CN-0267

Circuit Note

Rev. B | Page 4 of 8

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.

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 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-

The results of two of the tests involved the output noise during

silence and the analog rate of change.

200

kΩ

DN2540

BSP129

LOOP–

COM

AD5421

DRIVE

20MΩ

1MΩ

4700pF

L

L

TVS

40V

LOW

LEAKAGE

4.7V

LOW

LEAKAGE

TO

HART

INPUT

FILTER

LOOP+

LOOP–