![]() Optocoupler CTR not only has a very wide tolerance, it also degrades with time and temperature. ![]() The CTR is the ratio of the collector current that appears at the phototransistor to the current through the LED. One of the most important optocoupler specifications is the current transfer ratio (CTR). 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. Optocouplers use an LED to generate light across an isolation barrier to turn a phototransistor on and off. ![]() 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. Up until about ten years ago, optocouplers were one of the few practical solutions for isolating digital signals. 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. 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. Isolation amplifiers were an initial solution to this problem, but have been outdated with the need for measurements with higher bandwidth and resolution. Using an Isolated Front End to Measure the Voltage of a Single Cell in a High Voltage Stack. In this example, the system must measure millivolt signal resolution while rejecting high levels of common-mode 60 Hz noise and protecting operators from any hazardous voltages.įigure 1. A similar situation occurs when measuring the temperature of a current-carrying conductor with a thermocouple. 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. 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. Figure 1 shows a typical example of such a measurement. 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. Introductionĭesigning isolated measurement instruments is challenging and sometimes frustrating. Utilizing digital isolators, engineers can optimize isolated system designs for reduced power consumption and guaranteed system performance without resorting to excessive design margin to make up for missing or incomplete device specifications. Digital isolators offer a reliable and easy to use alternative to older technologies such as optocouplers. Measurement devices used in industrial environments often require isolation for user and system safety, and to ensure accurate measurements in the presence of high common-mode voltages. Digital Isolators Simplify Design and Ensure System Reliability
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