Susie Maestre

user Electronics Engineer

city Innovuze Solutions Inc.


  • 16-Bit Fully Isolated 4 mA to 20 mA Output Module

  • Created: Jan 11, 2016

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Summary

This reference design provides a 16-bit fully isolated output using the AD5662 DAC, ADuM1401 digital isolator, and external amplifiers. Isolation circuits are specifically designed to withstand the robustness of industrial environment. Industrial electronic equipment commonly uses galvanic isolators to protect systems and users from potentially hazardous voltages. It is well known that industrial equipment must operate reliably in the harshest environments, where strong electromagnetic fields, surges, fast transients, and high noise floors are the norm.


This design provides a solution for industrial control output modules where analog output current ranges from 4 mA to 20 mA. The AD5662 nanoDAC is a 5 V16-bit DAC in a SOT-23 package. The ADuM1401 four-channel digital isolator provides all the necessary signal isolation between the microcontroller and the DAC. The AD5662 provides a 0 V to 5 V output, which sets the current through the sense resistor R5 and, therefore, the current through R1.This current is mirrored (10×) to R2, using the second half of the ADA4622-2. The ADA4622-2 is the next generation of the AD822 single-supply, rail-to-rail output (RRO), precision JFET input op amp. The ADA4622-2 includes many improvements that make it desirable as an upgrade without compromising the flexibility and ease of use that makes the AD822 useful for a wide variety of applications. The first stage of the current mirror needs to operate at 0 V input to ensure there is no dead band when the DAC operates at zero-scale output. This requires that the negative supply to the ADA4622-2 be at least −1 V so that there is sufficient headroom on the output of the first stage ADA4622-2. The two diodes in the second stage, which are in series with the positive power supply, ensure that the output voltage of the second stage does not go to the positive supply rail of the ADA4622-2, which can be as high as 35V. The output of the FET is also protected by a series diode. Note that this series diode will increase the power dissipation of the circuit, which may be an issue in some applications.


This reference design is applicable for industrial applications such as process control programmable logic controllers (PLCs), distributed control system (DCS) modules and transmitters that require a 4 mA to 20 mA current output range.

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