Six Isolation Technologies for Reliable Industrial Measurements

Understanding Isolation

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:

• Protects expensive equipment, user life, and data from transient voltages

• Improves noise immunity

• Eliminates ground loops

• Improves common-mode voltage rejection

An isolated measurement system provides separate ground planes for the analog front end and the system backplane to separate the sensor measurement from the rest of the system. The ground connection for 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 ground loops and rejects any noise on the sensor lines.

Figure 1 Channel-isolated analog input circuit

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Necessity for IsolationIsolation

is necessary for any measurement system that involves any of the following situations:

• Close proximity to hazardous voltages

• Industrial environments where transient voltages may be present

• Environments where common-mode voltages or ground potential fluctuations exist

• Electrically noisy environments, such as those with industrial motors

• Transient-sensitive applications where voltage spikes must be prevented from transmitting through the measurement systemCommon

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

Depending on your voltage and data requirements, you can choose from several methods for isolating 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

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Isolation implementation method

Isolation technology requires that there cannot be any direct electrical contact when the signal is transmitted across the isolation barrier. Light-emitting diodes (LEDs), capacitors and inductors are three commonly available components that support non-direct contact electrical signal transmission. 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 a 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 method of data conversion 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 interference from electrical and magnetic noise. There are also some disadvantages to this technology, 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 measured signal value.

Figure 4 Capacitive isolation technology

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

Inductive Coupling Isolation

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

Figure 5 Inductive coupling

Inductive isolation 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 current to be induced in the second coil across the insulating layer. Inductive isolation 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.
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Analog Isolation vs. Digital Isolation

Today, many commercial off-the-shelf (COTS) components contain one of the above isolation implementation technologies. 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 in the circuit, you need to design a different circuit based on one of the two technologies. You can choose analog or digital isolation technology based on the performance, cost, and physical requirements of your data acquisition system. Figures 6a and 6b show the different stages of implementing isolation.

Figure 6a Analog isolation technology

Figure 6b Digital isolation technology

The following describes more details of analog isolation and digital isolation, and explains the different technologies for implementing the two.

Analog Isolation Technology

Isolation amplifiers are generally used to provide isolation functions in the analog front end of data acquisition equipment. The "isolation amplifier" in Figure 6a represents an isolation amplifier, which is one of the precursors of analog circuits 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 labeled “isolation” in Figure 7 uses one of the techniques discussed in the previous sections (optical, capacitive, or inductive coupling) to pass the signal across the isolation barrier. The modulator circuit preconditions the signal for the isolation circuit. For optical coupling, you need to quantize the signal or convert it to varying light intensities. For capacitive and inductive coupling, you need to convert the signal to 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.

Because you perform analog isolation before the signal is quantized, this is a good 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, while 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 using flexible signal conditioning circuits with isolation amplifiers. Another benefit of implementing isolation in the analog front end is protecting 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. A

digitally isolated

ADC is one of the key components of any analog input data acquisition device. For best performance, the input signal to the ADC should be as close to the original analog signal as possible. Analog isolation can introduce errors including gain, nonlinearity, and offset before the signal reaches the ADC. Placing the ADC closer to the signal source can achieve better performance. At the same time, analog isolation components are more expensive and may have a long settling time. Although digital isolation can achieve better performance, one of the reasons for using analog isolation in the past was to provide protection for expensive ADCs. As ADC prices have dropped dramatically, measurement equipment vendors are choosing to trade ADC protection for the better performance and lower cost offered by digital isolators, as shown in Figure 9.

Figure 9 Price decline curve of 16-bit analog-to-digital converters

Compared to isolation amplifiers, digital isolation components are lower cost and provide higher data rates. Digital isolation technology also provides analog designers with greater flexibility in selecting components and developing the best analog front end for their measurement devices. Products with digital isolation use current-limiting and voltage-limiting circuits to protect the ADC. Digital isolation components follow the same basic principles as optical coupling, capacitive coupling, and inductive coupling, which are also the basis of analog isolation technology.

Optocouplers

Optocouplers, digital isolation devices based on the principle of optical coupling, are one of the oldest and most commonly used digital isolation methods. They can withstand high voltages and provide high immunity to electrical and magnetic noise. Optocouplers are commonly used in industrial digital I/O products, such as the NI PXI-6514 isolated digital I/O module (shown in Figure 10) and the NI PCI-7390 industrial motion controller.

Industrial Digital I/O Industrial Digital I/O, Optpcouplers Optocouplers, Digital Input Digital Input, Digital Output Digital Output.

Figure 10 Industrial digital IO products use optocouplers

However, for high-speed analog measurements, optocouplers are subject to limitations associated with optical coupling, such as speed, power consumption, and LED losses. Digital isolators based on capacitive and inductive coupling can alleviate many of the limitations of optocouplers.