Many of the commercial off-the-shelf (COTS) components available today incorporate one of the above technologies to provide isolation. For analog I/O channels, you can implement isolation either in the analog section of the device before the analog-to-digital converter (ADC) has digitized the signal (analog isolation) or after the ADC has digitized the signal (digital isolation). You need to design different circuitry around one of these techniques based on your isolation implementation location in the circuit. 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 isolation.
Figure 6a. Analog Isolation
Figure 6b. Digital Isolation
The following sections cover analog and digital isolation in more detail and explore the different techniques for implementing each.
The isolation amplifier is generally used to provide isolation in the analog front end of data acquisition devices. “ISO Amp” in Figure 6a represents an isolation amplifier, which, in most circuits, is one of the first components of the analog circuitry. 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. Figure 7 represents the general layout of an isolation amplifier.
Figure 7. Isolation Amplifier
In an ideal isolation amplifier, the analog output signal is the same as the analog input signal. The section labeled “isolation” in Figure 7 uses one of the techniques discussed in the previous section (optical, capacitive, or inductive coupling) to pass the signal across the isolation barrier. The modulator circuit prepares the signal for the isolation circuitry. For optical methods, you need to digitize or translate this signal into varying light intensities. For capacitive and inductive methods, you need to translate the signal into varying electric or magnetic fields. The demodulator circuit then reads the isolation circuit output and converts it back into the original analog signal.
Because you perform analog isolation before the signal is digitized, it is the best method to apply when designing external signal conditioning for use with existing non-isolated data acquisition devices. In this case, the data acquisition device performs the analog-to-digital conversion and the external circuitry provides isolation. With the data acquisition device and external signal conditioning combination, measurement system vendors can develop general-purpose data acquisition devices and sensor-specific signal conditioning. Figure 8 shows analog isolation being implemented with flexible signal conditioning that uses isolation amplifiers. Another benefit to isolation in the analog front end is protection for the ADC and other analog circuitry from voltage spikes.
Figure 8. Use of Isolation Amplifiers in Flexible Signal Conditioning Hardware
There are several options available on the market for measurement products that use a general-purpose data acquisition device and external signal conditioning. 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 need isolation, you can use the M Series devices with external signal conditioning, such as the National Instruments SCXI or SCC modules. These signal conditioning platforms deliver the isolation and specialized signal conditioning you need for direct connection to industrial sensors such as load cells, strain gages, and pH sensors.
ADCs are 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 add errors such as gain, nonlinearity, and offset before the signal reaches the ADC. Placing the ADC closer to the signal source can lead to better performance. Analog isolation components are also costly and can suffer from long settling times. Despite better digital isolation performance, one of the reasons for using analog isolation in the past was to provide protection for the expensive ADCs. Because ADC prices have significantly declined, 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. Declining Price of 16-Bit Analog-to-Digital Converters
Graph Source: National Instruments and a Leading ADC Supplier
Compared to isolation amplifiers, digital isolation components are lower in cost and offer higher data transfer speeds. Digital isolation techniques also give analog designers more flexibility to choose components and develop optimal analog front ends for measurement devices. Products with digital isolation use current- and voltage-limiting circuits to provide ADC protection. Digital isolation components follow the same fundamental principles of optical, capacitive, and inductive coupling that form the basis of analog isolation.
Leading digital isolation component vendors such as Avago Technologies (www.avagotech.com), Texas Instruments (www.ti.com), and Analog Devices (www.analog.com) have developed their isolation technologies around one of these basic principles. Avago Technologies offers digital isolators based on optical coupling, Texas Instruments bases its isolators on capacitive coupling, and Analog Devices isolators use inductive coupling.
Optocouplers, digital isolators based on the 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. Optocouplers are often used on 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.
Figure 10. Industrial Digital I/O Products Use Optocouplers
For high-speed analog measurements, optocouplers, however, suffer from speed, power dissipation, and LED wear limitations associated with optical coupling. Digital isolators based on capacitive and inductive coupling can alleviate many optocoupler limitations.
Texas Instruments offers digital isolation components based on capacitive coupling. These isolators provide high data transfer rates and high transient immunity. Compared to capacitive and optical isolation methods ,inductive isolation offers lower power consumption.
iCoupler technology, introduced by Analog Devices in 2001 (analog.com/iCoupler), uses inductive coupling to offer digital isolation for high-speed and high-channel-count applications. iCoupler devices can provide 100 Mb/s data transfer rates with 2,500 V isolation withstand for a 16-bit analog measurement system that implies sampling rates in the megahertz range. Unlike optocouplers, iCoupler devices 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 has three main parts – a transmitter, transformers, and a 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 decoded on the other side by the receiver circuitry, shown in Figure 11. The small size of the transformers, about three-tenths of a millimeter, makes them practically impervious to external magnetic noise. iCoupler devices can also lower measurement hardware costs by integrating up to four isolated channels per integrated circuit (IC) and, compared to optocouplers, they require fewer external components.
Figure 11. Induction Coupling-Based iCoupler Technology from Analog Devices
Source: Analog Devices (analog.com/iCoupler)
Measurement hardware vendors are using iCoupler devices to offer high-performance data acquisition systems at lower costs. National Instruments industrial data acquisition (DAQ) devices intended for high-speed measurements, such as the industrial M Series multifunction DAQ devices, use iCoupler digital isolators, shown in Figure 12. These devices provide 60 VDC continuous isolation and 1,400 Vrms/1,900 VDC channel-to-bus isolation withstand for 5 s on multiple analog and digital channels and support sampling rates up to 250 kS/s. National Instruments C Series modules used in the NI PAC platform, NI CompactRIO, NI CompactDAQ, and other high-speed NI USB, Ethernet, and wireless devices also use iCoupler technology.
Figure 12. Industrial NI M Series multifunction DAQ uses digital isolators.