CN0267 Datasheet, PDF (3/8 Pages) Analog Devices – Devices Connected

Circuit Note
Output Protection
A transient voltage suppressor (TVS) protects the 4 mA to 20 mA
HART interface from overvoltage. Its voltage rating should prevent
exceeding the AD5421 absolute maximum voltage of 60 V on
the REGIN pin. Note that the TVS leakage current can affect the
current output accuracy; therefore, pay attention to the leakage
current at a given loop voltage and temperature range when
selecting this component.
An external depletion-mode FET can be used with the AD5421
to increase the loop voltage maximum
The circuit is protected against reversed polarity by a pair of
diodes in series with loop output.
The ferrite beads in series with the loop together with the 4700 pF
capacitor improve the system EMC performance. Do not use a
higher capacitance across the loop terminals because of the HART
network specifications.
The 4.7 V, low leakage, Zener diode protects the AD5421 on-chip,
50 Ω loop sense resistor in the event of an accidental external
voltage between the AD5421 COM pin and LOOP− pin (for
example, when programming the ADuCM360 or debugging
the circuit).
Power Supplies and Power Management
The complete field instrument circuitry, including the sensor drive
current, must operate on the limited amount of power available
from the 4 mA to 20 mA loop. This is a common challenge in
any loop powered field instrument design. The circuit in Figure 1
provides an example of delivering both a low power and high
performance solution. All three integrated circuits used in the
application are designed for low power, and the circuit leverages
their integrated features to deliver a flexible power management
structure and an optimum loop-powered solution.
The AD5421 is powered by the 4 mA to 20 mA loop voltage and
provides a regulated low voltage for the rest of the circuit. The
AD5421 REGOUT voltage is pin programmable from 1.8 V to 12 V
depending on circuit requirements. The circuit in Figure 1 uses
the 3.3 V supply voltage option as an example for the input sensors
used. However, the ADuCM360 and the AD5700 have a wider
power supply voltage range; therefore, a different power supply
voltage can be used to suit the application.
The REGOUT RC filter (10 µF/10 Ω/10 µF) helps to prevent any
interference coming from the loop affecting the sensor analog
front-end. It also prevents any interference generated by the
circuit, specifically by the controller and the digital circuitry,
from coupling back to the loop, which is important for a reliable
HART communication.
CN-0267
The AD5700 HART modem is supplied through an additional
RC filter (470 Ω/1 µF). This filter is very important in the loop
powered application because it prevents current noise from the
AD5700 from coupling to the 4 mA to 20 mA loop output, which
would otherwise affect the HART communication. The 4 mA to
20 mA loop noise performance is specifically addressed by the
HART in-band, noise during silence test. The AD5700 modem
uses the external crystal with 8.2 pF capacitors to ground on the
XTAL1 and XTAL2 pins, which is the option using the least
possible power.
The ADuCM360 has very flexible internal power management,
with many options for powering and clocking all the internal
blocks and, when utilized by the software, allows an optimal
balance between the required function, performance, and power
for the specific instrument application. Refer to the ADuCM360
product page and the AN-1111 Application Note.
The analog front-end AVDD is supplied from another filter
(10 µF/ferrite bead/1.6 Ω/10 µF) to minimize power supply noise
for better performance with respect to low voltage sensor signals.
The GND_SW ground switch pin of the ADuCM360 controls
the excitation/power supply for the primary sensor. The switch
is off as a default at the instrument power up. This default allows
the system to be fully configured, including appropriate power
modes, before turning on the sensor, and thus minimizes any
possible power-up spikes on the 4 mA to 20 mA loop output.
Similarly, the secondary sensor is supplied from the programmable
current source of the ADuCM360, and therefore, its power is
fully controlled by the software.
ADuCM360 Software
A basic code example that demonstrates the functionality and
performance of the circuit can be found in the CN-0267 Design
Support Package.
The code example includes a basic HART slave command response
to demonstrate the hardware function and capability. However,
the code example does not include the protocol layers of the
HART communication.
COMMON VARIATIONS
The ADuCM360 has a high performance and very flexible analog
front-end, with 12 analog input pins and extra pins for voltage
reference and ground switch. It allows direct interface to multiple
analog sensors of varying types, such as any resistive bridge sensors,
resistive temperature sensors, or thermocouples. Therefore, do
not limit the field instrument solution to temperature-compensated
pressure measurement only because it can be used for almost any
sensor field instrument.
The ADuCM361 can be used as an alternative to the ADuCM360
in applications that need only one Σ-Δ ADC in the analog front-
end. Aside from the second ADC, the ADuCM361 contains all
the features of the ADuCM360.
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