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Complete 4 mA to 20 mA HART Solution with Additional Voltage Output Capability (CN0278)
The circuit shown in Figure 1 uses the AD5700, the industry’s lowest power and smallest footprint HART®1-compliant IC 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 IOUT and VOUT pins to be shorted together, thus reducing the 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.
Figure 1. AD5422 HART-Enabled Circuit Simplified Schematic
Application Note AN-1065describes a manner in which theAD5420IOUTDAC can be configured for HART communication compliance.AN-1065outlines how the AD5700 HART modem output can be attenuated and ac coupled into the AD5420 via the CAP2 pin. The same is true of theAD5422. 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 RSETresistor and involves coupling the HART signal into the RSETpin of the AD5420 orAD5422. TheCN-0270describes 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.
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