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

　　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 transfer rates. Digital isolation technology also provides analog designers with greater flexibility in selecting components and developing the optimal analog front end for measurement equipment. 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 around 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 isolators 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 trigger encoding and converts rising and falling edges on the digital line into 1 ns pulses. These pulses are transmitted across the isolation barrier using transformers and decoded by the receiver circuit on the other side, 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

　　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 its surrounding environment. Applications that require a single general-purpose data acquisition device to connect to sensors with different characteristics can benefit from external signal conditioning circuits with analog isolation; however, low-cost, high-performance analog input applications can benefit from measurement systems with digital isolation technology.