EVAL-AD421EB【PDF 11/14 页】Loop-Powered 4 mA to 20 mA DAC

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AD421

–

REV. C

REF IN

DATA OUT

SCLK

DATA IN

AGND

DGND

MCLK IN

MCLK OUT

AD7714/

AD7715

ANALOG

DIGITAL

CONVERTER

SENSORS

RTD

mV TC

4.7 F

REF OUT1

BOOST

COMP

DRIVE

LOOP

RTN

REF OUT2

REF IN

CLOCK

LATCH

DATA

COM

LOOP

POWER

0.01 F

DN25D

2.2 F

3.3V

1.25V

4.7 F

AMBIENT

TEMP

SENSOR

AD421

MICROCONTROLLER

GND

0.01 F

1000pF

0.1 F

100k

Figure 14. AD421 in Smart Transmitter Application

Figure 16 shows a block diagram of a smart and intelligent

transmitter. An intelligent transmitter is a transmitter in which

the functions of the microprocessor are shared between deriving

the primary measurement signal, storing information regarding

the transmitter itself, its application data and its location and

also managing a communication system which enables two way

communication to be superimposed on the same circuit that

carries the measurement signal. A smart transmitter incorporat-

ing the HART protocol is an example of a smart intelligent

transmitter.

4mA TO 20mA

MEASUREMENT

CIRCUIT

MICRO-

PROCESSOR

D/A

CONVERTER

A/D

CONVERTER

MEMORY

SENSORS

COMMUNICATION

SYSTEM

Figure 16. Smart and Intelligent Transmitter

Figure 17 shows an example of the AD421 in a HART transmit-

ter application. Most of the circuit is as outlined in the smart

transmitter as shown in Figure 14. The HART data transmitted

on the loop is received by the transmitter using a bandpass filter

and modem and the HART data is transferred to the micro-

controller’s UART or asynchronous serial port. HART data to

be transmitted on the loop is sent from the microcontroller’s

UART or asynchronous serial port to the modem. It is then

waveshaped before being coupled onto the AD421’s output at

the C3 pin. The value of the coupling capacitor C

is determined

by the waveshaper output and the C3 capacitor of the AD421. The

blocks containing the Bell 202 Modem, waveshaper and bandpass

filter come in a complete solution with the 20C15 from Symbios

Logic, Inc., or HT2012 from SMAR Research Corp.

For a more complete AD421-20C15 interface, please refer to

Application Note AN-534 on the Analog Devices’ website

www.analog.com or contact your local sales office.

HART Interfacing

The HART protocol uses a frequency shift (FSK) keying tech-

nique based on the

Bell 202 Communication Standard

which is

one of several standards used to transmit digital signals over

the telephone lines. This technique is used to superimpose

digital communication on to the 4 mA to 20 mA current loop

connecting the central system to the transmitter in the field.

Two different frequencies, 1200 Hz and 2200 Hz, are used to

represent binary 1 and 0 respectively, as shown in Figure 15.

These sine wave tones are superimposed on the dc signal at a

low level with the average value of the sine wave signal being

zero. This allows simultaneous analog and digital communica-

tions. Additionally, no dc component is added to the existing

4 mA to 20 mA signal regardless of the digital data being sent

over the line. Consequently, existing analog instruments con-

tinue to work in systems that implement HART as the low-pass

filtering usually present effectively removes the digital signal. A

single pole 10 Hz low-pass filter effectively reduces the commu-

nication signal to a ripple of about

0.01% of the full-scale

signal. The HART protocol specifies that master devices like a

host control system or a hand held terminal transmit a voltage

signal whereas a slave or field device transmits a current signal.

The current signal is converted into a corresponding voltage by

the loop load resistor.

APPROX

+0.5mA

APPROX

–

0.5mA

1200Hz

“

”

2200Hz

“

”

Figure 15. HART Transmission of Digital Signals

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