Isolation technology for reliable industrial measurements and different implementation methods

Overview

Measurements of voltage, current, temperature, pressure, strain, and flow are important components in industrial and process control applications. These applications often involve hazardous voltages, transient signals, common-mode voltages, and unstable ground potentials that can damage the measurement system and reduce measurement accuracy. To overcome these drawbacks, electrical isolation is added to the design of measurement systems in industrial applications. This white paper focuses on isolation in analog measurements, answers common questions in isolation applications, and covers the technical implementation of different isolation methods.

Understanding Isolation

Isolation in an electrical sense separates sensor signals that are exposed to hazardous voltages1 from the low-voltage backplane of the test system. Isolation provides many advantages, including:

Protect expensive equipment, users and data from hazardous transient voltages.

Improved noise suppression

Eliminate ground loops

Improve common-mode voltage rejection

An isolated measurement system provides a separate ground plane for the analog front end and the system backplane, isolating the sensor measurement from the rest of the system. The ground of the isolated front end is a floating pin that can operate at voltages other than earth ground. Figure 1 shows an analog voltage measurement device. Any common-mode voltage between the sensor ground and the test system ground can be suppressed, thus avoiding the introduction of ground loops and the impact of noise on the sensor line.

Figure 1. Bank-isolated analog input circuit.

1Hazardous voltages are voltages greater than 30 Vrms, 42.4 Vpk, or 60 VDC.

The need for isolation

The following measurement systems require isolation considerations:

Adjacent to hazardous voltage

Industrial environments where transient voltages may occur

Environments with common-mode voltage or unstable ground potential

Electrically noisy environments, such as those with industrial motors

Transient-sensitive applications where voltage spikes must be avoided in the measurement system

Industrial measurement, process control, and automated testing are typical applications where common-mode voltage, high transient voltage, and electrical noise exist. Measurement equipment with isolation can provide reliable measurements in harsh environments. For medical devices that come into direct contact with patients, isolation can effectively prevent the transmission of transient power in the device.

There are many options for isolated measurements, depending on the voltage and data rate requirements. Users can choose plug-in boards for notebooks, desktop PCs, industrial PCs, PXI, tablet PCs, and CompactPCI, which have built-in isolation or external signal conditioning. Users can also achieve isolated measurements through programmable automation controllers (PACs) and USB measurement systems.

Figure 2. Isolated data acquisition system[page]

Methods for achieving isolation

Isolation requires that the signal be transmitted through an isolation barrier without direct electrical connection. Commonly used non-contact signal transmission devices include light-emitting diodes (LEDs), capacitors, inductors, etc. The basic principles of such devices are the three most common isolation technologies: photoelectric, capacitive, and inductive coupling.

Optical Isolation

LEDs emit light when powered. Photoelectric isolation uses LEDs and photoelectric detection devices to achieve an isolation barrier and transmit signals through light. The photoelectric detection device receives the light signal emitted by the LED and converts it into the original electrical signal.

Figure 3. Optical isolation

Optical isolation is the most commonly used isolation method. The advantage of using optical isolation is that it can avoid electrical and magnetic field noise. The disadvantage is that the transmission speed is limited by the switching speed of the LED, high power scattering and LED wear.

Capacitive isolation

Capacitive isolation is based on the change in electrical potential caused by charge accumulation on the plates of a capacitor. This change is measured across the isolation barrier and is proportional to the magnitude of the measured signal.

Figure 4. Capacitive isolation.

The advantage of capacitive isolation is that it can suppress electromagnetic interference. Because capacitive isolation does not require LED switching like optical isolation, it can support faster data transmission rates. However, since capacitive coupling uses electric potential to transmit data, it is susceptible to interference from external electric fields.

Inductive coupling isolation

In the early 19th century, Danish physicist Hans Oersted discovered that a magnetic field is generated when an electric current is passed through a coil. He later discovered that a current can be induced in another coil close to the coil that produces the changing magnetic field. The voltage and current induced in the other coil depend on the alternating current in the first coil. This phenomenon is called mutual induction, and it is the basic principle of inductive coupling.

Figure 5. Inductive coupling.

Inductive coupling uses a pair of coils with an insulating layer. The insulating layer isolates the transmission of physical signals. The changing current on one end of the coil will induce a similar current on the coil on the other end of the insulation barrier, and the signal is transmitted in this way. Inductive isolation provides high-speed transmission similar to capacitive technology. However, since inductive coupling transmission signals involve magnetic fields, they are easily interfered by external magnetic fields. [page]

Analog Isolation vs. Digital Isolation

Many commercial off-the-shelf devices now integrate one of the above technologies to achieve isolation. For analog I/O channels, isolation can be implemented in the analog part before the analog-to-digital converter (ADC) digitizes the signal (analog isolation) or after the ADC digitizes the signal (digital isolation). Depending on where the isolation is implemented, users need to design different circuits for the above three technologies. Users can decide to choose analog isolation or digital isolation based on the performance, cost, and physical requirements of the data acquisition system. Figure 6a and Figure 6b show the implementation of the two isolations.

Figure 6a. Simulated isolation

Figure 6b. Digital isolation

The following sections will analyze analog isolation and digital isolation in detail and discuss the different implementation techniques for the two.

Analog Isolation

Isolation amplifiers are commonly used isolation devices in the analog front end of data acquisition equipment. The "ISO Amp" in Figure 6a is an isolation amplifier, which is the first device in the analog circuit section in most circuits. The analog signal of the sensor first passes through the isolation amplifier, and then is sent to the analog-to-digital conversion circuit after isolation. Figure 7 shows a common layout of an isolation amplifier.

Figure 7. Isolation amplifier [page]

In an ideal isolation amplifier, the analog output signal should be identical to the analog input signal. The portion labeled “isolation” in Figure 7 uses the techniques discussed above (photoelectric, capacitive, inductive coupling) to pass the signal through the isolation barrier. The modular circuit already provides the isolation circuit for the signal. For the photoelectric method, the signal needs to be digitized or converted to different light intensities. For the capacitive and inductive methods, the signal needs to be converted to different electric or magnetic fields. The demodulation circuit reads the output of the isolation circuit and converts it back to the original analog signal.

Since analog isolation precedes signal digitization, external signal conditioning is the best solution for existing non-isolated data acquisition devices. In this case, the data acquisition device performs the analog-to-digital conversion, while the external circuitry provides isolation. Combining data acquisition devices with external signal conditioning allows measurement system vendors to develop multi-purpose data acquisition devices and sensor-specific signal conditioning. Figure 8 shows a flexible signal conditioning design using an isolation amplifier to achieve analog isolation. Another advantage of placing isolation at the analog front end is that it protects the ADC and other analog circuits from voltage spikes.

Figure 8. Isolation amplifiers for flexible signal conditioning hardware

There are many measurement products on the market that use general-purpose data acquisition devices and external signal conditioning, such as several non-isolated general-purpose multi-purpose data acquisition devices in the NI M series, which provide high-performance analog I/O and digital I/O. For applications that require isolation, you can choose M series devices with external signal conditioning, such as SCXI or SCC modules from National Instruments. These signal conditioning platforms provide isolation and specialized signal conditioning functions, and can directly connect to industrial sensors such as load cells, strain gauges, and pH sensors.

Digital Isolation

ADC is one of the key components of analog input data acquisition equipment. In order to obtain the best performance, the input signal of ADC should be as consistent as possible with the original analog signal. Analog isolation may introduce errors such as gain, nonlinearity, and offset during the signal transmission to ADC. The closer the ADC is to the signal source, the better the performance. Because analog isolation components are expensive and take a long time to adjust, in the past, although digital isolation has better performance, in order to protect expensive ADCs, analog isolation had to be chosen. As ADC prices continue to drop, suppliers of measurement equipment are turning from protecting ADCs to pursuing digital isolators to achieve better performance and lower costs, as shown in Figure 9.

Figure 9. 16-bit ADCs continue to fall in price.

Image source: National Instruments and major ADC suppliers [page]

Compared with isolation amplifiers, digital isolation devices are lower in cost and have higher data transmission rates. Digital isolation technology also provides greater flexibility for analog circuit engineers in selecting devices and developing optoelectronic analog front ends for measurement equipment. Products with digital isolation use current-limiting and voltage-limiting protection circuits to protect the ADC. The basic principles of digital isolation devices are similar to those of optoelectronic, capacitive, and inductive coupling in analog isolation.

Major digital isolation device suppliers, 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 the above basic principles. Avago Technologies provides digital isolators based on optocouplers; Texas Instruments' isolators are based on capacitive coupling principles; and Analog Devices' isolators use inductive coupling.

Optocoupler

Optocouplers are digital couplers based on the principle of photoelectric coupling. They have been used for the longest time and are the most common digital isolation method. They can withstand high voltages and have high immunity to electrical and magnetic field noise. Optocouplers are often used in industrial digital I/O products, such as the NI PXI-6514 isolated digital I/O module (Figure 10) and the NI PCI-7390 industrial motion controller.

Figure 10. Industrial digital I/O products using optocouplers.

For high-speed analog measurements, optocouplers have speed and power dissipation issues, and optocouplers are also limited by LED wear. Compared to optocouplers, digital isolators based on capacitors and inductors have fewer limitations.

Capacitive isolation

Texas Instruments offers digital isolation devices based on capacitive coupling. This type of isolator has high data rates and transient suppression capabilities. Compared to capacitive and optical isolation, inductive isolation consumes less power.

Inductive Isolation

Analog Devices’ iCoupler technology, developed in 2001 (analog.com/iCoupler), uses inductive coupling to achieve digital isolation for high-speed and high-channel-count applications. iCoupler devices can provide 100 Mb/s data rates on 16-bit analog measurement systems, with 2,500 V of voltage isolation, which means that sampling rates can reach megabits. Unlike optocouplers, iCoupler devices have lower power consumption, an operating temperature range of up to 125 °C, and transient voltage suppression of 25 kV/ms.

iCoupler technology is based on small chip-scale transformers. An iCoupler device consists of three main parts: transmitter, transformer, and receiver. The transmitter circuit uses an edge-triggered encoder to convert rising and falling edges into 1ns pulses. The pulses are transmitted through the isolation barrier by the transformer and decoded at the receiving circuit end, as shown in Figure 11. The small transformer of approximately 0.3 mm is almost unaffected by external magnetic fields. iCoupler devices integrate four isolation channels on each integrated circuit (IC), thereby reducing the cost of measurement hardware. Fewer external components are required compared to optocouplers.

Figure 11. Analog Devices’ iCoupler technology based on inductive coupling

Source: Analog Devices (analog.com/iCoupler) [page]

Measurement hardware vendors can use iCoupler devices to provide low-cost, high-performance data acquisition systems. National Instruments' industrial data acquisition (DAQ) devices for high-speed measurements, such as the industrial M Series multifunction DAQ device, use iCoupler digital isolators, as shown in Figure 12. This device provides 60 VDC continuous isolation, 1,400 Vrms/1,900 VDC channel-to-bus isolation for up to 5 seconds on multiple analog and digital channels, and sampling rates up to 250 kS/s. C Series modules in the NI PAC platform, NI CompactRIO, NI CompactDAQ, and other high-speed NI USB devices all use iCoupler technology.

Figure 12. Industrialized NI M Series multifunction DAQ using digital isolators.

Summarize

Isolated data acquisition systems provide reliable measurements in harsh industrial environments with hazardous voltages, transient voltages, etc. The measurement application and surrounding environment determine whether isolation is required. If the application requires a separate general-purpose data acquisition system to connect to various special sensors, choose analog isolation that can perform external signal conditioning; on the contrary, if low-cost, high-performance analog inputs are sought, choose a measurement system using digital isolation technology.