Programmable logic controllers (PLCs) use fast, deterministic functions, such as logic, sequencing, timing, counting, and arithmetic algorithms, to control machines and processes. They use analog and digital signals to communicate with end nodes (reading sensors and controlling actuators, for example). Typical methods of communication include current/voltage loops, Fieldbus, and industrial Ethernet protocols.
The industry has a continuing tendency to increase the number of sensor and control nodes in the remote area, causing a corresponding increase in the number of I/O module nodes in the controller—and some distributed control systems (DCS) can handle thousands of nodes. This concentration of nodes brings increased temperature-related challenges, especially for systems that implement the 4-mA to 20-mA loop communications standard.
Perhaps the biggest and most relevant challenge to the system designer is the need for greater efficiency and reduced power consumption, as the inefficiency of existing systems results in wasted power and increased operating costs. This article explains the challenges of designing such systems for greater efficiency and introduces the AD5755, a versatile, 4-channel, 16-bit digital-to-analog converter (DAC) as a more integrated solution to help resolve these issues.
The article is presented as a pdf file, in two parts (no registration required):
Part 1: System overview, and power dissipation concerns, click here.
Part 2: System error checking and diagnostics under fault conditions, flexible output-range programmability, communicating additional information over the 4- to 20-mA current loop, AD5755 complete solution, click here.
About the author
a system applications engineer for the Industrial and Instrumentation segment
at Analog Devices, Inc. (Limerick, Ireland). After working in test-development
engineering at Microsemi, he joined ADI in 1998. He spent three years in an
applications role in Shanghai. He graduated in 1995 from the University of
Limerick, Ireland, with a bachelor's degree in electronic engineering.
In this case we have a circuit that worked well at one operating point, but reliably self-destructed at another operating point. The active load circuits were the same. The transistor power dissipation was the same. The engineer pulls out his hair and cries out to the heavens: ‘What is going on?’