1. Definition of electrical isolation

Galvanic isolation is the separation of components with non-ideal effects from other components. In electronic circuits, dielectrics isolate by blocking direct current. How do isolation circuits work in a larger electrical system? The answer to this question is the subject of this article. As the number of products introduced by Texas Instruments and other suppliers continues to increase, so too are the options for transmitting isolated signals, making the designer's choice of product more complicated. This article introduces the important characteristics of isolators and explains the similarities and differences between the products.

After reviewing the need for circuit isolation, three methods of dielectric signal transmission and analog-to-digital isolators are discussed, and examples of digital isolators for each are described and compared. In other words, electrical isolation is a corrosion control method. Conductors are susceptible to corrosion from stray currents from dissimilar metals. Providing good isolation for these conductors can significantly control corrosion.

Electrical isolation is achieved by a mechanical switch that isolates part of the circuit from the main power system when required.

2. The significance of electrical isolation

The main reason to isolate a circuit is to protect it from dangerous voltages and currents. In the medical application example in Figure 1, even a small amount of AC current can be fatal, so an isolation barrier is needed to protect the patient. Isolation can also protect sensitive circuits from the high voltages that appear in industrial applications. The industrial example in Figure 2 is simply a high-voltage measurement. Isolating the sensor from the actual high voltage allows the measurement of the low-voltage circuit.

Figure 1: Example of a medical application

Figure 2: Example of industrial application

The principle of protection is to isolate high potentials that may occur in various systems or circuits. In the cable application shown in Figure 3, a remote driver is isolated from the receiver. Over such a long distance, the grounds may be at different voltages. With isolation, the voltage difference is formed in the isolator instead of the sensitive circuit.

Figure 3: Ground voltage difference between devices

As shown in Figure 4, isolation breaks the loop formed by high impedance circuit paths associated with other circuit elements. By breaking the loop, the noise voltage appears across the isolation barrier instead of at the receiver or more sensitive components. High noise voltages can be coupled by external current or voltage sources such as induction motors and lightning.

Figure 4: Isolation interrupts the loop formed by the circuit path

3. Circuit Isolator

Circuit isolators prevent low-frequency currents between circuits while allowing analog or digital signals to be transmitted over electromagnetic or optical links. Digital isolators transmit binary signals, while analog isolators transmit continuous signals across the isolation barrier. In both analog and digital isolators, working and peak voltage ratings and common-mode transient immunity are important characteristics of the isolation barrier. When isolating digital signals, these important characteristics of the isolation circuit are input and output logic voltage levels, signal rates, data run lengths, and self-protection responses.

Traditionally, transformers, capacitors or photodiodes, transistors, and discrete circuits have been conditioned to fit specific needs based on input and output signals. This approach is effective, but it is not transferable from one application to another. While this may preserve analog isolators, a new generation of digital isolators has emerged on the market that use innovative circuitry to isolate standard digital signals at DC signal rates exceeding 100 Mbps. Each of these general-purpose digital isolators has advantages and disadvantages. The following sections will introduce various technologies and compare specific products with TI's new ISO72x family.

• 3.1 Optical Coupling Technology

Optical coupling refers to the transmission of light across a transparent insulating layer (such as an air gap) for isolation. Figure 5 shows the main components of a digital isolator. The current driver uses the digital input and converts the signal into a current to drive the light emitting diode (LED). The output buffer converts the current output of the photodetector into a digital output.

Figure 5: Basic optical coupling mechanism

The main advantage of optical coupling technology is that light is inherently unaffected by external electronic or magnetic fields, and optical coupling technology allows for constant information transmission. The disadvantages of optical coupling are mainly reflected in speed limitation, power consumption and LED aging.

The maximum signal rate of the optocoupler depends on the speed of the LED switch. From the current existing products, the fastest optocoupler HCPL-0723 can reach a signal rate of 50Mbps.

The input-to-output current transfer ratio (CTR) is an important characteristic of optocouplers and LEDs, which typically require an input current of 10mA to achieve high-speed digital transmission. This ratio adjusts the current used to drive the LED and the current produced by the phototransistor. Over time, LEDs have become less efficient, requiring more current to produce the same brightness with the same output current from the phototransistor. In many digital isolators, internal circuitry controls the LED drive current, and the user cannot compensate for the reduced CTR. The advantage of the LED is reduced, and the isolator no longer works as efficiently as before.

• 3.2 Inductive Coupling

Inductive coupling technology uses the changing magnetic field between two coils to communicate across an isolation barrier. The most common example is a transformer, where the magnetic field depends on the coil configuration (number of turns per unit length) of the primary and secondary windings, the dielectric constant of the core, and the current amplitude. Figure 6 shows a transformer with a signal conditioning circuit block.

Figure 6: Inductive isolation

The advantage of inductive coupling is the possible common-mode differential and differential transmission characteristics. A well-designed transformer allows noise and signal frequencies to overlap, but will present a high noise common-mode impedance and low signal differential impedance. Another advantage is that the signal energy transfer efficiency can be close to 100%, making low-power isolators possible.

The main disadvantage of inductive coupling technology is magnetization (noise) from external magnetic fields. Industrial applications often require magnetic field isolation. For example, motion control. Another disadvantage of digital transformer transmission is the run length of the data. Signal converters transmit signals within a certain frequency and amplitude range with acceptable distortion. In order to keep the signal within the available transformer bandwidth, data run length limitation or clock encoding is required.

General-purpose digital isolators using inductive coupling require signal processing to transmit and reconstruct digital signals while transmitting low-frequency signals (long 1 or 0 characters). NVE/Avago's Isoloop and ADI's (Analog Devices) iCoupler use encoding functions and provide digital isolation solutions that support DC-100Mbps operation.

The ADUM1100 is an example of analog device IC coupler technology. The ADUM1100 uses a basic transformer to transfer information across an isolation barrier. This Isoloop technology, such as the HCPL-0900, replaces the secondary coil with a resistor network, as shown in Figure 7. The resistors are made of giant magnetoresistance (GMR) material, which changes in response to a magnetic field. The circuit senses the change in resistance and conditions its output. This technology was introduced to the market when AC performance improved and exceeded the performance of existing optocouplers. Now, with the introduction of ADI's digital isolators and TI's ISO72x series of devices, the performance of these Isoloop devices has been surpassed.

Figure 7: GMR structure

• 3.3 Capacitive coupling

Capacitive coupling technology uses a changing electric field to transfer information across an isolation layer. The material between each capacitor plate is a dielectric isolator, forming a barrier. The size of the plates, the spacing between the plates, and the dielectric material all determine the electrical performance.

Figure 8: Capacitive coupling

The advantages of using a capacitive isolation layer are size and energy transfer efficiency, as well as immunity to magnetic fields. The former makes it possible to integrate low-power, low-cost isolation circuits; the latter makes it possible to operate in saturated or high-density magnetic fields.

The disadvantage of capacitive coupling technology is that there is no differential signal and noise, the signals share the same transmission channel, which is different from the transformer. This requires that the frequency of the signal is significantly higher than the expected frequency of the noise so that the isolation capacitor presents a low impedance to the signal and a high impedance to the noise. In the case of inductive coupling, capacitive coupling cannot transmit steady-state signals and requires clock-encoded data.

• 3.3.1 TI introduces ISO72x--Electrical Isolation Test

TI introduced the ISO72x series of isolators using capacitive coupling technology. Capacitive coupling solutions use mature, low-cost manufacturing processes and are inherently immune to magnetic fields.

To provide constant information transfer, the ISO72x uses high signal rate and low signal rate channels for communication, as shown in Figure 9. The high signal rate channel is not encoded, it transmits data via single-ended differential conversion across the isolation layer. The low signal-to-noise ratio channel encodes data in a pulse width modulation format and transmits differentially across the isolation layer to ensure accurate communication under constant conditions (long 1 and 0 characters).