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Benefits of integrated signal, power isolation

Posted: 13 Aug 2013 ?? ?Print Version ?Bookmark and Share

Keywords:industrial sensors? RTD? thermocouples? isolation? opto-couplers?

In far field, the flux from one half of the transformer would cancel out the flux from the other half by design each half with opposite phase. While the radiation emissions from the transformers are negligible, we need to take precaution in designing PCB for using isoPower products. Besides good bypassing to contain noises within the part, isolated islands of PCB need to be carefully designed not to form efficient dipole antenna. CISPR Class A or B can be met for the isoPower parts with properly designed PCB.

Signal, power isolation needs for measurement, control
To make reliable and safe measurements of current, voltage, temperature, pressure, flow rate or other parameters for a variety of industrial applications in the harsh environments, isolation is essential to maintain data integrity and equipment or user safety by eliminating the ground loops and the direct exposure of the user or equipment to hazardous voltages.

Besides signal isolation, power isolation is also needed to establish bias for the ADCs or DACs to interface with various sensors or transducers. There is significant challenge to pack isolation components such as those based on opto-couplers in a small area considering the fact that there are many analogue inputs or outputs that not only require isolation from the back plane but also between those analogue channels. Integrated signal and power isolation is the idea solution to deliver compact and robust isolation to each of the analogue I/O port.

An example of implementation of isolated signal and power for use in industrial instrument or process control equipment is shown in figure 6.

Figure 6: Fully isolated industrial measurement and control system Using isoPower.

A single isoPower component such as ADuM5211, a two channel digital isolator with 150mW isolated power, would satisfy all the isolation needs for a given analogue input channel. One isolated data channel is used to send the system clock from the micro-controller to the ADC, and another data channel in the opposite direction would send the acquired data from the sensor back to the micro-controller. Multiple ADuM5211 can be used for multiple sensors that require isolation between them.

Similarly, a single isoPower component such as ADuM5210, a two channel digital isolator with 150mW isolated power, can handle all the isolation needs for a given analogue output channel. The two data channels are used to send both the clock and the digital commands from the micro-controller to the DAC to control the actuator. Multiple ADuM5210 can be used for multiple actuators or analogue outputs that require isolation between them. The red horizontal dashed lines illustrate the isolation barriers between various sensors and actuators and the long red vertical dashed line to the right indicates the isolation between the system and analogue I/O ports.

For communication between many pieces of process control equipment, additional isolation as shown by the short red vertical dashed line is needed between the system controller and communication bus such as RS485, RS232, CAN and others. Again, a single isoPower component such as ADuM5287, full integrated isolated RS-485 transceiver with 500mW isolated power, can be used for the complete bus interface isolation for a single piece of equipment. Here ADuM2587 provides integrated RS485 transceiver and three channels of digital isolation, of which two are used to send Drive and Drive Enable to the micro-controller to the bus and the other one is used to send received data Receive from the bus to the controller. As we can see, integrated signal and power isolation significantly simplifies the measurement, control and communication interface for industrial equipment.

There are similar signal and power isolation needs for battery measurement or test equipment. Voltages and Currents for each battery cell need to be monitored continuously to maintain battery life span but the whole battery stack in series can be hundreds or even thousands of volts in total. Battery monitor ICs to interface with various battery packs in different locations of the stack need to be isolated from system controllers and isolated power is needed to allow proper system shut down in the event of battery failure.

Conclusion
Integrated signal and power isolation using micro-transformers in a small package dramatically simplify the design for industrial measurement and process control system and it significantly reduces board space and cost compared to those based on opto-couplers and discrete transformers. It provides reliable and accurate measurement and control in harsh industrial environments.

About the author
Baoxing Chen is an ADI fellow. He joined Analog Devices in 1997 after he graduated from University of Michigan with a Ph. D in Physics and M.S. in Electrical Engineering. Baoxing also has a B.S. in Physics from Nanjing University in China. Baoxing pioneered the use of micro-transformers for transmitting signal and power through the isolation barrier. At ADI, he has been leading iCoupler and isoPower technology developments including circuit architectures and process developments. Baoxing has published over 25 papers in areas ranging from material, device, circuits and applications. He holds 20 US patents.

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