Complete 4 mA to 20 mA HART Solution (CN0270)
The circuit shown in Figure 1 uses the AD5700, the industry’s lowest power and smallest footprint HART1-compliant IC modem and the AD5420, a 16-bit current-output DAC, to form a complete HART-compatible 4 mA to 20 mA solution.
For additional space savings, the AD5700-1 offers a 0.5% precision internal oscillator.
Figure 1. AD5420 HART-Enabled Circuit Simplified Schematic
This circuit adheres to the HART physical layer specifications as defined by the HART Communication Foundation, for example, the analog rate of change and noise during silence 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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