown
in Figure 1 as 18 Ω, is that during normal operation the output
level at I
OUT
must remain within its voltage compliance limit of
AV
DD
− 2.5 V, and the two protection diodes and resistor must have
the appropriate power ratings. With 18 Ω, for a 4 mA to 20 mA
output, the compliance limit at the terminal is decreased by
V = I
MAX
× R = 0.36 V. There is also a 10 kΩ resistor shown at
the positive input of the OP184 buffer. This protects the amplifier
by limiting the current during a transient event. Further protection
can be provided with transient voltage suppressors (TVS) or
transorbs. These are available as both unidirectional and
bidirectional suppressors, and in a wide range of standoff
and breakdown voltage ratings. Size the TVS with the lowest
breakdown voltage possible while not conducting in the
functional range of the current output. It is recommended
that all remotely connected nodes be protected.
In many process control applications, it is necessary to provide
an isolation barrier between the controller and the unit being
controlled to protect and isolate the controlling circuitry from
any hazardous common-mode voltages that may occur.
The iCoupler family of products from Analog Devices, Inc., provides
voltage isolation in excess of 2.5 kV. Further information on
iCoupler products is available at www.analog.com/icouplers. To
reduce the number of isolators required, nonessential signals, such
as CLEAR, can be connected to GND;
FAULT
and SDO can be
left unconnected, reducing the isolation requirements to only three
signals. However, note that either
FAULT
or SDO are required
to provide access to the fault detection features of the
AD5422.
COMMON VARIATIONS
A common variation on the circuit shown in Figure 1 is to use
the AD5420, which is similar to the AD5422, but contains only
a current output. It therefore does not contain the OP184 buffer
configuration at the output. This AD5420 and AD5700 HART
modem circuit is described in more detail in CN-0270. Circuit
Note CN-0065 provides extra information on an IEC 61000-
compliant solution for a fully isolated output module using the
AD5422 and the ADuM1401 digital isolator. Circuit Note CN-0233
contains information on providing power and data isolation using
the ADuM3471 PWM controller and transformer driver with
quad-channel isolators.
If multiple channels are required, the AD5755-1 quad voltage
and current output DAC may be used. This product has innovative
on-chip dynamic power control that minimizes package power
dissipation in current mode. Each channel has a corresponding
CHARTx pin so that HART signals can be coupled to the
current output of the AD5755-1.
The AD5421 and the AD5700 HART modem can be combined if
the requirement is a loop powered, 4 mA to 20 mA HART solution.
Such a HART enabled smart transmitter reference demo circuit
was developed by Analog Devices and uses the AD5421, the
ADuCM360, and the AD5700 modem. This circuit has been
compliance tested, verified, and registered as an approved
HART solution by the HART Communication Foundation.
CIRCUIT EVALUATION AND TEST
To build this circuit, it requires the use of the AD5422 evaluation
board (EVAL -AD5422EBZ, LFCSP version) and the AD5700-1
evaluation board (EVAL-AD5700-1EBZ), see Figure 11. As well as
the two evaluation boards, the circuit also requires three external
capacitors (C1, C
H
, and C
L
), a resistor (R
H
), a load resistor (R
L
),
a buffer amplifier, and a UART interface.
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