Digital isolators offer significant advantages over optocouplers in terms of size, speed, power consumption, ease of use, and reliability.
For many years, designers of industrial, medical and other isolation systems have had limited means of achieving safe isolation. The only reasonable option was optocouplers. Today, digital isolators offer advantages in performance, size, cost, efficiency, and integration. Understanding the characteristics of the three key elements of a digital isolator and their interrelationships is very important for the correct selection of a digital isolator. The three elements are: insulation material, structure and data transmission method.
Designers introduce isolation to meet safety regulations or to reduce ground loop noise. Galvanic isolation ensures that data transmission is not through electrical connections or leakage paths, thus avoiding security risks. However, isolation introduces limitations in terms of latency, power consumption, cost, and size. The goal of digital isolators is to meet safety requirements while minimizing adverse effects.
The traditional isolator-optocoupler will have a very large adverse impact, the power consumption is extremely high, and the data rate is lower than 1 Mbps. While higher efficiency and higher speed optocouplers exist, they also cost more.
Digital isolators were introduced more than 10 years ago to reduce the adverse effects associated with optocouplers. Digital isolators use CMOS-based circuits to achieve significant cost and power savings while greatly increasing data rates. Digital isolators are defined by the above elements. Insulating materials determine their inherent isolation capabilities, and the materials selected must meet safety standards. The structure and data transmission method should be chosen with the aim of overcoming the adverse effects mentioned above. All three elements must work together to balance design goals, but there is one goal that must be achieved without compromise, and that is compliance with safety regulations.
Insulating materials
Digital isolators are manufactured using wafer CMOS processes and are limited to commonly used wafer materials. Non-standard materials can complicate production, resulting in poorer manufacturability and higher costs. Commonly used insulating materials include polymers (such as polyimide PI, which can be spin-coated into thin films) and silicon dioxide (SiO2). Both have well-known insulating properties and have been used in standard semiconductor processes for many years. Polymers are the basis for many optocouplers and have a long history as high voltage insulators.
Quasi-standards typically specify a 1-minute withstand voltage rating (typically 2.5 kV rms to 5 kV rms) and operating voltage (typically 125 V rms to 400 V rms). Some standards will also specify shorter duration, higher voltages (e.g. 10 kV peak for 50 μs) as part of the requirements for reinforced insulation certification. Polymer/polyimide-based isolators provide the best isolation characteristics, as shown in Table 1. Polyimide-based digital isolators are similar to optocouplers and have longer life at typical operating voltages. SiO2-based isolators have relatively weak protection against surges and cannot be used in medical and other applications.
The inherent stress of various films is also different. The stress of polyimide film is lower than that of SiO2 film, and the thickness can be increased as needed. The thickness of the SiO2 film is limited, and thus the isolation capability is also limited; above 15 μm, stress may cause the wafer to crack during processing or delaminate during use. Polyimide-based digital isolators can use isolation layers up to 26 μm thick.
Isolator Structure
Digital isolators use transformers or capacitors to magnetically or capacitively couple data to the other end of the isolation barrier, and optocouplers use LEDs. Light. As shown in Figure 1, a transformer current pulse passes through one coil, forming a small local magnetic field, thereby generating an induced current in the other coil. The current pulses are very short (1 ns), so the average current is very low. The transformer is connected differentially, providing excellent immunity to ***mode transients up to 100 kV/μs (typically around 15 kV/μs for optocouplers).
Magnetic coupling is also less dependent on the distance between transformer coils than capacitive coupling is on the distance between plates, so the insulation layer between transformer coils can be thicker, resulting in higher isolation capabilities. Combined with the low-stress properties of polyimide film, transformers using polyimide can more easily achieve advanced isolation performance than capacitors using SiO2.
The capacitor is a single-ended connection and is more susceptible to ***mode transients. While it is possible to compensate with a differential capacitor pair, this increases size and cost. One of the advantages of a capacitor is that it uses low current to create a coupling electric field. When the data rate is higher (above 25 Mbps), this advantage is quite obvious.
Data transmission method
The optocoupler uses the light emitted by the LED to transmit data to the other end of the isolation barrier: when the LED is on, it indicates a logic high level, and when it is off, it indicates a logic low level. level. When the LED lights up, the optocoupler consumes power; for applications where power consumption is a concern, the optocoupler is not a good choice. Most optocouplers leave signal conditioning at the input and/or output to the designer, which is not necessarily a straightforward task.
Digital isolators use more advanced circuits to encode and decode data, support faster data transfer speeds, and are able to handle complex bidirectional interfaces such as USB and I2C.
One approach is to encode the rising and falling edges as double or single pulses to drive the transformer. (Figure 2) These pulses are decoded on the secondary side as rising or falling edges. This approach consumes 10 to 100 times less power than an optocoupler because, unlike an optocoupler, power does not need to be continuously supplied to the device. Refresh circuitry can be included in the device to update the DC levels periodically.
Another approach is to use an RF modulated signal in much the same way an optocoupler uses light, a logic high signal will cause continuous RF transmission. The power consumption of this method is higher than that of the pulse method because the logic high signal requires continuous power consumption. Differential techniques can also be used to provide *** mode suppression, however, these techniques are best used with differential components such as transformers.
Parameters of digital isolator ADUM4160BRWZ-RL
Digital isolator
Manufacturer Analog Devices Inc.
Series iCoupler?
Packaging? Tape and Reel (TR) ?
Parts Status On Sale
Technology Magnetic Coupling
Type USB
Isolated power supply None
Number of channels 2
Input - input side 1/input side 2 2/2
Channel type bidirectional
Voltage - Isolation 5000Vrms
***mode transient immunity (minimum) 25kV/μs
Data rate 12Mbps
Propagation delay tpLH / tpHL ( Maximum) 70ns, 70ns Pulse Width Distortion (Max) - Rise/Fall Time (Typical) 20ns, 20ns (Max) Voltage - Power Supply 3.1 V ~ 5.5 V Operating Temperature -40°C ~ 105°C
Mounting Type Surface Mount
Package/Case 16-SOIC (0.295", 7.50mm wide)
Supplier Device Package 16-SOIC
Basic Part Number ADUM4160
Featured Product: ADuM4160 Digital Isolator
Standard Packaging: 47
Input - 1 Side/2 Sides: 2/2
Supply voltage: 3.1 V ~ 5.5 V
Data rate: 12Mbps
Output type: Logic
Operating temperature: -40°C ~ 105°C
Number of channels: 2
Voltage - Isolation: 5000Vrms
Propagation delay: 60ns
Package/casing: 16-SOIC (0.295 ", 7.50mm wide)
Packing: pipe fittings