How to Reduce Radiated Emissions of Digital Isolators for Systems with RF

Analog Design Journal Signal Chain How to reduce radiated emissions of digital isolators for systems with RF modules By Koteshwar Rao Applications Lead, Isolation, Interface Products Introduction modules. This could lead to lower RF-module sensitivity, A typical industrial system has a dedicated subsystem to thereby reducing the error-free communication range that enable communication with other systems known as a the modules can support. This article describes methods communication module. Communication modules interact to minimize the impact of digital isolators on the sensitiv- with other systems either through wired media or wire- ity of RF modules so that they can support the expected lessly. Applications using wireless communication include error-free communication range. ® ® human machine interfaces (HMIs) (Wi-Fi /Bluetooth ), Operation of a digital isolator energy meters (Global System for Mobile Communication On-off keying (OOK) is a common circuit architecture [GSM]/ Long-Term Evolution [LTE]) and patient monitor- implemented in high-performance digital isolators to ing (Wi-Fi/Bluetooth). These applications often use a establish data communication over an isolation barrier. For digital isolator for ground-loop isolation in wired communi- example, TI’s ISO7741 high-speed, robust-EMC, reinforced cations, or for human/component safety isolation. digital isolator employs an OOK modulation scheme to Digital isolators employ proprietary modulation tech- transmit digital data across a silicon dioxide-based isola- niques to transmit data across an isolation barrier over a tion barrier. carrier signal. The chosen carrier frequency is typically a Figure 1 is a conceptual block diagram of an OOK-based few hundred megahertz to support high data rates. When digital isolator, ISO7741. Figure 2 shows the correspond- used in the vicinity of wireless radio-frequency (RF) ing modulation scheme. modules, the digital isolator’s carrier and its harmonics may interfere with radio signals from the communication

Figure 1. ISO7741 conceptual block diagram

Transmitter Receiver

OOK EN Modulation TX SiO2-Based TX Signal RX Signal Envelope IN Capacitive RX Conditioning Isolation Conditioning Detection OUT Barrier

Emissions Oscillator Reduction Techniques

Figure 2. Modulation scheme for an OOK digital isolator ( TX IN TX IN

Carrier Signal Carrier Signal Through Through Isolation Barrier Isolation Barrier

RX OUT RX OUT

(a) Default low-state digital isolator, ISO7741F (b) Default high-state digital isolator, ISO7741

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In the OOK-based architecture, an internal clock modu- discernible signal (MDS), which is the smallest signal level lates the incoming digital bit-stream to generate OOK that a radio receiver can detect. In other words, an MDS signaling. One of the input states is represented by the signal level is the smallest signal that can be processed transmission of a carrier signal, and the other state is through the analog and digital signal chain and demodu- represented by no transmission. This modulated signal lated by the receiver to provide usable information at the couples through the isolation barrier and appears in an output. attenuated form on the receive side. The receive path In digital isolators, data transmits through a carrier consists of a preamplifier to gain up the incoming signal, signal across an isolation barrier. Since this is only a signal followed by an envelope detector that serves as a demodu- transmission and not a power transmission, the power lator to regenerate the original digital pattern. The levels of the carrier signal being transmitted will be very ISO7741 also incorporates spread-spectrum modulation low. Therefore, most digital isolators that transmit only techniques to minimize radiated emissions from the high- data do not pose any challenges in meeting standard frequency carrier. emissions requirements for most industrial (like Comité International Spécial des Perturbations Radioélectriques RF modules [CISPR] 22) and automotive (like CISPR 25) applications. An RF module is usually a small electronic device used to RF-module sensitivity is typically very high; its MDS is transmit and/or receive radio signals between two devices. usually expressed in decibel-milliwatts (0 dBm = 1 mW). There are various types of RF modules; some can transmit When a digital isolator and an RF module are used on a up to a few tens of feet and others can transmit as far as a same printed circuit board (PCB) side by side, it is possi- few kilometers. Good electronic radio design is notoriously ble for the isolator’s carrier signal to affect RF module complex, and radio circuits are usually subject to limits on sensitivity, thereby degrading its MDS to some extent. A radiated emissions. Radio circuits also require confor- degraded RF-module MDS may result in a lower range mance testing and certification by standards bodies such than what it can originally support. If the supported range as the Federal Communications Commission (FCC). For is lower than the acceptable range, then the impact of the these reasons, design engineers will often drop a readily isolator on module sensitivity must be minimized. available radio module into their board rather than attempt a discrete design, which saves development time and money. Improving RF-module sensitivity Commercially available RF modules use several carrier There are many ways to minimize the impact of a digital frequencies that include the industrial, scientific and isolator on RF-module sensitivity. A few of the most effec- medical (ISM) radio bands such as 433.92 MHz, 915 MHz tive solutions to improve RF-module sensitivity so that it and 2,400 MHz. The GSM open and digital cellular technol- can support its original communication range include: ogy operates in the 850-MHz, 900-MHz, 1,800-MHz and • Implementing an interlayer PCB stitching 1,900-MHz frequency bands. capacitor. In the TI application report identified by RF modules may comply with a defined protocol for RF Reference 1, Section No. 1 describes how a carrier sig- ® communications such as Zigbee , Bluetooth low energy or nal transmitted across an isolation barrier could create Wi-Fi, or may implement a proprietary protocol. common-mode currents, resulting in radiated emissions. Performance impact on sensitivity of RF module To address potential interference with any RF modules in the vicinity, a capacitor can be used across the isola- RF sensitivity is one of the key specifications of any radio tion barrier to shorten the common-mode current loop. receiver, whether it is used for Wi-Fi, cellular telecommu- Section No. 4 of the same application report nication broadcasts, or any other form of wireless commu- (Reference 1) describes how a high-voltage Y2 capacitor nication. A radio receiver’s ability to pick up the required can reduce digital-isolator emissions. It also covers the level of radio signals enables it to operate more effectively capacitor’s limitations because of its lead inductances within its application. and how an interlayer PCB stitching capacitor over- One of the parameters used to represent the receiver comes such limitations by having zero lead inductance. sensitivity of an RF module is the minimum detectable or

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The report also explains in detail the design and imple- The data modulation technique can be implemented mentation of an interlayer PCB stitching capacitor on a in software if the communication across the isolation multilayer PCB. Figure 3 shows a cross-section of an barrier is happening between two microcontrollers interlayer PCB stitching capacitor. (MCUs) or programmable devices. This approach • Data modulation. As previously described, one of the doesn’t use any external components to reduce radiated input states of a digital isolator is represented by the emissions by tenfold or more. Thus, it is one of easiest transmission of a carrier signal, while the other input and most efficient approaches to reduce emissions and state is represented by no transmission. In Figure 2a for improve sensitivity. the ISO7741F, a logic 1 input transmits a carrier signal Alternatively, data modulation can be implemented across the isolation barrier, while a logic 0 (zero) input in hardware by using external components without doesn’t transmit any carrier signal. For a fixed data-rate affecting the system software. The key components application, if the input logic 1 data bit can be repre- required for implementing data modulation are an oscil- sented by a pattern that has a 10% high state followed lator with an adjustable duty cycle to generate a modu- by a 90% low state constituting one data bit, the amount lated data pattern and an envelope detector to recover of time a carrier signal transmits across the barrier will the original data stream. This approach is useful for be significantly reduced, as shown in Figure 4a. Once it applications where the software is large and sophisti- passes through the barrier, such a data pattern is easily cated and involves critical system interrupts with the recoverable to get back the original data bit. This highest execution priority. approach reduces radiated emissions tenfold and pro- • Choosing an isolator with the right default state. portionally improves RF-module sensitivity. It’s even Many applications use isolators on various communica- possible to reduce emission further by reducing the tion interfaces/protocols, including Serial Peripheral high-state duration to values lower than 10%. Interface (SPI), Inter-Integrated Circuit (I2C), RS-485 In the ISO7741, a logic 0 input transmits a carrier and Controller Area Network (CAN). Many of these signal across the isolation barrier and a logic 1 input applications require intermittent data transmission in doesn’t transmit any carrier signal, as shown in Figure 2b. the form of packets. There are long periods of time Emissions can be reduced by following the approach between data-packet transmissions when the communi- shown in Figure 4b. cation bus is idle. The communication interface may

Figure 3. Cross-sectional view of an interlayer PCB stitching capacitor

Top Layer 0.51 mm GND

VCC 6.57 mm 0.35 mm

Bottom 0.51 mm

K EO D L W C (pF) –12 4.2 8.85 x 10 0.35 77.3 6.5 53.38

Figure 4. Data modulation approach to reduce radiated emissions (

Input Data 0 1 0 0 1 Input Data 0 1 0 0 1

Modulated Modulated Data Data

(a) Default low-state digital isolator, ISO7741F (b) Default high-state digital isolator, ISO7741

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require the bus to be in a particular state (either high, Conclusion low or custom) during this idle state. Various applications for industrial, automotive and Choosing an isolator whose default state matches personal electronics use digital isolators for ground-loop the communication bus idle state will prevent the isola- isolation, safety isolation or high-side driving. Many appli- tor from contributing any radiated emissions or distur- cations also include an RF module for wirelessly communi- bances. The correct choice will leave the sensitivity of cating with other systems. RF modules, being very RF modules unaffected, especially during communica- sensitive to high-frequency radiations, are prone to distur- tion bus idle time. Along with data-packet transmission, bance by the very-low radiated emissions of a carrier careful isolator selection may be sufficient to meet the signal in digital isolators, thus affecting their sensitivity required RF module sensitivity for an application. and supported range. Test results The various approaches described in this article can minimize the impact of a carrier signal in digital isolators Various methods discussed here were tested with the on RF-module sensitivity and restore the supported range. ISO7741EVM digital-isolator evaluation module that was placed close to a high-sensitivity RF receiver. Table 1 lists References the results of these various emission-reduction techniques, 1. Anand Reghunathan, Koteshwar Rao, Anant Kamath, which show that it is possible to completely recover any “Low Emissions Designs With ISOW7841 Integrated degradation of the receiver’s RF sensitivity that was Signal and Power Isolator,” Texas Instruments caused by a digital isolator. Application Report (SLLA368A), August 2017. Table 1. Summary of RF-module sensitivity tests with various 2. “Digital Isolator Design Guide,” Texas Instruments improvement techniques Developer’s Guide (SLLA284A), November 2014. Test Condition MDS (dBm) Degradation (dB) Without isolator –124 0 Related Web sites Product tool: Isolator with low –110 14 inputs ISO7741 evaluation module Product information: Isolator with high –124 0 inputs ISO7741 ISO7762 With stitching –124 0 capacitor, 50 pF ISO7841 ISO7821LLS With 10% duty cycle –124 0 ISO1211 ISOW7841

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