How to understand isolation technology? Introduction to the six major isolation technologies in industrial measurement

　　Testing voltage, current, temperature, pressure, strain, and flow rate is an integral part of industrial and process control applications. Often, these applications are located in environments with hazardous voltages, transient signals, common-mode voltages, and ground potential fluctuations that can damage the measurement system and compromise the accuracy of the measurement. To address these challenges, measurement systems designed for industrial applications use electrical isolation technology. This white paper focuses on isolation technology for analog measurements, answers common isolation questions, and covers technical content on different isolation implementation technologies.

　　Understanding Isolation Technology

　　Electrical isolation separates sensor signals, which may be exposed to hazardous voltages, from the low-voltage backplane of the measurement system. Isolation provides many advantages, including:

　　Protect expensive equipment, user life and data from transient voltage threats Improve noise immunity Eliminate ground loops Improve common-mode voltage rejection

　　An isolated measurement system provides separate ground planes for the analog front end and the system backplane to isolate the sensor measurement from the rest of the system. The ground connection of the isolated front end is a floating pin that can operate at a different potential than earth ground. Figure 1 shows an analog voltage measurement device. Any common-mode voltage that exists between the sensor ground and the measurement system ground is rejected. This prevents the formation of ground loops and removes any noise on the sensor lines.

Figure 1. Channel-isolated analog input circuit.

　　The need for isolation

　　Measurement systems involving any of the following situations warrant consideration of isolation technology:

　　Industrial environments in the immediate vicinity of hazardous voltages where transient voltages may be present Environments where common mode voltages or ground potential fluctuations are present Electrically noisy environments, such as those with industrial motors Applications sensitive to transient voltages where the transmission of voltage spikes through the measurement system must be prevented

　　Common-mode voltage, transient high voltage, and electrical noise are common in industrial control, process control, and automotive testing. Measurement equipment with isolation can provide reliable measurements in these harsh environments. For medical devices that come into direct contact with patients, isolation helps prevent transient currents from the power line from being transmitted through the device.

　　Depending on your voltage and data requirements, you can choose from several methods for isolated measurements. You can use plug-in cards for laptops, desktop PCs, industrial PCs, PXI, Tablet PCs, and CompactPCI, and choose between built-in isolation or external signal conditioning. You can also make isolated measurements using programmable automation controllers (PACs) and measurement systems for USB.

Figure 2 Data acquisition system with isolation function

　　How to implement isolation

　　Isolation technology requires that the signal be transmitted across the isolation barrier without any direct electrical contact. Light emitting diodes (LEDs), capacitors, and inductors are three commonly available components that support the transmission of electrical signals without direct contact. The principles on which these devices are based are at the heart of the three most common isolation technologies – optical coupling, capacitive coupling, and inductive coupling.

　　Optical Isolation Technology

　　When voltage is applied to both ends, the LED will emit light. Optical isolation technology uses an LED and a photodetection device to use light as a data conversion method to achieve signal transmission across the isolation barrier. A photodetector receives the light emitted by the LED and converts it back to the original signal.

Figure 3 Optical isolation technology

　　Photodiode

　　Optical isolation technology is one of the most commonly used isolation methods. One of the advantages of using optical isolation is its ability to resist electrical and magnetic noise interference. This technology also has some disadvantages, including transmission rate limited by the LED switching rate, high power consumption and LED loss.

　　Capacitive Isolation Technology

　　Capacitive isolation technology is based on an electric field that changes with the amount of charge on the capacitor plates. This charge is detected across an isolation barrier and is proportional to the value of the measured signal.

Figure 4 Capacitive isolation technology

　　One advantage of capacitive isolation technology is its immunity to magnetic noise interference. Compared with optical isolation technology, capacitive isolation can support higher data transmission rates because the LED does not need to be switched. Since capacitive isolation technology involves the use of electric fields for data transmission, it is susceptible to interference from external electric fields.

　　Inductive coupling isolation technology

　　In the early 1800s, Danish physicist Hans Oersted discovered that a magnetic field is generated when an electric current is passed through a coil. Later, it was discovered that a current is induced in a coil next to the changing magnetic field of another coil. The induced voltage and current in the second coil depend on the rate of change of the current in the first coil. This principle is called mutual induction and forms the basis for inductive isolation technology.

Figure 5 Inductive coupling

　　Inductive isolation technology uses a pair of coils separated by an insulating layer. The insulating layer prevents any physical signal transmission. Signals can be transmitted by changing the current flowing through one of the coils, which causes a similar induced current in the second coil across the insulating layer. Inductive isolation technology can provide high-speed transmission similar to capacitive technology. However, because inductive coupling involves the use of magnetic fields for data transmission, it is susceptible to interference from external magnetic fields.

　　Analog Isolation Technology and Digital Isolation Technology

　　Many commercial off-the-shelf (COTS) components today include one of these isolation implementation techniques. For analog I/O channels, you can implement isolation in the analog portion of the device before (analog isolation) or after (digital isolation) the analog-to-digital converter (ADC) completes the signal quantization process. Depending on where your isolation implementation is located in the circuit, you will need to design a different circuit based on one of these techniques. You can choose analog or digital isolation technology based on your data acquisition system performance, cost, and physical requirements. Figures 6a and 6b show the different stages of implementing the isolation function.

Figure 6b Digital isolation technology

　　The following sections cover analog and digital isolation in more detail and explain the different techniques used to implement them.

　　Analog Isolation Technology

　　Isolation amplifiers are generally used to provide isolation in the analog front end of data acquisition equipment. The "isolation amplifier" in Figure 6a represents an isolation amplifier, which is one of the front elements of the analog circuit in most circuits. The analog signal from the sensor is passed to the isolation amplifier, which provides isolation and passes the signal to the analog-to-digital conversion circuit. Figure 7 shows an isolation amplifier.

Figure 7 Isolation amplifier

　　In an ideal isolation amplifier, the analog output signal is identical to the analog input signal. The portion of Figure 7 labeled "Isolation" uses one of the techniques discussed in the previous sections (optical, capacitive, or inductive) to pass the signal across the isolation barrier. The modulator circuit pre-processes the signal for the isolation circuit. For optical coupling, you need to quantize the signal or convert it into varying light intensity. For capacitive and inductive coupling, you need to convert the signal into varying electric or magnetic fields. The demodulator circuit then reads the output of the isolation circuit and converts it back to the original analog signal.

　　Since you perform analog isolation before the signal is quantized, this is the best approach when designing external signal conditioning circuits that need to be used with existing non-isolated data acquisition devices. In this case, the data acquisition device performs the analog-to-digital conversion, and the external circuitry provides the isolation function. Using this combination of data acquisition devices and external signal conditioning circuits, measurement system vendors can develop general-purpose data acquisition devices and sensor-specific signal conditioning methods. Figure 8 shows analog isolation implemented with flexible signal conditioning circuits using isolation amplifiers. Another advantage of implementing isolation in the analog front end is that it protects the ADC and other analog circuits from voltage spikes.

Figure 8. Using isolation amplifiers in flexible signal conditioning hardware.

　　There are several configurations available for measurement products that use general-purpose data acquisition devices and external signal conditioning hardware. For example, the NI M Series includes several non-isolated, general-purpose, multifunction data acquisition devices that provide high-performance analog I/O and digital I/O. For applications that require isolation, you can use M Series devices with external signal conditioning circuits such as SCXI modules or SCC modules from NI. These signal conditioning platforms provide the isolation and specialized signal conditioning you need to directly interface with industrial sensors such as load cells, strain gauges, and pH sensors.