A brief discussion on the selection and application of digital isolation devices

introduction

Digital isolation technology is often used in fieldbus, military electronic systems and aerospace electronic equipment in industrial network environments, especially in some harsh application environments. Digital isolation circuits are mainly used for the transmission of digital signals and switching signals. One of the primary reasons for using isolation circuits is to eliminate noise. Another important reason is to protect devices (or people) from high voltage hazards. The isolation rating listed in the manufacturer's product manual should comply with the relevant isolator standards established by the Underwriters Laboratories (UL 1577), the International Electrotechnical Commission (IEC 60747-5-2, IEC 61010-1) and the Canadian Standards Association (CSA Component Acceptance Notice 5A).

There are many manufacturers of digital isolation devices, such as Avago, TI, ADI, NVE (nonvolatile electronics Inc), Silicon Laboratories, etc. The products of each manufacturer have been widely used. According to the production process, electrical structure and transmission principle of digital isolation circuits, digital isolation circuits are mainly divided into digital isolation devices with optical, inductive and capacitive coupling technologies. Products are available at multiple levels such as consumer equipment, industrial control, military, and aerospace for users to choose from.

Working principles and characteristics of various digital isolation devices

Opto-isolator

Optical coupler is also called photoelectric isolator or optocoupler, or simply optocoupler. It is a device that uses light as a medium to transmit electrical signals. The light emitter (infrared light emitting diode LED) and the light receiver (photosensitive semiconductor tube) are packaged in the same tube shell. When an electrical signal is applied to the input end, the light emitter emits light, and the light receiver generates photocurrent after receiving the light, which flows out from the output end, thus realizing the "electric-light-electric" conversion. The optical coupler that uses light as a medium to couple the input signal to the output end has been widely used in digital circuits due to its advantages such as small size, long life, no contact, strong anti-interference ability, insulation between output and input, and unidirectional signal transmission. Its circuit structure is relatively simple, mainly composed of gallium arsenide infrared light emitting diode and photosensitive diode or triode used as a detector. Some products add some processing circuits in the post-stage of the photosensitive diode or triode to make its characteristics suitable for some special applications or to realize some standard interfaces. The principle block diagram of Avago's high-speed CMOS interface optical coupler HCPL-0723 is shown in Figure 1.

Figure 1 Schematic diagram of the HCPL-0723 coupler from Avago

Optical coupling has long been used in industrial networks, and early reference designs for electrical layer interfaces often include optocouplers. Its main advantages are that light has inherent immunity to external electromagnetic interference, and optical coupling allows for steady-state information transmission. Disadvantages are limited transmission speed, high power consumption, and the susceptibility of light-emitting diodes (LEDs) to aging over time and temperature.

Inductive Isolators

Like optical coupling, inductive coupling also has a long history of application, but it is usually only used for power supplies or analog isolators, not digital devices. However, with the advancement of manufacturing technology and the improvement of R&D and design levels, inductive digital isolation devices have been rapidly developed and widely used.

Inductive coupling uses a changing magnetic field to communicate across an isolation barrier. One advantage of inductive coupling is that the common-mode noise of the transformer can be minimized without significantly degrading the differential-mode signal. Another advantage is that the conversion efficiency of signal energy is very high, which allows for low-power isolators. One disadvantage is that it is susceptible to interference from external magnetic fields (noise). Industrial applications such as motor control often require isolation in magnetic field environments. Another issue of concern with inductive coupling is the transmission of digital data and data run-length (the number of consecutive "1" or "0"). The coupling between the primary and secondary windings can pass signals over a certain frequency range with acceptable attenuation. The limitation of data run-length or clock encoding requires that the signal must be kept within the available bandwidth of the transformer. General-purpose digital isolators using inductive coupling require signal processing to transmit and reconstruct digital signals, as well as transmit low-frequency signals representing a long string of "1" or "0", or even DC levels.

The transformer is the most common example: the structure of the primary and secondary windings (number of turns per unit length), the dielectric constant of the core, and the current strength determine the magnetic field strength. Depending on the encoding and decoding of digital signals, there are two main types of products, those using pulse modulation (ADI) and those using RF modulation (Silicon Labs). Digital isolation devices designed using giant magnetoresistance (GMR) effect technology are another example, represented by NVE and Avago.

Pulse Modulation Transformer Isolation Devices

ADI's iCoupler isolator is a magnetic coupler based on chip-size transformers and is a digital isolation device implemented by pulse modulation. The planar transformer uses a CMOS metal layer with a layer of gold plated on the top for passivation. The high-breakdown voltage-resistant polyimide layer under the gold layer isolates the transformer coil on the top from the bottom coil. High-speed CMOS circuits connected to the top and bottom coils provide an interface between each transformer and its external signals. Wafer-level signal processing provides a low-cost method to integrate multiple isolation channels and other semiconductor functions in a single chip. iCoupler technology eliminates the uncertain current transfer ratio, nonlinear transfer characteristics, and drift over time and temperature associated with optocouplers, reduces power consumption by 90%, and does not require external drivers or discrete devices.

The transmission of digital signals is achieved by sending short pulses of about 1ns width to the other end of the transformer. Two consecutive short pulses represent a rising edge and a single short pulse represents a falling edge. The signal transmission block diagram is shown in Figure 2. A non-repeatable triggering monostable circuit generates a detection pulse on the secondary side. If two pulses are detected, the output is set to a high level. On the contrary, if a single pulse is detected, the output is set to a low level. The use of an input filter helps to improve noise immunity. If no signal edge is detected for about 1ms, a refresh pulse signal is sent to the transformer to ensure DC correctness (DC correction function). If the input is high, two consecutive short pulses are generated as refresh pulses, and if the input is low, a single short pulse refresh is generated. This is important for power-on states and input waveforms with low data rates or constant DC inputs. To complement the refresh circuit on the driver side, a watchdog timer is used on the receiver side to ensure that the output is in a fail-safe state when no refresh pulse is detected. The block diagram of the ADuM1100 device of Analog Devices is shown in Figure 3.

Figure 2 Block diagram of ADI's iCoupler series digital signal transmission


Figure 3 Principle block diagram of ADI's ADuM1100 device

RF Modulation Transformer Isolation Devices

Silicon Labs is a typical representative of digital isolation devices developed and produced using RF modulation transformer technology. Its Si844x series devices are based on a patented architecture and use standard full CMOS processes to manufacture multiple sets of chip-level transformers, which can provide the most integrated 4-channel isolation function. The RF encoding and decoding mechanism used in the product allows a reliable isolated data path to be provided without special consideration or initial setting. The advantages of Silicon Labs' products are similar to those of ADI's products, but there is also a very obvious disadvantage. Due to the use of RF modulation, there is an internal 2.1GHz carrier generation and detection. The carrier and harmonics will generate electromagnetic radiation to the outside world, but the electromagnetic radiation value meets the FCC (US Communications Commission) standard requirements. The implementation principle block diagram of the company's RF modulation isolation device is shown in Figure 4.

Figure 4 RF modulation isolation device implementation principle block diagram

Giant Magnetoresistance Isolation Device

NVE's IL series and Avago's HCPL-90XX/09XX series high-speed digital isolation devices are high-speed CMOS devices integrated with giant magnetoresistance technology. In a GMR isolator, the input signal induces current in a low-inductance coil, generating a proportional magnetic field. The total magnetic field changes the resistance of the GMR, and through CMOS integrated circuit analysis, the output is an accurate reproduction of the input signal. The advantages of this type of device are similar to other inductive devices, but there are several obvious disadvantages: the input and output may be inconsistent at power-on or initial state; sensitive to input noise, accompanied by a noise spike, the output is unstable, may be inconsistent with the input, may be consistent, and may oscillate; for a slower pulse rising edge, the output may change with the input, may not change, and may oscillate; the output has overshoot; there is no DC correction function, and DC signals cannot be transmitted. The implementation schematic diagram of NVE's giant magnetoresistance isolation device IL710 is shown in Figure 5.

Figure 5 NVE IL710 implementation principle block diagram

Capacitively coupled isolation devices

Capacitive coupling uses a changing electric field to transfer information across an isolation barrier. The material between the capacitor plates is a dielectric insulator (silicon dioxide), or isolation barrier, which is a high-performance insulator with very stable reliability and durability, as well as resistance to magnetic interference and transient voltages. The size of the plates, the distance between the plates, and the dielectric material determine the electrical characteristics. The advantage of using a capacitive isolation barrier is high efficiency, both in terms of volume, energy conversion, and resistance to magnetic field interference. This high efficiency makes it possible to implement low-power and low-cost integrated isolation circuits. The interference immunity allows the device to operate in saturated or dense magnetic field environments. Unlike transformers, the disadvantage of capacitive coupling is that there is no differential signal and noise and signal share the same transmission channel. This requires that the signal frequency should be much higher than the possible noise frequency so that the isolation barrier capacitor presents low impedance to the signal and high impedance to the noise. Like inductive coupling, capacitive coupling also has bandwidth limitations and requires clock-encoded data.

TI's ISO72x series of digital isolators use capacitive coupling technology. Capacitive coupling solutions use proven low-cost manufacturing processes and provide inherent immunity to magnetic field interference. ISO72x uses two channels, "AC" and "DC", for communication, as shown in Figure 6. The "AC" channel is not encoded, but transmits data directly across the isolation barrier after single-ended to differential conversion. The advantage of differential signal transmission is that it can suppress common-mode noise at the receiver. Common-mode rejection and the coupling medium (high impedance to noise and low impedance to high-frequency data) together achieve transient interference immunity. The "DC" channel converts the input data into a pulse width modulation (PWM) format and transmits the data through the isolation barrier using a differential method. The pulse width demodulator (PWD) on the receiving side of the PWM and isolation barrier ensures correct communication under steady-state conditions (long strings of 1 or 0). In addition, the "DC" channel also provides auto-protection. Auto-protection refers to the judgment of the output state in the event of an input fault. The ISO72x series of devices use carrier detection to determine whether the power supply of the input structure is "on" and whether the structure is operating. If the carrier detector does not detect a pulse within 4ms, it sets the output to a logic high level.

Figure 6 TI ISO72X series implementation block diagram

Performance comparison of various digital isolation devices

Table 1 summarizes and compares the performance indicators of various digital isolation devices for reference by R&D designers when designing products. As long as the isolation devices of different companies have the same number of channels, they all use the same package and are pin compatible. Only some pin definitions are slightly different, and in most cases they can be replaced with each other. Product designers can choose products from different companies according to specific needs, or replace them during debugging, leaving more room for choice in product design.

Application Examples

ADuM1100 Applications

When designing a portable product, the transceiver module needs to provide a 0, 1 level state line and a 40MHz square wave clock to the signal processing module. The system requires the signal processing module to isolate and receive these two signal lines. In the initial design, the photoelectric isolator HCPL-063 of Avago was selected to achieve the isolation of the state line, and the IL710 of NVE was selected to achieve the isolation of the clock. When improving the design, the circuit board remains unchanged, and the DC correction function of the ADuM series isolation devices of ADI is fully utilized. The HCPL-063 and IL710 are directly replaced by ADuM1100, and the experimental results show good performance. Of course, TI's ISO721 can also be used as a substitute. If the circuit board is redesigned, dual-channel isolation devices with DC correction function, such as ADI's ADuM1200 or TI's ISO7220, can be selected to replace the single-channel HCPL-063 and IL710 devices.

ADuM5241 Applications

The control module of a communication device needs to communicate with external devices through the RS-422 asynchronous serial port. The system requires that the asynchronous serial port in the control module of the communication device needs to be isolated from other circuits. At the beginning of the design, TI's isolated power conversion chip DCR01 was selected to convert the internal +5V power supply into a single +5V to power the isolation chip IL712 and the level conversion chip RS422. The current of the power supply required for isolation is about 5mA. When improving the design, ADI's ADuM5241, in addition to realizing the isolation of digital signals, also has a built-in DC/DC converter, which can provide power to the isolation end of the chip and provide up to 10mA of current to various applications using 5V power supply. Therefore, the ADuM5241 device is used to replace the DCR01 and IL712 devices, achieving the same function while saving space, reducing devices and reducing costs.

Conclusion

With the rapid development of digital circuits and communication industries, digital isolation devices have been widely used. This article introduces the working principles, advantages and disadvantages of the latest digital isolation technologies and devices. Circuit designers can choose appropriate digital isolation devices according to the specific circuit characteristics.

References

　　1. Chris Sterzik. Principles of digital isolation technology and new capacitive coupled digital isolators. TI

　　2. Scott Wayne. iCoupler Digital Isolators Protect RS-232, RS-484, and CAN Buses in Industrial, Instrumentation, and Computer Applications

　　3. Avago Corporation. HCPL-0723 Data Sheet. AV01-0566EN.pdf, September 2007

　　4. Avago Corporation. HCPL-0900 Data Sheet. AV02-0137EN.pdf, May 2007

　　5. Analog Devices. ADuM1100 Data Sheet. ADuM1100.pdf, REV.E

　　6. NVE Corporation. IL710 Data Sheet.IL710.pdf, May 2005

　　7. TI. ISO72X Manual.SLLS629B.pdf, May 2006