The circuit shown in Figure 1 is an isolated intelligent industrial field instrument that can interface with many types of analog sensors, such as temperature sensors (Pt100, Pt1000, thermocouples) or bridge pressure sensors. The meter communicates via a 4 mA to 20 mA analog output and a Highway Addressable Remote Transducer (HART® ⁾ interface. HART is a digital two-way communication that modulates a 1 mA peak-to-peak Frequency Shift Keying (FSK) signal on top of a standard 4 mA to 20 mA analog current signal. The HART interface enables functions such as remote calibration, troubleshooting and process variable transmission; these functions are necessary in applications such as temperature and pressure control.

This circuit uses an ultra-low power, precision, 24-bit analog-to-digital converter (ADC) AD7124-4 , which includes all the features required for temperature and pressure systems. The circuit also includes the AD5421 16-bit 4 mA to 20 mA loop-powered digital-to-analog converter (DAC) , the industry's lowest power and smallest HART-compatible IC modem AD5700 , which provides an ultra-low power serial peripheral interface (SPI) ) isolated ADuM1441 , CMOS switch ADG5433 , and low-power 3.3 V regulator ADP162 located in an isolated power circuit .