To gain better insight into the mechanical processes of expensive machinery as well as predictive maintenance, several industrial automation, process control, and test applications monitor plant efficiency by measuring pressure, strain, flow, voltage, and current signals. Traditionally, high-performance measurement systems were not designed for industrial environments involving hazardous voltages, transient signals, high common-mode voltages, and fluctuating ground potentials. This resulted in the ubiquitous use of programmable logic controllers (PLCs) for both measurement and control tasks. Because PLCs are not designed to take accurate measurements at high sampling rates, speed and accuracy are often sacrificed.
To overcome these challenges, National Instruments designed high-performance industrial measurement systems with built-in digital isolation technologies. With digital isolation, you can take safer measurements without sacrificing speed or accuracy and keep measurement system costs low. National Instruments Industrial M Series and S Series data acquisition devices with isolation offer built-in high-speed digital isolation with a variety of input and output channels, including 250 kS/s high-speed current and voltage analog channels with 16-bit accuracy and 24 V digital I/O. This article discusses how isolation works, the benefits isolation provides, and the technologies behind high-speed digital isolation used in NI M and S Series devices with isolation.
Figure 1. Industrial M and S Series devices offer built-in digital isolation, ±20 mA current, and 24 V digital I/O channels.
Isolation offers safety by electrically separating the sensor signals, which can be exposed to hazardous voltages, from the measurement system’s low-voltage backplane by way of a layer of insulation. Signals are transmitted across the insulation barrier using light, inductance, or capacitance without the use of any physical contact. Isolation offers several benefits, including:
• Protection for expensive equipment, the user, and data from transient voltages
• Improved noise immunity
• Ground-loop removal
• Increased common-mode voltage rejection
Isolated measurement systems provide separate ground planes for the analog or digital front end and the system backplane. The ground reference of the isolated front end is a floating pin that can operate at a different potential than the earth ground, as shown in Figure 2. Any common-mode voltage that exists between the sensor ground and the measurement system ground is rejected. This rejection prevents ground loops from forming and removes any noise common to the sensor lines and ground.
Figure 2. Isolated analog inputs offer safety from voltage spikes, reject common-mode voltages, and prevent ground loops from forming.
High-Speed Digital Isolation Technologies
For analog I/O channels, you can apply isolation either in the analog or the digital sections of the circuit. Isolation applied in the analog section of the circuit before the analog-to-digital converter (ADC) has digitized the signal is called analog isolation. Isolation applied after the ADC has digitized the signal is called digital isolation. Figure 3 shows both analog and digital isolation.
Figure 3a. Analog isolation applies isolation before the signal passes through the ADC and can add gain, nonlinearity, and offset to the signal.
Figure 3b. Digital isolation applies isolation after the signal passes through the ADC, so it can receive the original analog sensor signal.
Analog isolation is generally implemented by using an isolation amplifier in the analog front end of the data acquisition device. The ISO amp in Figure 3a represents an isolation amplifier. The analog signal from a sensor is passed to the isolation amplifier, which provides isolation and passes the signal to the analog-to-digital conversion circuitry.
For best performance, the input signal to the ADC should be as close to the original sensor signal as possible. Passing the signal through an isolation amplifier can add errors such as gain, nonlinearity, and offset before the signal reaches the ADC. You can improve performance by placing the ADC close to the original sensor signal. Digital isolation is based on this principle.
Along with higher performance, digital isolation components are also lower in cost, offer higher data transfer speeds, and give analog designers more flexibility to choose components and develop optimal analog front ends for measurement devices.
Optocouplers, digital isolators based on optical coupling principles, are one of the oldest and most commonly used methods for digital isolation. They can withstand high voltages and offer high immunity to electrical and magnetic noise. Engineers often use optocouplers in industrial digital I/O products, such as the National Instruments PXI-6515 isolated digital I/O module. For high-speed analog measurements, however, optocouplers suffer from speed, power dissipation, and LED lifetime limitations. Isolation based on capacitive and inductive coupling can provide higher data transfer rates and higher transient immunity. Compared with optical and capacitive isolation methods, inductive isolation offers lower power consumption.
iCoupler technology, introduced by Analog Devices in 2001, uses inductive coupling to offer digital isolation for high-speed and high-channel-count applications. iCouplers can provide 100 Mb/s data transfer rates with 2,500 V isolation withstand. Compared to optocouplers, iCouplers offer other benefits such as reduced power consumption, high operating temperature range up to 125 °C, and high transient immunity up to 25 kV/ms.
iCoupler technology is based on small, chip-scale transformers. An iCoupler contains three main parts – a transmitter, transformer, and receiver. The transmitter circuit uses edge trigger encoding and converts rising and falling edges on the digital lines to 1 ns pulses. These pulses are transmitted across the isolation barrier using the transformer and are decoded on the other side by the receiver circuitry (see Figure 3). The transformers’ small size, about three-tenths of a millimeter, makes them practically impervious to external magnetic noise. iCouplers also lower measurement hardware cost by integrating up to four isolated channels per IC and require few external components.
Figure 4. Induction coupling-based iCoupler technology from Analog Devices uses chip-scale transformers to offer high-speed digital isolation.
Source: Analog Devices (analog.com/iCoupler)
National Instruments M and S Series data acquisition devices with isolation use iCoupler digital isolators (see Figure 4). With iCouplers, Industrial M and S Series devices can provide 60 VDC continuous isolation and 1,950 VDC withstand isolation for 5 s, making these devices ideal for applications requiring high-speed, accurate measurements in hazardous environments. National Instruments C Series modules used in NI CompactRIO, NI CompactDAQ, and high-speed NI USB devices also use the iCoupler technology.
Figure 5. National Instruments M Series multifunction devices with isolation use Analog Devices iCoupler technology to deliver reliable and accurate measurements.
Safe and Accurate Measurements
By integrating digital isolation with high-speed, accurate data acquisition, National Instruments M and S Series devices with isolation offer a reliable, low-cost measurement solution for applications involving hazardous environments. Isolation offers safety from voltage spikes and removes common analog measurement problems such as ground loops and common-mode voltages. High-speed digital isolation provides safety with no sacrifice in speed or accuracy.
Industrial Data Acquisition
Learn more about the many technologies available for implementing isolation by reading the “Isolation Technologies for Reliable Industrial Measurements” white paper.
View a 45-minute Webcast on Demand on “M Series Data Acquisition for Control” to learn about the technology behind analog output, counter/timer, and digital I/O functions in an M Series device.