Datasheet

Data Sheet AD5700/AD5700-1
Rev. F | Page 17 of 24
APPLICATIONS INFORMATION
SUPPLY DECOUPLING
It is recommended to decouple the V
CC
and IOV
CC
supplies with
10 F in parallel with 0.1 F capacitors to ground. For many
applications, 1 F in parallel with 0.1 F ceramic capacitors to
ground should be sufficient. The REG_CAP voltage of 1.8 V is
used to supply the AD5700/AD5700-1 internal circuitry and is
derived from the V
CC
supply using a high efficiency clocking
LDO. Decouple this REG_CAP supply with a 1 µF ceramic
capacitor to ground. It is also required to decouple the REF pin
with a 1 µF ceramic capacitor to ground. Place decoupling
capacitors as close to the relevant pins as possible.
For loop-powered applications, it is recommended to connect a
resistance in series with the V
CC
supply to minimize the effect of
any noise, which may, depending on the system configuration, be
introduced onto the loop as a result of current draw variations
from the AD5700/AD5700-1. For typical applications, 470 Ω of
resistance has proven most effective. However, depending on the
application conditions, alternative values may also be acceptable
(see R1 in Figure 31).
TRANSIENT VOLTAGE PROTECTION
Many industrial control applications have requirements for
HART-enabled current input and output modules. Figure 28
shows an example of a HART-enabled current input module
that contains transient voltage protection circuitry, which is
very important in harsh industrial control environments.
The module is powered from a 24 V field supply, and the 250 Ω
load is within the low impedance module itself. This configuration
is in contrast to Figure 29, which demonstrates a secondary HART
device, in which the load is outside of the module. For transient
voltage protection, a 10 V unidirectional (for protection against
positive high voltage transients) transient voltage suppressor (TVS)
is placed at the connection point of the current input module.
The TVS component that is used in a given application circuit
must have power ratings that are appropriate to the individual
system. When choosing the TVS, low leakage current is also an
important specification for maintaining the accuracy of the analog
current input. In the event of a transient spike, the 22 Ω series
resistor acts as a current limiting resistor for the FSK output pin.
The FSK input pin is inherently protected by the 150 kΩ resistor,
which forms part of the recommended external filter circuitry
at the FSK input. The voltage divider, made up of both a 75 kΩ
resistor and a 22 kΩ resistor, is used to maintain a 0.75 V dc bias
at the field side of the FSK output switch.
Figure 28. Current Input Module, HART Circuit
Figure 29. Secondary HART Device
10435-031
HART_OUT
ADC_IP
AGND
TXD
RXD
RTS
CD
V
CC
AD5700/
AD5700-1
ADC
REF
10nF
3.3V
75kΩ
22kΩ
2.2µF
22Ω
6.8nF
3.3V
1.2MΩ
1.2MΩ
300pF
150kΩ
150pF
10µF
10V
400W
MICRO-
CONTROLLER
V
LOOP
24V
FIELD
INSTRUMENT
20kΩ
250Ω
1µF
10435-030
HART_OUT
ADC_IP
AGND
TXD
RXD
RTS
CD
AD5700/
AD5700-1
REF
10nF
3.3V
75kΩ
22kΩ
2.2µF
50V
4.7Ω
0.5W
6.8nF
50V
3.3V
1.2MΩ
1.2MΩ
300pF
150kΩ
150pF
39V
1500W
20Ω
10V
400W
HOST
1µF
V
CC