In industrial automation and process control applications, the analog output module transmits analog signals (voltage or current) that operate controls such as hydraulic actuators, solenoids, and motor starters. Figure 1 shows a typical configuration for an output module regulating a process control plant. The design requirements for these systems center on the selection of digital-to-analog converter (DAC), signal conditioning, and isolation to perform the conversion from the digital domain to the analog domain and control the valves and actuators.
Figure1. Analog Output Control Module Controls Valves/Actuators in Process Control
Designs range in resolution from 12 to 16 bits, offering 0.1% accuracy over the industrial temperature range. Analog output ranges include 5 V, 10 V, 0"10 V, 4"20 mA, and 20 mA, with DAC settling time requirements varying from 10 ¼s to 100 ms, depending on the application. Opto-couplers or digital isolators provide galvanic isolation between the field side (I/O, digital-to-analog converters, and their associated signal conditioning circuitry) and the bus side (controller and the digital circuitry). Some system designs may also require full isolation on the analog side; this necessitates the use of single channel converters to maximize the isolation between channels, and transformers to provide isolated power.
Process Technology Supporting Industrial Automation and Process Control Designs
Silicon process technology development plays an integral part in providing a platform for component development, enabling products that directly target specific applications to be developed. Newer high-voltage industrial processes enable the development of signal processing converters and amplifiers for these high analog content, high accuracy applications—and of products that provide signal isolation when required. Figure 2 shows where these different process technologies address the different areas of the industrial automation and process control signal chain.
Fig. 2: Process Technology used in Analog Output Module Solutions.
One example of a process technology that combines high-voltage devices, submicron CMOS, and complementary bipolar transistors is iCMOS from Analog Devices. This combination allows a single chip
design to mix-and-match 5-V CMOS and higher voltage
16-, 24-, or 30-V CMOS
circuitry—with multiple supply voltages running to the same chip. These submicron devices can thus have higher performance, a more integrated feature set, lower power consumption, and significantly smaller board area than previous generation-ns of high-voltage products.
The ability to mix and match components and operating voltages makes the process especially flexible. The bipolar technology provides excellent references, matching, and stability for accuracy in ADCs, DACs, and low-offset amplifiers. Thin-film resistors enable the design of high-precision, high-accuracy digital-to-analog converters. The thin-film resistors provide 12-bit raw accuracy and up to 16-bit ratio matching, and exhibit temperature and voltage coefficients approximately 20 times lower than conventional polysilicon resistors. On-chip thin-film fuses enable digital techniques to calibrate integral nonlinearity, offset, and gain in high-precision converters.
In most analog output module architectures, digital signals are transmitted from a central controller in the digital domain to the digital-to-analog converter (DAC), which converts them to analog to drive valves and actuators. To maintain safe voltage at the user interface and to prevent transients from being transmitted by the output sources, galvanic isolation is implemented to introduce a barrier between the analog and digital domains. The most commonly used isolation technology is optical (optocouplers), but magnetic (transformer based) and capacitive (capacitively coupled) isolators are also used in these applications.
Optocouplers rely on light emitting diodes to convert electrical signals to light, and on photo detectors to convert the light back to electrical signals. The low conversion efficiencies for electrical-to-light conversion and the slow response of photo detectors lead to optocoupler limitations in terms of lifetime, speed, power consumption, and performance variation over temperature,. They are generally limited to one or two channel configurations and require external components to configure.
One example of a process that combines chip scale transformers with integrated CMOS inputs and outputs to provide solutions that offer reduced size, cost, and power relative to optocouplers is iCoupler technology from Analog Devices. These products do not contain LEDs and photodiodes, so they operate at power levels as low as 2% that of optocouplers, reducing heat dissipation, improving reliability, and reducing cost. They have standard CMOS interfaces, require no external components, and provide high stability over temperature, supply voltage, and lifetime.
Yet another challenge is finding a way to transmit power from the non-isolated system side to the isolated field side in the design of a fully isolated system. Traditional techniques employed in transferring power across an isolation barrier include a separate dc-to-dc converter, which is relatively large and expensive, and has insufficient isolation; or a discrete approach using transformers that are difficult to design and interface.