ANALOG DEVICES CN-0278 Technical data

Circuit Note

CN-0278

AD5700,

Low Power HART Modem

AD5422

16-Bit Current and Voltage Output DAC

Rev. 0

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I

OUT

LATCH

AD5422

AD5700

SCLK
SDIN
SDO

GND

CLEAR

FAULT

REFOUT

REFIN

4mA TO 20mA

CURRENT LOOP

0.1µF

0.1µF

10µF

DIGITAL

INTERFACE

UART

INTERFACE

10.8V TO 26.4V

2.7V
TO

5.5V

HART_OUT

DGND

TXD
RXD

RTS
CD

V

CC

ADC_IP

REF

150pF

0.1µF

1µF

18Ω

10kΩ

1.2MΩ

1.2MΩ

300pF

150kΩ

C1

4.7nF

*C2

*NC

R

SET

DV

CC

AV

DD

CAP2 CAP1

R

L

15kΩ

R

H

27kΩ

C

H

8.2nF

C

L

4.7nF

AGND

0.1µF

10µF

V

OUT

+V

SENSE

–V

SENSE

*OP184

0.1µF

10µF

0V TO –26.4V

AV

SS

500Ω

*OP184 WAS USED F OR THESE MEASUREM E NTS
BUT AN ALTERNATIVES SUCH AS THE O P 1177
COULD ALSO BE US E D FOR THIS PURPOSE.

10kΩ

D1

D2

D3

D4

AV

SS

10803-001

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/CN0278.

Complete 4 mA to 20 mA HART Solution with Additional Voltage Output Capability

EVALUATION AND DESIGN SUPPORT

Circuit Evaluation Boards

AD5422 Circuit Evaluation Board (EVAL-AD5422EBZ,

LFCSP version)

AD5700-1/AD5700 Evaluation Board (EVAL-AD5700-1EBZ)

Design and Integration Files

Schematics, Layout Files, Bill of Materials

CIRCUIT FUNCTION AND BENEFITS

The circuit shown in Figure 1 uses the AD5700, the industry’s
lowest power and smallest footprint HART
modem, and the AD5422, a 16-bit current output and voltage
output DAC, to form a complete HART-compatible 4 mA to
20 mA solution. The use of the OP184 in the circuit allows the
I

OUT

number of screw connections required in programmable logic
control (PLC) module applications. For additional space savings,
the AD5700-1 offers a 0.5% precision internal oscillator.

Devices Connected/Referenced

AD5700-1

and V

pins to be shorted together, thus reducing the

OUT

®

1

-compliant IC

Figure 1. AD5422 HART-Enabled Circuit Simplified Schematic

1

HART is a registered trademark of the HART Communication Foundation

Circuits from the

each circuit, and their function and performance have been tested and verified in a lab environment at

be liable for direct, indirect, special, incidental, consequential or punitive damages due to any cause

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: [phone]

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CN-0278 Circuit Note

STOP

START

8-BIT DATA + PARITY

TXD

HART_OUT

"1" = MARK

1.2kHz

"0" = SPACE

2.2kHz

10803-002

Application Note AN-1065 describes a manner in which the
AD5420 I

DAC can be configured for HART communication

OUT

compliance. AN-1065 outlines how the AD5700 HART modem
output can be attenuated and ac coupled into the AD5420 via the
CAP2 pin. The same is true of the AD5422. However, if the
application involves a particularly harsh environment, an
alternative circuit configuration can be used which offers better
power supply rejection characteristics. This alternative circuit
requires the use of the external R
the HART signal into the R

resistor and involves coupling

SET

pin of the AD5420 or AD5422.

SET

The CN-0270 describes this solution for the AD5420, typical of
line-powered transmitter applications. The current circuit note
is relevant to the AD5422, which, unlike the AD5420, offers
both a voltage and a current output pin, and so is particularly
useful in PLC/distributed control system (DCS) applications.
The AD5422 is available in both 40-lead LFCSP and 24-lead
TSSOP packages and the relevance of this, to the circuit
characteristics, is examined in the Circuit Description section.

This circuit adheres to the HART physical layer specifications as
defined by the HART Communication Foundation, for example,
the output noise during silence and the analog rate of change
specifications.

For many years, 4 mA to 20 mA communication has been used
in process control instrumentation. This communication method is
reliable and robust, and offers high immunity to environmental
interference over long communication distances. A limitation,
however, is that only 1-way communication of one process
variable at a time is possible.

The development of the highway addressable remote transducer
(HART) standard provided highly capable 2-way digital
communication, simultaneously with the 4 mA to 20 mA analog
signaling used by traditional instrumentation equipment. This
allows for features such as remote calibration, fault interrogation,
and transmission of additional process variables. Put simply,
HART is a digital two-way communication in which a 1 mA
peak-to-peak, frequency-shift-keyed (FSK) signal is modulated
on top of the 4 mA to 20 mA analog current signal.

CIRCUIT DESCRIPTION

Figure 1 shows the manner in which the AD5422 can be combined
with the AD5700 HART modem and a UART interface to
construct a HART-capable 4 mA to 20 mA current output, typical
of PLC and DCS systems. The buffer connected to the +V
pin is not necessary if the application does not require the I
and V

AD5700 is attenuated and ac-coupled into the R
AD5422. If the external R

alternative method of connecting the AD5422 and the AD5700
via
previously described. This method is only relevant to the 40-lead
LFCSP package option of the AD5422 because the lower pin­count 24-lead TSSOP package does not contain a CAP2 pin.

While the method described in the current circuit note requires
the use of the external R
power supply rejection performance than the alternative
application note solution. The use of either solution results in
the AD5700 HART modem output modulating the 4 mA to 20 mA
analog current (as shown in Figure 2) without affecting the dc level
of the current. The diode protection circuitry (D1 to D4) is
discussed in more detail in the Transient Voltage Protection
section.

pins to be shorted. The HART_OUT signal from the

OUT

pin of the

SET

resistor is not being used, an

SET

the CAP2 pin can be found in Application Note AN-1065, as

resistor, in return, it provides better

SET

Figure 2. AD5700/AD5700-1 Sample Modulator Waveform

SENSE

OUT

Rev. 0 | Page 2 of 10

Circuit Note CN-0278

CAP1

AD5422

OP184

C1

CAP2

BOOST

AV

DD

I

OUT

FAULT

R

SET

R

L

R

SET

AV

SS

C

COMP

R

SET

+V

SENSE

–V

SENSE

–V

OUT

C

H

C

L

R3

R2

12-/16-BIT

DAC

10564-003

R

H

V

HART

RANGE

SCALING

GND

–50
–55
–60
–65
–70
–75
–80
–85
–90

–95
–100
–105
–110

1

10 100 1k 10k 100k

I (I

OUT

) /HART (V) (dB)

FREQUENCY ( Hz )

10803-004

↓ ↓ ↓ ↓

↓

↓ ↑ ↓ ↓

CH1 200mV CH2 2.00V M 500µs CH2 1.76V

<10Hz

2

1

CH1
MAX

280mV

CH1
MIN

–288mV

CH1

p-p

568mV

CH2
MAX

NONE

CH2
MIN

NONE

MEASURE

10803-005

Determining the Values of the External Components

Capacitors, C1 and C2, can be used in conjunction with the digital
slew rate control functionality of the part to control the slew
rate of the I

signal of the AD5422. In determining the absolute

OUT

values of the capacitors, ensure that the FSK output from the
modem is passed undistorted. Thus, the bandwidth presented to
the modem output signal must pass the 1200 Hz and 2200 Hz
frequencies. Figure 3 shows a circuit that achieves this requirement.
In this case, C2 (shown in Figure 1) is left open-circuit.

Table 1. Effect on Frequency Response of Individual
Component Value Increase

Component C1 CH CL RH

fL (Hz)
fH (kHz)
G (dB)

No change No change No change

The output of the modem is an FSK signal consisting of 1200 Hz
and 2200 Hz shift frequencies. This signal must translate to a
1 mA p-p current signal. To achieve this, the signal amplitude at
the R

pin must be attenuated. This is due to the internal current

SET

gain configuration in the AD5422 design. Assuming that the
modem output amplitude is 500 mV p-p, its output must be
attenuated by 500/150 = 3.33. This attenuation is achieved by
means of R

and CL.

H

The measurements in this circuit note were completed using the
following component values:

• C1 = 4.7 nF

• R

= 27 kΩ

H

• C

= 4.7 nF

L

• C

= 8.2 nF

H

Figure 5 shows the individual 1200 Hz and 2200 Hz shift
frequencies measured across a 500 Ω load resistor. Channel 1
shows the modulated HART signal coupled into the AD5422
output (set to output 4 mA), while Channel 2 shows the AD5700
TXD signal.

Figure 3. AD5422 and AD5700 HART Modem Connection

The low-pass and high-pass filter circuitry is formed through
the interaction of R

, CL, CH, and C1, along with some internal

H

circuitry in the AD5422. In calculating the values of these
components, the low-pass and high-pass frequency cutoff point
targets were >10 kHz and <500 Hz, respectively. Figure 4 shows
a plot of the simulated frequency response, while Tab l e 1 shows
the effect on the frequency response of increasing each component
while the remaining component values are kept constant.

Figure 4. Simulated Frequency Response

Figure 5. FSK Waveforms Measured Across a 500 Ω Load

HART Compliance

For the circuit in Figure 1 to be HART-compliant, it must meet
the HART physical layer specifications. There are numerous
physical layer specifications included in the HART specification
documents. The two that are most important in this case are the
output noise during silence and the analog rate of change.

Rev. 0 | Page 3 of 10