Isolation Technology for Reliable Industrial Measurements

Overview

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

• Improved noise immunity

• Eliminate ground loops

• Improved 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:

• Proximity to hazardous voltage

• Industrial environments where transient voltages may be present

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

• Electrically noisy environments, such as those with industrial motors

• Transient voltage sensitive applications where voltage spikes must be prevented from being transmitted through the measurement system

Common-mode voltages, transient high voltages, 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 into direct contact with patients, isolation helps prevent transient currents from the power supply lines 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 signals be transmitted across the isolation barrier without any direct electrical contact. Light emitting diodes (LEDs), capacitors , and inductors are three commonly available components that enable electrical signal transmission without direct contact. The principles underlying these devices 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 across the two ends , the LED will emit light. Optical isolation technology uses an LED and a photodetector device to transmit signals across the isolation barrier using light as a data conversion method. 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 in the circuit , you will need to design a different circuit based on one of these techniques. You can choose analog or digital isolation 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 a data acquisition device. 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 labeled “Isolation” in Figure 7 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.

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. 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. 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 NI's SCXI modules or SCC modules. 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.

Digital Isolation

The 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 may cause 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 expensive and may have the disadvantage of 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 the price of ADCs has dropped significantly, suppliers of measurement equipment are choosing to trade the better performance and lower cost provided by digital isolation devices for the protection of ADCs, as shown in Figure 9.

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

Compared to isolation amplifiers, digital isolation components have lower costs and provide higher data rates. Digital isolation technology also provides analog designers with greater flexibility in selecting components and developing the optimal analog front end for the measurement device. Products with digital isolation function use current limiting circuits and voltage limiting circuits to provide ADC protection. 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.

Leading suppliers of digital isolation components, such as Avago Technologies (www.avagotech.com), Texas Instruments (www.ti.com), and Analog Devices (www.analog.com), have developed their own isolation technologies based on these basic principles. Avago Technologies provides digital isolation based on optical coupling. Texas Instruments' isolation devices are based on capacitive coupling, while Analog Devices' isolation devices use inductive coupling.

Optocoupler

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 often 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, Optpcouplers, Digital Input, Digital Output

Figure 10 Industrial digital I/O products using 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.

Capacitive Isolation Technology

Texas Instruments offers digital isolation components based on capacitive coupling. These isolation devices provide high data transfer rates and high immunity to transient signals. Inductive isolation consumes less power than capacitive isolation methods and optical isolation methods.

Inductive Isolation Technology

iCoupler technology (analog.com/iCoupler), introduced by Analog in 2001, uses inductive coupling to provide digital isolation for high-speed, high-channel-count applications. iCoupler devices can provide 100 Mb/s data rates with 2500 V isolation for a 16-bit analog measurement system with sampling rates in the megahertz range. Unlike optocouplers, iCoupler devices offer other technical advantages such as reduced power consumption, a high operating temperature range of up to 125°C, and high immunity to transient signals up to 25 kV/ms.

iCoupler technology is based on small, chip- sized transformers. An iCoupler consists of three main parts—a transmitter, transformer, and receiver. The transmitter circuit uses edge triggers to encode and convert rising and falling edges on the digital lines into 1 ns pulses. These pulses are transmitted across the isolation barrier using transformers and decoded on the other side by the receiver circuit, as shown in Figure 11. The small size of these transformers (about three-tenths of a millimeter) makes them virtually immune to external magnetic noise. iCoupler devices can also reduce the cost of measurement hardware by integrating up to four isolated channels on each integrated circuit (IC), and they require fewer external components than optocouplers.

Figure 11. iCoupler technology from Analog based on inductive coupling

Source: Analog Devices (analog.com/iCoupler)

Measurement hardware vendors are using iCoupler devices to provide low-cost, high-performance data acquisition systems. NI industrial data acquisition (DAQ) devices for high-speed measurement applications, such as the industrial M Series multifunction DAQ devices, use iCoupler digital isolation devices as shown in Figure 12. These devices provide 60 VDC continuous isolation on analog and digital channels and 1,400 Vrms/1,900 VDC channel-to-bus isolation withstand for up to 5 seconds, and support sampling rates up to 250 kS/s. NI C Series modules for NI PAC platforms, NI CompactRIO, NI CompactDAQ, and other high-speed NI USB devices also use iCoupler technology.

Figure 12 Industrial NI M Series multifunction DAQ uses digital isolation device

Summarize

An isolated data acquisition system ensures reliable measurements in harsh environments with hazardous voltages and transient signals. Your need for isolation depends on the measurement application and the environment around it. Applications that require a single general-purpose data acquisition device to interface with sensors of varying characteristics may benefit from external signal conditioning circuits with analog isolation, whereas low-cost, high-performance analog input applications may benefit from a measurement system with digital isolation technology.