Segregation separates one part from the non-ideal influences of other parts. In electronic circuits, the dielectric is isolated by blocking direct current (DC). How does the isolated circuit operate in a larger electrical system? The answer to this question is the subject of this application report. With the increasing number of products introduced by Texas Instruments and other suppliers, the transmission options for isolated signals have also increased, making designers more complex in product selection. This report describes the important features of the isolator and explains the differences and similarities between the products. After reviewing the necessity of circuit isolation, we discussed three methods of dielectric signal transmission and analog to digital isolators and described and compared examples of each type of digital isolator. 2. Necessity of circuit isolation The main reason for isolating the circuit is to protect the circuit from dangerous voltage and current damage. In the medical application example of Fig. 1, even a small amount of AC current may cause fatal injuries, so a barrier is needed to protect the patient. Isolation also protects sensitive circuits from high voltage damage in industrial applications. The industrial example of Figure 2 is only a high pressure measurement. Separating the sensor from the actual high voltage makes it possible to measure low voltage circuits. The protection principle is to isolate high voltage potentials, which may appear in various systems or circuits, as shown in the cable application in Figure 3, where long distances can isolate one driver from the receiver. After such a long distance, the grounding may be at different voltages. By isolating, a voltage difference is created in the isolator rather than in the sensitive circuit. As shown in FIG. 4, the isolation interrupts the loop formed by the circuit path through high impedance relative to other circuit components. By interrupting the loop, noise voltages appear on the isolation layer rather than on the receiver or more sensitive components. The high level of the noise voltage can be coupled by an external current or voltage source (eg, inductive motor and lightning). 3. Circuit Isolator Circuit isolators prevent low frequency currents between circuits while allowing analog or digital signal transmission through electromagnetic or optical links. Digital isolators transmit binary signals, and analog isolators transmit continuous signals on the isolation layer. In analog and digital isolators, the operating and peak voltage ratings as well as common-mode transient immunity are important characteristics of this isolation layer. When isolating digital signals, these important characteristics of the isolation circuit are input and output logic voltage levels, signal rates, data run lengths, and Auto-Protect responses. Traditionally, transformers, capacitors, or photodiode transistors and discrete circuits are conditioned on input and output signals to meet special requirements. This method is effective, but it cannot be transferred from one application to another. Although this may hold analog isolators, there has been a new generation of digital isolators in the market that use innovative circuitry to isolate standard digital signals at data rates exceeding 100 Mbps DC. These universal digital isolators have their respective advantages and disadvantages. The following paragraphs will introduce various technologies and compare specific products with TI's new ISO72x series. 3.1 Optical Coupling Technology Optical coupling technology is the transmission of light over a transparent insulating barrier (eg air gap) for isolation purposes. Figure 5 shows the main components of a digital isolator. The current driver uses a digital input and converts the signal into a current to drive a light emitting diode (LED). The output buffer converts the current output of the photodetector into a digital output. The main advantage of optical coupling technology is that light has inherent immunity to external electrons or magnetic fields, and optical coupling technology allows the use of constant information transmission. The shortcomings of optocouplers are mainly reflected in speed limitations, power consumption, and LED aging. The maximum signal rate of an optocoupler depends on the speed at which the LED can turn on and off. From the current available products, the fastest optocoupler is HCPL-0723, which can reach 50Mbps signal rate. The current-to-output ratio (CTR) from input to output is an important characteristic of optocouplers. LEDs typically require 10mA of input current for high-speed digital transmission. This ratio adjusts the current used to drive the LED and the current generated by the phototransistor. Over time, LEDs become more inefficient and require more current to produce the same level of brightness and the same level of phototransistor output current. In many digital isolators, the internal circuitry controls the LED drive current, and the user cannot compensate for the decreasing CTR. The advantages of LEDs have diminished, and the isolators are no longer as effective as they used to be over time. 3.2 Inductive coupling technology Inductive coupling technology uses a changing magnetic field between two coils to communicate on a separate layer. The most common example is a transformer whose magnetic field size depends on the coil structure (number of turns/unit length) of the main and secondary windings, the dielectric constant of the magnetic core, and the current amplitude. Figure 6 shows a transformer with a signal conditioning circuit module. The advantage of inductive coupling is that there may be common-mode differences and differential transmission characteristics. The careful design of the transformer allows the noise and signal frequencies to overlap, but exhibits high noise common mode impedance and signal low differential impedance. Another advantage is that signal energy transmission can be nearly 100% efficient, making low-power isolator possible. The main drawback of inductive coupling technology is the magnetization of the external magnetic field (noise). Industrial applications often require magnetic isolation, such as motor control. Another drawback in digital transformer transmission is the length of data run. A signal converter transmits signals over a range of frequencies and amplitudes, and its distortion is acceptable. Data run length restrictions or clock coding are required to keep this signal within the available transformer bandwidth. Universal digital isolators using inductive coupling techniques require signal processing to transmit and reconstruct digital signals along with methods that transmit low-frequency signals (1 or 0 long characters). The Isoloop from NVE/Avago and the iCoupler from ADI (Analog Devices, USA) all use coding capabilities and provide a digital isolation solution that supports operation from DC to 100Mbps. The ADuM1100 is an example of ADI's iCoupler technology. The ADuM1100 uses a basic transformer to transmit information over an isolation layer. This Isoloop technology (eg, HCPL-0900) uses a resistor network as shown in FIG. 7 to replace the secondary coil. The resistor consists of a GMR (giant magnetoresistive) material, which changes when the magnetic field is applied. The circuit senses changes in resistance and satisfies its conditions for output. When this technology was introduced to the market for the first time, AC performance was actually improved, exceeding the performance of existing optocouplers. Now, as ADI recently introduced more digital isolators and the introduction of TI's ISO72x family of devices, the performance of these Isoloop devices has been exceeded. 3.3 Capacitive coupling technology Capacitive coupling technology uses a constantly changing electric field to transmit information on the isolation layer. The material between the capacitor plates is a dielectric separator and forms the isolation layer. The plate size, spacing between plates, and dielectric materials all determine electrical performance. The benefits of using a capacitive isolation layer are high efficiency in size and energy transfer, as well as immunity to magnetic fields. The former makes it possible to integrate isolation circuits with low power consumption and low cost, while the latter makes it possible to operate in a saturated or high-density magnetic field environment. The disadvantage of capacitive coupling is that it does not have differential signals and noise, and the signals share the same transmission channel, unlike transformers. This requires the signal frequency to be much higher than the noise expected frequency, so that the isolation capacitor exhibits a low impedance of the signal and a high impedance of the noise. After inductive coupling is used, capacitive coupling cannot transmit a steady state signal and requires clock coded data. 3.3.1 TI introduced the ISO721 TI's ISO72x series of isolator use capacitive coupling technology. Capacitive coupling solutions use proven, low-cost manufacturing processes with inherent immunity to magnetic fields. In order to provide constant information transmission, the ISO72x uses a high signal rate and low signal rate channel for communication, as shown in Figure 9. The high signal rate channel is not encoded and it transmits data on a separate layer after single-ended to differential conversion. The low-signal-rate channel encodes data in a pulse-width modulation format and differentially transmits data over the isolation layer to ensure accurate communication at constant states (long characters of 1 and 0). The differential transmission of single-ended logic signals on the isolation layer allows the use of low-level signals and small coupling capacitors. This presents a high impedance to common-mode noise and, through the receiver's common mode noise suppression, brings about excellent transient immunity, which is the main problem that signal capacitive coupling needs to solve. 3.4 Isolation performance Three major standards validate the need for isolation protection, which are UL 1577, IEC 60747-5-2, and CSA, respectively. Although each standard is slightly different, it provides a standard for comparing isolation performance. Tests by IEC, UL, and CSA confirmed the voltage outside the dielectric breakdown between input and output. The use of these standards is very simple, because the test criteria and the isolation method are irrelevant. Figure 10 shows how the isolation test treats the isolator as a two-terminal device. Although there is a difference in the physical structure of each device, the isolation test is measured on the dielectric breakdown voltage. UL 1577, IEC 60747-5-2, IEC 61010-1, and CSA tested the ISO72x family of isolation performance. Table 1 shows the isolation performance of these five devices illustrating the three isolation techniques. All three tests, namely UL, CSA, and IEC, tested the quality of the isolation layer. Both UL and CSA tests are stress tests that use a specified time set by the manufacturer to test dielectric breakdown voltage. Breakdown of the dielectric is a failure that occurs during this test. The IEC test uses a phenomenon known as partial discharge to detect voids in the dielectric. A large voltage is applied to the device, which is a function of the operating voltage defined by the manufacturer and then reduced to another voltage level, Vm. In this low voltage application, the device under test is subjected to ineffective partial discharge monitoring within the dielectric. These invalidities will result in the final breakdown of the entire dielectric. 3.5 Transient immunity High slew rate (high frequency) transients can disrupt data transmission on one isolation layer. This isolation capacitor provides a channel as shown in Figure 11 that causes transient events to pass through the isolation layer and disrupt the output waveform. A Faraday shield can keep this part of the displacement current in the optocoupler or inductive coupler away from the important output structure. In capacitive coupling solutions, Faraday shield is not a viable solution. In addition to transients, Faraday shields also block the electric field used for data transmission. In order to provide transient immunity, the ISO72x series capacitive isolators transmit only fo signals (data signals that represent only the highest frequency energy in the signal). This allows a small coupling capacitor with a high noise frequency. Other noise comes from the differential technique of transmitting data on the isolation layer. Figure 9 shows the four signals passing through the capacitive isolation layer; two contain low signal rate information and the other two contain high signal rate information. By using differential techniques, any remaining common-mode transients through the isolation layer can be seen in the real and compensated signals, and the differential receiver rejects it. As shown in Table 2, the transient immunity of the ISO72x series is as high as all comparable devices up to 25 kV/us. 3.6 Automatic Protection The data line circuit and the digital isolator need to pay attention to the output state of the input signal loss. Input loss may occur when the cable is disconnected or removed directly from the isolator input. Auto-protection refers to a deterministic or known output state under input loss conditions. The ISO72x family uses a periodic pulse to determine if the input structure is powered and is working. If the output of the isolator does not receive a pulse after 4 us, the output is set to a high state. The ADum1100 from ADI also integrates an automatic protection circuit in the output section of the IC. Avago Technologies' optical solutions (HCPL-0721 and -0723) do not mention auto-protection, and the inductive GMR solution (HCPL-0900) clearly describes the indeterminate nature of the output during power sequencing. 3.7 Power Consumption In addition to the efficiency of signal transmission at the isolation layer, the design of the input and output conditioning circuits is most relevant to power consumption. As shown in Table 3, optocouplers consume more power than inductors or capacitors. 3.8 Reliability The mean time to failure (MTTF) is a standard measure of semiconductor device reliability. For digital isolators, this measurement represents the reliability of the integrated circuit and isolation mechanism. Table 4 shows the MTTF of an optical, inductive and capacitive digital isolator. Compared to inductors and optical solutions, ISO721 is very reliable. The ADuM1100 reliability data sheet does not explicitly describe the MTTF, but it provides the results of a reliability test. Table 5 shows the parameters of the ISO721 and ADuM1100 reliability tests. 3.9 External Magnetic Field Immunity Figure 12 compares the magnetic field immunity of the ADuM1100 and ISO72x (data not found for the HCPL-0900) . Relatively speaking, although both of these examples have a certain degree of immunity to magnetic fields, ISO72x offers a greater margin. As previously described, the optically coupled isolation layer circuit has an inherent magnetization immunity to the external magnetic field. 4. Summary of this article Noise reduction and noise protection make isolators widely used in electronic circuits where those isolators interrupt the ground loop and isolate the ground voltage differential. Designers now have many options for digital signal isolation, including the TI's ISO72x family of key features such as signal rate, dielectric breakdown voltage, transient immunity, power consumption, magnetic field immunity, and reliability. All have performed well. Table 6 summarizes these characteristics of the examples discussed in this report. bibliography 1. "electronic noise reduction technology system in the second edition," the authors: HW Ott; Pin Header Connector,0.80Mm Header Connector,3.96Mm Pin Header Connector,Smt Vertical Type Connector Shenzhen CGE Electronics Co.,Ltd , https://www.cgeconnector.com
Figure 1: Possible circuit paths between the power supply and the patient. 
Figure 2: Isolation between high and low voltage circuits. 
Figure 3: Ground voltage difference between devices. 
Figure 4: Ground loop between isolated disconnected nodes. 
Figure 5: Basic optical coupling mechanism. 
Figure 6: Inductive isolation. 
Figure 7: GMR structure diagram. 
Figure 8: Capacitive coupling. 
Figure 9: Block diagram of ISO72x and ISO72xM. 
Figure 10: Isolation voltage test at both ends. 
Figure 11: Spacer Capacitance. 
Figure 12: Sensitivity to an external magnetic field.
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10. "sensor", January 1999 edition, "System Design Guide isolation device" by: P Pickering;
11. "40ns propagation delay CMOS optical coupler reliability product specification", July 2002 edition, Agilent Technologies (Avatech);
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13. "ADuM1100Fab Transmission, Reliability Report", December 2002, Analog Devices, USA.
Interpretation of Electronic Isolation Application Techniques in High Speed ​​Digital Circuits
1 Introduction