Isolator vs. Optocoupler Technology 1.0. Introduction Optocouplers have been the unchallenged signal isolation solution for more than four decades, but digital isolators fabricated in complementary metallic oxide semiconductor (CMOS) process technology are gaining favor in the design community because of their superior performance, reliability and integration. This white paper explains the underlying technologies of digital optocouplers and digital CMOS isolators with side-by-side performance and reliability comparisons. Table of Contents Digital Optocouplers and CMOS Digital Isolator Basics Device Performance Device Reliability Common Mode Transient Immunity (CMTI) Electromagnetic Interference (EMI) Ease of Use Optocoupler Replacement Upgrades Summary 2.0. Digital Optocouplers and CMOS Digital Isolator Basics Digital optocouplers are available in several versions, from 4-pin devices costing pennies to expensive higher-speed devices. The most popular, moderately priced optocouplers are typically 8-pin LED-based devices that require an output side bias voltage and use input-side current to control the output state. Referring to Figure 1A, most optocouplers are composed of a light-emitting diode (LED), photo detector and output driver (usually a transistor or buffer). Current flowing though the LED causes emitted light to strike the photo detector, creating a current flow from VDD to the base of the output device, driving the output low. Conversely, the absence of LED current inhibits current flow to the output device, resulting in output logic high. Figure 1A. Optocoupler Figure 1B. CMOS Digital Isolator Figure 1. Basic Operation of Optocoupler vs. CMOS Digital Isolator Figure 2A. Optocoupler Package X-Ray Figure 2B. Decapsulated 6-Channel CMOS Digital Isolator Figure 2A shows an x-ray view of a single-channel optocoupler where the LED and photo coupler die are attached to a split lead frame separated by a physical gap (distance through insulation [DTI] 80 µM to 1,000 µM gap between the LED and optical receiver die) and a transparent insulating shield. More recent optocoupler packaging also includes the addition of silicone filler (see Figure 3). Optocouplers are fabricated in gallium-arsenide (GaAs) process technologies that are notorious for wide parametric variation over temperature, high input current, relatively short time-dependent device breakdown (TDDB) and inherent wear-out mechanisms. Figure 3. Cross Sectional View of an Optocoupler The basic operating principle of the CMOS digital isolator is somewhat analogous to that of an optocoupler, with the exception that output logic state control is determined by the presence or absence of a high-frequency (HF) carrier instead of light. Figure 2B shows the die configuration and bonding of a 150 Mbps, six-channel CMOS digital isolator. The base isolator die is designed and fabricated such that wire-bonding two identical die together (Figure 2B) forms a transmitter and receiver separated by a differential capacitive isolation barrier. The isolator output state is determined by either an input logic level or an input current (similar to the optocoupler), depending on the part number. The heart of any digital isolator is the isolation barrier that safely withstands the applied high-voltage stress. The optocoupler isolation barrier relies on a combination of a physical gap (i.e., distance through insulator or DTI); polyimide tape, silicon filler and plastic mold compound for insulation (see Figure 3). This hybrid methodology not only makes increased optocoupler integration difficult, but increases fabrication complexity and thus cost and reliability. That said, many international standards still specify isolation barrier requirements based on optocoupler DTI. Fortunately, the standards agencies tests are based on barrier withstand voltage regardless of device implementation. Please see Section 4.0 in this document for a discussion regarding insulation material and its effect on isolation performance. CMOS construction and careful attention to design enable CMOS digital isolators to achieve higher degrees of performance, reliability and ease-of-use, primarily by virtue of these key enabling technologies: Mainstream, low-power CMOS process technology instead of GaAs: CMOS is the highest performance, most reliable and cost-effective process technology available. Devices fabricated in CMOS exhibit superior timing, integration, operating stability and reliability characteristics compared to gallium arsenide processes used in optocouplers. High-frequency carrier modulation instead of light: The combination of the precision, high-frequency carrier and narrow receiver pass band provides tight frequency discrimination for outstanding noise rejection and thus higher data integrity. Fully differential isolation path instead of single-ended: Optocouplers are single-ended devices and are subject to CMT perturbations, whereas the differential signal path and high receiver selectivity of CMOS digital isolators provides high rejection of common-mode transients up to 50 kV/µS, external RF field immunity as high as 300 V/m and magnetic field immunity beyond 1000 A/m. Proprietary EMI design techniques: CMOS digital isolators meet the FCC Part B emission standards for conducted and radiated EMI. For more information on CMOS isolator emissions, susceptibility and reliability vs. optocouplers, see the Silicon Labs white paper, “CMOS Isolators Supersede Optocouplers in Industrial Applications” available at www.silabs.com/isolation. 2.1. Isolator Requirements: Isolator parametric requirements are dictated by end applications, and designers tend to stress the importance of certain isolator parameters over others. For example, isolators operating in noisy industrial environments might cause a designer to prioritize high common-mode transient immunity (CMTI) over low-power operation, whereas a high-precision ADC application might cause the designer to prioritize low jitter over pulse width distortion, and so on. In short, many applications will care most about just a few key isolator parameters, and designers will often verify these parameters independently to ensure they meet the system requirements. A list of the more common isolator parameters and their definitions is shown in Table 1 for reference.