Digital isolators simplify design and ensure system reliability
Luis Orozco - July 9, 2012
Designing isolated measurement instruments is challenging and sometimes frustrating. An isolated front end protects users against potentially lethal voltages present on measurement systems and allows engineers to make accurate measurements in the presence of high common mode voltages. Figure 1 shows a typical example of such a measurement.
In a high voltage fuel cell or battery stack, knowing the individual cell voltages helps ensure the system is operating safely and obtains the longest possible battery life. When determining the voltage of a single cell, we must measure it in the presence of up to several hundred volts of common mode voltage.
A similar situation occurs when measuring the temperature of a current-carrying conductor with a thermocouple. In this example, the system must measure millivolt signal resolution while rejecting high levels of common mode 60Hz noise and protecting operators from any hazardous voltages.
Isolation amplifiers were an initial solution to this problem, but have been outdated with the need for measurements with higher bandwidth and resolution. Today, the most accurate, economical, and efficient technique for performing these measurements is to isolate the entire measurement front end, including the Analog-to-Digital Converter (ADC), and to implement an isolated serial link to the rest of the system, as shown in Figure 1. This link can be either a local bus such as SPI, or an industrial protocol like RS-485 to send the measurement data across long distances to a controller unit.
Designing for reliability
Up until about ten years ago, optocouplers were one of the few practical solutions for isolating digital signals. However, ask any engineer who has had to design with them, and you will quickly learn how challenging it is to develop an efficient and reliable system, especially when trying to keep costs to a minimum. Optocouplers use an LED to generate light across an isolation barrier to turn a phototransistor on and off.
When designing with optocouplers, you have to guarantee that the LED will generate enough light to turn on the receiving phototransistor, and that the output rise and fall times will be fast enough to support operation at the desired frequency. One of the most important optocoupler specifications is the current transfer ratio (CTR). The CTR is the ratio of the collector current that appears at the phototransistor to the current through the LED.
Optocoupler CTR not only has a very wide tolerance, it also degrades with time and temperature. In order to guarantee that the optocoupler will continue to operate after several years of service, and at elevated temperatures, engineers have to assume the worst possible CTR, which can be challenging in itself, since optocoupler datasheets list CTR specifications only at room temperature. For example, a typical optocoupler’s specification table lists a guaranteed CTR of 50%-600% at 25°C. In addition, most datasheets include a typical chart showing that the CTR at 80°C is only about 50% of the CTR at 20°C. Virtually no datasheet includes a minimum CTR at 85°C so you have to make an assumption as to what it will be.
Additionally, some studies exist that model CTR degradation with time, but this is another specification you will not find on a datasheet, so you must make a decision as to how much additional design margin to add so the end product will operate reliably for its expected lifetime. Designing a robust isolator circuit means you must make many engineering assumptions, requiring tradeoffs in the form of increased current consumption and slower operating speeds to leave enough margin for reliable operation over the life of the product.
Digital isolators use non-optical means of sending data across the isolation barrier. For example, Analog Devices’ isolators use micro-transformer technology to send pulses across the isolation barrier, and do not suffer from the time and temperature degradation effects associated with optocouplers. This makes it possible to publish guaranteed minimum and maximum power consumption, propagation delays and pulse distortion specifications over the entire operating temperature range of the devices.
Having complete specifications removes the need to perform extensive characterization testing of optocouplers under your operating conditions in favor of using the datasheet information to calculate worst case system performance. You can simply look at the digital isolators’ guaranteed propagation delays, skews, and power consumption, and use the data to calculate your top-level system timing specifications like any standard digital integrated circuit. Other non-optical technologies, such as capacitive, radio frequency (RF) and giant magnetoresistive (GMR) coupling are also available.
Because magnetic digital isolators consume most of their power when switching from one state to the other, power consumption scales with operating frequency. Therefore, channels that are idle, or switch at very low speeds, consume very little power. Once you have decided what the maximum serial clock rate will be for an application, you can design a power supply to provide enough current to support this rate. When designing with optocouplers, you must make sure circuits always idle with the LED in the off state to minimize system power consumption.