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AD421 Fiches technique(PDF) 11 Page - Analog Devices |
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AD421 Fiches technique(HTML) 11 Page - Analog Devices |
11 / 14 page 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 |
Numéro de pièce similaire - AD421 |
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Description similaire - AD421 |
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