Circuit Note

CN-0267

Circuits from the Lab™ reference circuits are engineered and

tested for quick and easy system integration to help solve today’s

analog, mixed-signal, and RF design challenges. For more

information and/or support, visit www.analog.com/CN0267.

Devices Connected/Referenced

ADuCM360

Low Power, Precision Analog

Microcontroller

AD5421

16-Bit, Loop Powered, 4 mA to 20 mA

DAC

AD5700

Low Power HART Modem

Complete 4 mA to 20 mA Loop Powered Field Instrument with HART Interface

Rev. B

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EVALUATION AND DESIGN SUPPORT

Circuit Evaluation Board

CN0267 Circuit Evaluation Board (DEMO-AD5700D2Z)

Design and Integration Files

Schematics, Layout Files, Bill of Materials, Code Example

CIRCUIT FUNCTION AND BENEFITS

The circuit shown in Figure 1 is a complete smart industrial,

loop powered field instrument with 4 mA to 20 mA analog output

and a highway addressable remote transducer (HART®) interface.

HART is a digital 2-way communication in which a 1 mA peak-

to-peak frequency-shift-keyed (FSK) signal is modulated on top

of the standard 4 mA to 20 mA analog current signal. This allows

features such as remote calibration, fault interrogation, and

transmission of process variables, which are necessary in

applications such as temperature and pressure control.

This circuit has been compliance tested, verified, and registered by

the HART Communication Foundation (HCF). This successful

registration provides circuit designers with a high level of

confidence using one or all of the components in the circuit.

The circuit uses the ADuCM360, an ultralow power, precision

analog microcontroller, the AD5421, a 16-bit, 4 mA to 20 mA,

loop powered digital-to-analog converter (DAC), and the AD5700,

the industry’s lowest power and smallest footprint HART-

compliant IC modem.

Figure 1. 4 mA to 20 mA, Loop Powered Field Instrument with HART Interface (Simplified Schematic: All Connections and Decoupling Not Shown)

50Ω

DAC

LOOP–

HART_OUT

ADC_IP

DVDD

COM

AD5421

ADuCM360

PRIMARY

SENSOR

SECONDARY

SENSOR

AD5700

ADC

TEMPERATURE

SENSOR

REF

IOVDD

XTAL1

CORTEX

M3

+

SRAM

FLASH

+

DMA

UART

SPI

CLOCK

RESET

WATCHDOG

WATCHDOG

TIMER

20MΩ

1MΩ

4700pF

1.6Ω

10µF

10µF

470Ω

1µF

L*

L*

L*

TVS

40V

LOW

LEAKAGE

4.7V

LOW LEAKAGE

10Ω

10µF

10µF

1.2MΩ

1.2MΩ

300pF

150pF

150kΩ

0.068µF

0.22µF

1µF

SCLK

SDO

SDIN

SYNC

AIN4

AIN0

AIN3

AIN2

0.1µF

0.01µF

0.01µF

0.1µF

AIN7

1kΩ

1kΩ

1kΩ

1kΩ

1kΩ

0.1µF

1kΩ

1kΩ

0.1µF

0.01µF

0.01µF

AIN1

GND_SW

AVDD

AVDD

ADC1

RXD

TXD

CD

RTS

SCLK0

MISO0

MOSI0

CS0

SIN

P0.5

SOUT

LDAC

XTAL2

3.8664MHz

1µF

REG_CAP

AGND

DGND

EXPOSED

PAD

AGND

DGND

AGND

0.47µF

DVDD_

REG

AVDD_

REG

VREF–

VREF+

PT100

VREF

0.47µF

0.1µF

10µF

10µF

REFOUT1

REFOUT2

P0.4

ADC0

RREF

5.62kΩ

10ppm

SPI INTERFACE

UART INTERFACE

LOOP+

LOOP–

NOTES

1. L* = FERRITE BEAD, 0.3Ω @ DC, 1kΩ @ 100MHz.

2. THE ADuCM360 EXPOSED PAD IS CONNECTED TO DGND.

VREF

DVDD

VOLTAGE

REGULATOR