CN0267 Datasheet, PDF (4/8 Pages) Analog Devices – Devices Connected

CN-0267
The ADuCM361 on-chip DAC with an external transistor can
be used to control the 4 mA to 20 mA loop, refer to CN-0300
for details.
The AD5421 can be connected via the protection directly to the
loop. Alternatively, a depletion mode N-channel MOSFET can
be connected between the AD5421 and the loop power supply,
as shown in Figure 2. The use of the additional MOSEFT in
this configuration keeps the voltage drop across the AD5421 at
approximately 12 V, lowers the power dissipated in the AD5421
package, and therefore improves the 4 mA to 20 mA analog output
accuracy. It also increases the maximum voltage allowed in the
loop to the level of the MOSFET rating. The additional MOSFET
has no effect on the HART communication.
L ^LOOP+
AD5421
REG_IN
DN2540
BSP129
200kΩ
4700pF
DRIVE
V_LOOP
COM
20MΩ
1MΩ
TVS
40V
LOW
LEAKAGE
LOOP–
L ^LOOP–
4.7V
LOW
LEAKAGE
TO
HART
INPUT
FILTER
Figure 2. MOSEFT Connected to the AD5421 Loop Power Supply
The AD5700 is used with a 3.8664 MHz crystal in this circuit,
which is the configuration achieving the lowest power consumption.
Alternatively, the AD5700-1, with an integrated 0.5 %precision
internal oscillator, can be used. The internal oscillator increases
the modem power supply current by 225 µA maximum, compared
to the crystal oscillator, but because no external crystal is needed,
this option provides both cost savings and reduced board area
requirements.
For the applications that are not loop powered, the AD5410,
AD5420, AD5422, or AD5755 are good choices for the 4 mA to
20 mA DAC.
Circuit Note
CIRCUIT EVALUATION AND TEST
Circuit Hardware
The circuit shown in Figure 1 is built on the DEMO-AD5700D2Z
printed circuit board shown in Figure 3.
Figure 3. DEMO-AD5700D2Z Printed Circuit Board (Pressure Sensor Not Included)
The DEMO-AD5700D2Z circuit board includes some additional
features for easy system evaluation. The 0.1 inch-pitch connector
footprints allow optional primary and secondary sensor
connections. There are test points for HART RTS and CD
that may be needed for HART compliance tests.
A connector on the edge of the DEMO-AD5700D2Z makes the
ADuCM360 single wire and UART download/debug signals
accessible allowing easy software development, code download,
and in-circuit debugging and emulation. The connector, with a
small header extender included with the DEMO-AD5700D2Z
board, is compatible with all Analog Devices, Inc., Cortex-M3
based development tools, such as the EVAL-ADuCM360QSPZ
evaluation kit (the evaluation kit is not included with the
DEMO-AD5700D2Z board).
These features are not shown in the simplified diagram in Figure 1;
however, they can be seen in the complete circuit schematic in
the CN-0267 Design Support Package. The design support package
also includes a full field instrument C-code example, which
enables complete verification and evaluation of all hardware
blocks and features of the circuit, and a limited verification of
the HART interface functionality. For detailed information about
HART interface specifications and resources, contact the Hart
Communication Foundation.
HART Compliance
The DEMO-AD5700D2Z has been verified to be compliant with
HART FSK Physical Layer Specification (HCF_SPEC-054,
Revision 8.1), using methods and equipment specified in the
HART Physical Layer Test Specification (HCF_TEST-2,
Revision 2.2). The board was submitted to the Hart
Communication Foundation and was successfully registered.
The registered circuit can be found on the HART Communication
Foundation (HFC) web site in the product catalog as DEMO-
AD5700D2Z.
The results of two of the tests involved the output noise during
silence and the analog rate of change.
Rev. B | Page 4 of 8