The widespread popularity of general-purpose personal computers (PCs) is reshaping the way medical systems are built. The core of these medical systems is the PC, which is configured with specialized software and functions customized for medical applications. This method reduces the cost and shortens the development time, because the price of many PC components is already very low. This technology also enables better interoperability with other systems (such as service technician laptops) and peripherals (such as printers, keyboards, and mice). However, due to the lack of a direct, cost-effective isolation solution for standard PC interfaces, the application of this technology can sometimes be hindered. This is ADI's single-chip USB isolator, which can work with a 5V USB power supply or a 3.3V power supply provided by the system using an internal voltage regulator. RS-232 is a PC communication port that is easy to isolate, but is being phased out, giving way to a more robust and higher-speed interface USB, and there are already quite a lot of peripherals with this interface. However, this interface is different from RS-232, which is difficult to isolate because it is a differential and bidirectional interface. Until recently, USB isolation required the use of multiple USB controllers, isolators, and other components, which not only increased costs, but also extended development time. Now, new USB isolation technology has emerged, which can integrate all the functions required for USB in isolated medical devices without additional components, and can be directly inserted into the USB signal path without modifying the host or peripheral software in. Isolated interface Medical systems use isolation to protect the safety of operators, patients, or the system itself. Isolation can also isolate the noise generated by a component of the system from another component that is more sensitive to noise. In applications requiring safety, isolation devices are subject to standards set by organizations such as UL and IEC, and the applicable standards depend on the specific application. For example, IEC 60601 specifies the safety requirements for medical equipment, while IEC 60950 applies to information technology equipment. Here are some specific terms related to the isolation level or quality of the medical system in the safety standards: Isolation rating. The isolation rating is usually specified as an AC voltage, which refers to the instantaneous overvoltage that the isolator can withstand. The typical value is 2.5kV rms, 1 minute, but medical systems with higher isolation requirements may be specified as 5kV rms, 1 minute. Operating Voltage. Working voltage refers to the voltage continuously applied to the isolation. Like the isolation rating, the operating voltage is usually specified as an AC voltage, but the isolation barrier needs to be able to withstand this voltage during the entire working life. Typical operating voltage is about 400V rms. Enhanced isolation. Reinforced isolation is usually a requirement put forward by the medical system, and its specified isolation value is equivalent to the isolation of two independent systems. This equivalence needs to be determined by ensuring that the isolation barrier can withstand short-term continuous surge voltages, such as 10 kV. Reinforced isolation is commonly found in IEC standards, such as IEC 60601-1 for medical applications. Creepage distance. Creepage distance refers to the shortest distance along the package surface between two conductors on both sides of the isolation barrier. Void. The air gap refers to the shortest air distance between two conductors. The creepage distance and clearance required for a specific application depend on many factors, including safety standards, isolation type (basic / single or reinforced / double), operating voltage, etc. Medical equipment related to patient safety generally requires enhanced isolation, the working voltage is 125V rms or 250V rms, and the creepage distance and clearance are at least 8mm. The isolation level depends on how the system is divided. Figure 1 shows the block diagram of a general medical device with various interfaces and the locations where isolation can be achieved. The patient must be isolated from the main system, so at point B, C, or D, the patient is required to be safely isolated. In many cases, point D does not need to be isolated, because sensors or other devices must be directly connected to the patient. In other cases, such as ultrasonic equipment, the isolation at point D is provided by the plastic housing of the sensor head. The information at point C is still in the analog domain, so it is uneconomical to isolate and maintain accuracy. In this way, the isolation in medical equipment is usually implemented at point B, but this will leave the operator and peripherals in an unprotected state, so isolation at other interfaces is also required. Figure 1: A block diagram of a general medical device, which indicates the location of the interface where isolation can be implemented. Medical safety standards allow two types of isolation: patient protection (MOPP) and operator protection (MOOP). MOPP complies with IEC 60601 regulations, while MOOP complies with less stringent regulations, such as IEC 60950. In the above example, the system can be classified as requiring interface B to pass IEC 60601 certification, while interfaces A, E, F, and G may only require IEC 60950 certification. In order to ensure the highest level of safety in some medical systems, all interfaces are required to comply with IEC 60601, because these systems may allow patients to access peripheral devices. In addition, the part of the system that is connected to the patient may be regarded as a peripheral device that will be connected to any of the E, F, and G interfaces shown in the figure. IEC 60601 also specifies the safety when using high-voltage defibrillators. As long as any equipment connected to the patient is not certified to IEC 60601, it must be removed during defibrillation, regardless of whether there is time to do so. The adoption of USB The internal interfaces of the system, such as points A, B, and C in Figure 1, are usually UART, SPI, and I2C, depending on cost, performance, and size requirements. The system architect also chooses external connection ports based on interoperability. In the past, PC systems relied on RS-232 serial communication. However, RS-232 is becoming fewer and fewer on PCs, especially notebooks, and the number of peripherals with RS-232 is also rapidly decreasing. In contrast, USB has grown rapidly, partly because of the rapid popularity of the USB interface and the support of a large number of peripherals. The plug-and-play nature of USB also reduces development costs and the need for special software. In medical equipment, the use of USB is not limited to professionally trained operators, patients can also use USB devices at home to download data to USB storage, and then take it to the hospital for doctors to use. USB can also be used to connect sensors or other measuring devices to the host system. One of the advantages of USB is that it allows up to 127 devices to work on a bus, so even if there is only one USB port, multiple peripherals can be used. In contrast, the RS-232 serial communication port can only handle one device. USB isolation In short, USB has some obvious advantages over RS-232, including: Can be expanded to 127 peripherals. Plug and play operation Hot swap capability High data rate (1.5Mbps, 12Mbps and 480Mbps).
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