AD54-2 by

Analog Devices Inc Circuit Assembly Rochester Electronics LLC SEI Stackpole Electronics Inc SMC Corporation of America Viavi Solutions Inc

Set Alert Share

• Copy Link
• Print Page

Figure 1 shows the manner in which the AD5422 can be combined with the AD5700 HART modem and a UART interface to construct a HART-capable 4 mA to 20 mA current output, typical of PLC and DCS systems. The buffer connected to the +VSENSE pin is not necessary if the application does not require the IOUT and VOUT pins to be shorted. The HART_OUT signal from the AD5700 is attenuated and ac-coupled into the RSET pin of the AD5422. If the external RSET resistor is not being used, an alternative method of connecting the AD5422 and the AD5700 via the CAP2 pin can be found in Application Note AN-1065, as previously described. This method is only relevant to the 40-lead LFCSP package option of the AD5422 because the lower pin-count 24-lead TSSOP package does not contain a CAP2 pin.

Figure 1 shows the manner in which the AD5420 can be combined with the AD5700 HART modem and a UART interface to construct a HART-capable 4 mA to 20 mA current output, typical of PLC and DCS systems. The HART_OUT signal from the AD5700 is attenuated and ac-coupled into the RSET pin of the AD5420. If the external RSET resistor is not being used, an alternative method of connecting the AD5420 and the AD5700 can be found in Application Note AN-1065, where the AD5700 HART modem output is coupled into the AD5420 via the CAP2 pin. While the method described in this circuit note requires the use of the external RSET resistor, it provides better power supply rejection performance than the alternative application note solution. The use of either solution results in the AD5700 HART modem output modulating the 4 mA to 20 mA analog current (as shown in Figure 2) without affecting the dc level of the current. The diode protection circuitry (D1 − D4) is discussed in more detail in the Transient Voltage Protection section.

The AD5422 is a fully integrated, fully programmable 16-bit voltage and current output DAC, capable of programming ranges from 4 mA to 20 mA, 0 mA to 24 mA, 0 V to 5 V, 0 V to 10 V, ±5 V, ±10 V. The voltage output headroom is typically 1 V, and the current output needs about 2.5 V headroom. This means that the 20 mA current output can drive a load up to approximately 600 Ω with a 15 V supply.

This circuit provides a complete, fully isolated, analog output channel suitable for programmable logic controllers (PLCs) and distributed control system (DCS) modules that require standard 4 mA to 20 mA HART®1-compatible current outputs and unipolar or bipolar output voltage ranges. It provides a flexible building block for channel-to-channel isolated PLC/DCS output modules or any other industrial application that requires a fully isolated analog output. The circuit also includes external protection on the analog output terminals.

This circuit is a complete smart industrial, loop powered field instrument with 4 mA to 20 mA analog output and a highway addressable remote transducer (HART®) interface. HART is a digital 2-way communication in which a 1 mA peak-to-peak frequency-shift-keyed (FSK) signal is modulated on top of the standard 4 mA to 20 mA analog current signal. This allows features such as remote calibration, fault interrogation, and transmission of process variables, which are necessary in applications such as temperature and pressure control.<p>This circuit has been compliance tested, verified, and registered by the HART Communication Foundation (HCF). This successful registration provides circuit designers with a high level of confidence using one or all of the components in the circuit.<p>The circuit uses the ADuCM360, an ultralow power, precision analog microcontroller, the AD5421, a 16-bit, 4 mA to 20 mA, loop powered digital-to-analog converter (DAC), and the AD5700, the industry’s lowest power and smallest footprint HART compliant IC modem.