June Birth Magoncia

user Electronics Engineer

city Innovuze Solutions, Inc.

  • 4-20mA Current Loop Transmitter

  • Created: Feb 15, 2017

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This reference design features a 4-20mA current loop transmitter that can be used in a design as an industry standard communication protocol. It combines both analog and digital techniques to create an industry standard 4-20mA current loop transmitter. This design features ADuM1411 quad-channel digital isolator, MCP4921 12-bit digital-to-analog converter, and XTR116 4-20mA current loop transmitter. It uses SPI protocol to receive commands from a microcontroller.

The current output of this 4-20mA current loop transmitter can be controlled by an external microcontroller that supports SPI protocol. The XTR116 device produces the 4-20mA current to the loop. But its current output depends on the input current that comes from the MCP4921 device. The MCP4921 device is a digital-to-analog converter that has an analog output that can be controlled via its SPI interface with a microcontroller. The microcontroller sends data to MCP4921 through the ADuM1411 digital isolator. This ADuM1411 digital isolator protects the microcontroller from the high voltage within the loop that might affect the performance of the microcontroller. The output voltage of the MCP4921 serves as input to the XTR116 device. The current entering to the IIN pin of XTR116 controls the output current. The XTR116 device has a gain of 100, so the current flowing into the IIN pin produces an output current that is 100 times larger. A portion of the current flows into the V+ pin of the device and the remaining current flows in transistor Q1.

The transistor Q1 conducts most of the output current. With a 20mA loop current and 40V loop voltage, the power dissipated in Q1 can reach up to 0.8W, which makes it produce heat that might affect the XTR116 device. Q1 must be located away from the XTR116 and other sensitive analog circuitry in order to minimize the effects of heat. The whole circuit can operate in 3.3V or 5V supply. The supply voltage can be selected through header J1. This 4-20mA current loop transmitter can be used in any industrial smart sensor and other devices that use 4-to-20mA current transmission.