Simplify design and ensure system reliability with digital isolators

　　Measurement devices used in industrial environments often require isolation to ensure user and system safety, as well as to ensure accurate measurements in the presence of high common-mode voltages. Digital isolators provide a reliable, easy-to-use alternative to older technologies such as optocouplers. Using digital isolators, engineers can optimize isolation system designs to reduce power consumption and maintain system performance without resorting to additional design margins to compensate for missing or incomplete device specifications.

　　

　　Introduction

　　Designing isolated measurement instruments can be challenging and sometimes frustrating. An isolated front end protects the user from potentially lethal voltages present in the measurement system while allowing engineers to make accurate measurements in the presence of high common-mode voltages. A typical example of this type of measurement is shown in Figure 1. In a high-voltage fuel cell or battery stack, knowing the voltage of an individual cell helps ensure safe system operation while maximizing battery life. When determining the voltage of an individual cell, measurements must be made in the presence of common-mode voltages up to hundreds of volts. A similar situation occurs when measuring the temperature of a current-carrying conductor with a thermocouple. In this case, the system must be able to measure signals with millivolt resolution while rejecting high levels of 60 Hz common-mode noise and protecting the operator from any hazardous voltages. Initially, this problem was addressed with isolation amplifiers, but as measurement bandwidths and resolutions have grown, this solution has become obsolete. Today, the most accurate, cost-effective, and efficient technique for performing this type of measurement is to isolate the entire measurement front end, including the analog-to-digital converter (ADC), and implement an isolated serial link to the rest of the system, as shown in Figure 1. The link can be a local bus such as SPI or an industrial protocol such as RS-485 to transmit the measurement data over long distances to the controller unit.

　　

　　Reliability Design

　　Until about 10 years ago, optocouplers were one of the few viable solutions for isolating digital signals. However, ask any engineer who has had to design with optocouplers and you will learn how difficult it is to develop efficient, reliable systems with optocouplers, especially when costs need to be kept to a minimum. Optocouplers use LEDs to generate light across an isolation barrier to turn a phototransistor on and off. When designing with optocouplers, you must ensure that the LED produces enough light to turn on the receiving phototransistor, while also ensuring that the output rise and fall times are fast enough to support operation at the target frequency. One of the most important specifications for an optocoupler is the current transfer ratio (CTR). The CTR is the ratio of the collector current appearing at the phototransistor to the current through the LED.

　　Optocoupler CTR not only has a very wide tolerance, but also degrades over time and temperature. To ensure that optocouplers will continue to operate after years of use at high temperatures, engineers must assume worst-case CTR, which is challenging in itself because optocoupler data sheets only list CTR specifications at room temperature. For example, the specification sheet for a typical optocoupler lists a guaranteed CTR of 50%–600% at 25°C. In addition, most data sheets include a typical graph showing that the CTR at 80°C is only about 50% of the CTR at 20°C. In fact, no data sheet will list the minimum CTR at 85°C, so you must make an assumption about that value. In addition, some studies have simulated the degradation of CTR over time, but this specification is also not listed in the data sheet, so you must decide how much additional design margin to add to ensure that the end product will operate reliably over the expected lifetime. Designing a robust isolator circuit means that you must make many engineering assumptions and need to trade off increased power consumption and reduced operating speed to leave enough margin for reliable operation over the product's lifetime.

　　Digital isolators use non-optical means to send data across the isolation barrier. For example, Analog Devices isolators use micro-transformer technology to send pulses across the isolation barrier without the time and temperature degradation effects associated with optocouplers. This allows guaranteed minimum and maximum power consumption, propagation delay, and pulse distortion specifications to be published over the entire operating temperature range of the device. Having complete specifications eliminates the need to extensively characterize optocouplers under your operating conditions and allows you to use the data sheet data to calculate worst-case system performance. Simply look at the guaranteed propagation delay, skew, and power consumption of a digital isolator and use that data to calculate top-level system timing specifications, just like any standard digital integrated circuit. Other non-optical technologies such as capacitive, radio frequency (RF), and giant magnetoresistance (GMR) coupling are also available. [page]

　　

　　Because magnetic digital isolators consume most of their power when switching from one state to another, power consumption scales with operating frequency. Therefore, channels that are idle or switching very slowly consume very little power. Once the maximum serial clock rate for the application has been determined, the power supply can be designed to provide sufficient current to support that rate. When designing with optocouplers, it is important to ensure that the circuit is always idle when the LED is off to minimize power consumption.

　　Optocoupler technology has been on the market for more than 30 years; some engineers are wary of moving to new isolator technologies. Most manufacturers submit their products for regulatory approval and clearly demonstrate which standards their isolators pass. Devices such as Analog Devices’ digital isolators use polyimide as an insulator, a material also used in many optocouplers. In some cases, they are tested to the same safety standards as optocouplers, while in other cases, such as VDE V 0884-10, specific standards are developed specifically for digital isolators. For example, Table 1 shows the agency certifications for the ADuM140x family of isolators.

　　Other issues involve the ability of digital isolators to withstand overvoltage surges, as well as their immunity to transients in the form of common-mode voltage and magnetic field disturbances. Fortunately, with the help of polyimide insulation materials, Analog Devices' digital isolators can withstand surges of up to 6 kV for 10 seconds. Magnetic isolators also have excellent common-mode transient immunity (CMTI) relative to other technologies due to the very low parasitic capacitance across the isolation barrier. For example, typical high-speed optocouplers have a CMTI specification of 1 to 10 kV/μs, while magnetic digital isolators can suppress common-mode transients of more than 35 kV/μs.

　　At first glance, concerns about magnetic field interference seem reasonable, since isolators using microtransformers use magnetic fields to transmit pulses across the isolation barrier. One might think that a sufficiently strong field could interfere with the pulses, causing an output error. However, because the radius of the transformer and its air core is very small, only very large magnetic fields or very high frequencies can produce a fault. The maximum allowable currents and frequencies shown in Figure 2 still guarantee that the output of the AD344x isolator will not fail. For example, only currents in excess of 500 A (1 MHz, 5 mm from the device) could trigger a faulty output. Theoretically, the amplitude and frequency combinations required to produce an erroneous output are far beyond the range of most applications.

　　High-speed operation

　　Isolating a serial bus with an optocoupler can be a difficult task when the isolated measurement system uses high sampling rates. The parasitic capacitance of the receiver photodiode limits the speed at which the optocoupler can transmit digital signals. You can increase the speed at which this parasitic capacitance charges by increasing the amount of light from the LED, but doing so will increase power consumption. In addition, few optocouplers offer more than two channels per package in the same direction, and they typically do not include timing specifications related to channel-to-channel matching. While it is logical to assume good matching between optocouplers in the same package, the lack of printed specifications means that you must make engineering assumptions. As is the case with relying on unprinted specifications, most prudent engineers will choose to leave ample design margin and operate at performance far below what the data sheet indicates when using a single optocoupler.

　　

　　Another advantage of using digital isolators is that they are available in 4-channel devices with guaranteed speeds up to 150 Mbps. In addition, all digital isolator manufacturers provide guaranteed channel-to-channel matching specifications in the timing section of the data sheet. For example, Analog Devices’ ADuM344x isolators have a guaranteed channel-to-channel propagation delay mismatch of less than 2 ns over the entire operating temperature range. In practical terms, this means that digital isolators can be used at the speeds listed in the data sheet without having to derate system performance for larger or unknown device-to-device or channel-to-channel skews.

　　

　　integrated

　　Because digital isolator technology is compatible with standard CMOS processes, it is relatively easy to integrate additional functions to simplify system design. For example, a traditional thermocouple measurement device may use multiple optocouplers to implement a low-speed SPI interface and an isolation transformer with a driver and regulator to power the isolated front end. With a digital isolator with integrated isolated power supply (such as the ADuM5401), the entire isolation system becomes a single integrated circuit with four data channels and isolated power supply. This approach improves reliability, saves a lot of board space, and reduces cost compared to using discrete isolators and isolated power supplies.

　　Many instruments have built-in isolated RS-485 ports for remote monitoring or control. A few years ago, implementing such an isolated port required not only isolators for the data lines, but also a transceiver compatible with RS-485 differential signaling and power. Figure 3 shows how a single IC like the ADM2682E can integrate all of these functions into a single package.

　　Summarize

　　In the past, designing isolated measurement equipment was an expensive, difficult, and sometimes frustrating task due to the many technical issues with optocouplers. Advances in digital isolation technology over the past few years have greatly simplified this task. Digital isolation technology offers low cost, high performance, ease of use, and high integration, helping engineers meet development schedules. In addition, regulatory agency certifications and the ability to withstand high interference levels make it a good fit for long-life products common in industrial measurement systems.