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參數(shù)資料
| 型號： | AD421BRZRL7 |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 3/14頁 |
| 文件大?。?/td> | 0K |
| 描述： | IC DAC 16BIT LOOP 4-20MA 16SOIC |
| 產品培訓模塊： | Data Converter Fundamentals DAC Architectures |
| 標準包裝： | 1 |
| 設置時間： | 8ms |
| 位數(shù)： | 16 |
| 數(shù)據(jù)接口： | 串行 |
| 轉換器數(shù)目： | 1 |
| 電壓電源： | 單電源 |
| 功率耗散（最大）： | 450mW |
| 工作溫度： | -40°C ~ 85°C |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 16-SOIC（0.295"，7.50mm 寬） |
| 供應商設備封裝： | 16-SOIC W |
| 包裝： | 標準包裝 |
| 輸出數(shù)目和類型： | 1 電流，單極 |
| 其它名稱： | AD421BRZRL7DKR |
AD421
–11–
REV. C
DVDD AVDD
REF IN
CS
DATA OUT
SCLK
DATA IN
AGND
DGND
MCLK IN
MCLK OUT
AD7714/
AD7715
ANALOG
TO
DIGITAL
CONVERTER
SENSORS
RTD
mV
TC
4.7 F
REF OUT1
BOOST
VCC
LV
COMP
DRIVE
LOOP
RTN
REF OUT2
REF IN
CLOCK
LATCH
DATA
COM
C1
C2
C3
LOOP
POWER
0.01 F
DN25D
2.2 F
3.3V
1.25V
4.7 F
AMBIENT
TEMP
SENSOR
AD421
MICROCONTROLLER
VCC
GND
0.01 F
1k
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 CC 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
“1”
2200Hz
“0”
Figure 15. HART Transmission of Digital Signals
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