Application of USB Isolation Technology in Medical Systems

The widespread availability of general-purpose personal computers (PCs) is reshaping the way medical systems are built. At the heart of these medical systems is a PC that has been configured with specialized software and features tailored for medical applications. This approach reduces costs and development time because many PC components are already inexpensive. This technology also enables better interoperability with other systems (such as a service technician's laptop) and peripherals (such as printers, keyboards, and mice). However, the lack of direct, cost-effective isolation solutions for standard PC interfaces has sometimes hindered the adoption of this technology.

This is a single-chip USB isolator from Analog Devices that can operate from a 5V USB power supply or a system-supplied 3.3V power supply using an internal regulator.

RS-232 is an easily isolated PC communications port, but it is being phased out in favor of a more robust, higher-speed interface, USB, and there are already a large number of peripherals with this interface. However, unlike RS-232, this interface is difficult to isolate because it is differential and bidirectional. 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 that integrates all the functions required to isolate USB in medical devices, without the need for additional components and can be inserted directly into the USB signal path without modifying the host or peripheral software.

Isolated interface

Medical systems use isolation to protect operators, patients, or the system itself. Isolation can also isolate noise generated by one part of the system from another part that is more sensitive to noise. Where safety is required, isolation components are subject to standards set by organizations such as UL and IEC, and the applicable standard depends on the specific application. For example, IEC 60601 specifies safety requirements for medical equipment, while IEC 60950 applies to information technology equipment.

The following are some specific terms used in safety standards that relate to the level or quality of isolation in medical systems:

Isolation rating. Isolation rating is usually specified as an AC voltage and refers to the transient overvoltage that the isolator can withstand. A typical value is 2.5kV rms for 1 minute, but medical systems with higher isolation requirements may specify 5kV rms for 1 minute.

Working voltage. Working voltage is the voltage that is continuously applied to the isolation. Like the isolation rating, the working voltage is usually specified as an AC voltage, but the isolation barrier needs to be able to withstand this voltage throughout its operating life. The typical value of the working voltage is about 400V rms.

Reinforced isolation. Reinforced isolation is often a requirement for medical systems and specifies an isolation value equivalent to the isolation of two independent systems. This equivalence is determined by ensuring that the isolation barrier can withstand a short-duration, continuous surge voltage of, for example, 10 kV. Reinforced isolation is commonly found in IEC standards such as IEC 60601-1 for medical applications.

Creepage distance. Creepage distance is the shortest distance along the package surface between two conductors on either side of the isolation barrier.

Clearance. Clearance is the shortest air distance between two conductors. The creepage distance and clearance required for a specific application depends on many factors, including safety standards, isolation type (basic/single or reinforced/double), working voltage, etc.

Medical equipment related to patient safety generally requires reinforced isolation with a working voltage of 125V rms or 250V rms and a creepage and clearance of at least 8mm.

The level of isolation depends on how the system is partitioned. Figure 1 shows a general medical device block diagram with various interfaces and indicates where isolation can be implemented. The patient must be isolated from the main system, so patient safety isolation is required at points B, C, or D. In many cases, isolation is not required at point D because the sensor or other equipment must be connected directly to the patient. In other cases, such as ultrasound 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 not economical to isolate and maintain accuracy. As a result, isolation in medical equipment is usually implemented at point B, but this leaves the operator and peripherals unprotected, so isolation is also required at other interfaces.

Figure 1: Block diagram of a typical medical device showing where interfaces can be isolated.

Medical safety standards allow for 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 divided to require interface B to be certified to IEC 60601, while interfaces A, E, F, and G may only require IEC 60950 certification.

Some medical systems require compliance with IEC 60601 on all interfaces to ensure the highest level of safety, as these systems may allow patients to come into contact with peripheral devices. In addition, the part of the system that connects to the patient may be considered a peripheral device and connected to any of the E, F, and G interfaces shown in the figure. IEC 60601 also specifies safety when using high-voltage defibrillators. Any device connected to the patient that is not IEC 60601 certified must be removed during defibrillation, regardless of whether there is time to do so.

USB adoption

The system internal interfaces, such as points A, B, and C in Figure 1, are usually UART, SPI, and I2C, depending on cost, performance, and size requirements. System architects also need to select external connection ports based on interoperability. In the past, PC systems relied on RS-232 serial communication. However, RS-232 is becoming less and less common on PCs, especially notebooks, and the number of peripherals with RS-232 is also decreasing dramatically.

In contrast, USB has grown rapidly, partly due to the rapid popularity of the USB interface and the support of a large number of peripherals. The plug-and-play nature of USB can also reduce development costs and the need for specialized 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 memory and then take it to the hospital for doctors to use when diagnosing. 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, an RS-232 serial communication port can only handle one device.

USB Isolation

In summary, USB has some distinct advantages over RS-232, including:

Expandable to 127 peripherals.

Plug and Play Operation

Hot-swap capability

High data rates (1.5Mbps, 12Mbps, and 480Mbps).

Compatible with industry standards

Widely used on PC

Despite these advantages, USB has not been adopted as quickly in medical systems as it has been in other consumer applications. What differentiates the medical field from other fields is the isolation requirements. Although USB has many advantages over RS-232, isolating a USB interface has proven to be not as simple as isolating other interfaces.

USB is difficult to isolate because it is a differential, bidirectional interface and requires configuration (via pull-up and pull-down) resistors to indicate bus speed. The bidirectional nature alone is challenging because there must be some way to determine the direction of data transfer. In an isolated USB interface, this information must be able to cross the isolation barrier. Control flow is determined by the data structure, not the control signals.

The USB interface consists of the following 4 wires:

VDD

D+

D–

VSS

VDD is the 5V power supply, VSS is the reference ground, and D+ and D- are differential signals. To make things more complicated, D+ and D- can also be used to send single-ended data and can be used to determine the state of the bus. Pull-up and pull-down resistors on the peripheral side of the bus are used to set the speed and idle state of the USB interface. By definition, data can be transmitted at one of the following three rates:

1.5 Mbps (low speed).

12 Mbps (full speed).

480 Mbps (High Speed).

The USB 2.0 standard supports all three data rates (USB 1.1 only supports low-speed and full-speed data rates). It is worth noting that a device can be called USB 2.0 compliant even if it does not support 480Mbps.

Because standard optocouplers are inherently unidirectional devices, isolated interfaces using optocouplers or other unidirectional isolators must first convert the USB signals into a set of unidirectional signals, as shown in Figure 2 (EEPROMS are not shown, but are often used to store the code for signal conversion). In this example, the D+/D- lines from the microcontroller are converted into single-ended, unidirectional SPI signals. These signals are isolated and then converted back to USB signals using a USB serial interface engine or USB controller. This controller adds multiple components and more traces than a simple two-wire bus. This approach is not only expensive, it takes up considerable board space, and requires additional design time, in part because the microcontroller requires software configuration. The complexity of this implementation has been a major reason for the slow adoption of USB by medical system architects.

Figure 2: Alternative methods of isolating a USB interface.

The figure above shows the configuration method of using a microcontroller and a serial interface engine to convert the D+/D- signal into a unidirectional single-ended SPI. The figure on the right shows a simpler method where the ADuM4160 USB isolator can be inserted into the D+/D- signal path without the need for additional signal conversion components.

Single package USB isolation

A simpler, more cost- and area-effective approach to USB isolation is to use a dedicated USB isolator that can be inserted directly into the D+/D- USB signal path. Such isolation technology exists today, providing reinforced isolation up to 5kV rms and supporting both low-speed and full-speed data rates.

Unlike optocouplers, which use LEDs and phototransistors to transmit data across an isolation barrier via light, isolators based on newer technology use planar transformers to transmit data across a 20μm thick polyimide insulation layer that can withstand 6kV rms. Data transmission is accomplished by induction from one coil to another. Figure 3 shows the structure of such a transformer. Figure 4 illustrates how the rising and falling edges of the data stream are encoded as two or single 1ns pulses, respectively. These pulses are decoded on the receiver side to recover the transmitted data.

Figure 3: Schematic diagram of a planar transformer.

The coils are isolated by a 20μm thick polyimide insulation layer with an isolation rating of up to 6kV rms for 1 minute.

Figure 4: Method for sending data across the barrier.

The rising and falling edges are encoded as double and single pulses respectively. The receiver decodes these pulses to regenerate the data stream on the other side of the barrier. The refresh circuit constantly retransmits the data every 1ms to ensure the DC voltage is correct.

Benefits of Isolated USB

Using a dedicated USB isolator in a single package has many benefits over optocouplers. Using a transformer allows data to be transmitted in both directions across the isolation. While this technique uses dedicated transformers for both transmit and receive signals, all coils are identical and contained within a single package. This approach is not possible with optocouplers. A similar setup using optocouplers would require separate devices to handle each direction of communication.

The transformer is inherently faster than the LED-phototransistor combination used in the optocoupler, allowing the isolator to support the faster data rates and shorter propagation delays required by USB. At the same time, the isolator consumes less power, meeting the stringent standby power requirements of USB.

The key benefit of this isolation technology is the ability to integrate additional functionality into the isolator product. The space savings from this integration are shown in Figure 2, where the board area occupied by the USB isolator can be reduced by 75% compared to a multi-IC configuration of a USB transceiver plus optocoupler.

With this cost-effective and easily implemented USB isolation technology, medical devices can take full advantage of USB. For example, in medical systems, an isolated USB port on a home patient monitor can provide real-time connection between patients at home and doctors in the hospital, providing better and more accurate health care. With isolated USB, this home patient monitor can be connected to a PC and then transmit data to the hospital in real time via the Internet. As long as it is certified to IEC 60601 medical grade safety, a system with isolated USB can even maintain connection with the patient during defibrillation.

Conclusion

The widespread use of USB has created a challenge for medical system architects who want to take full advantage of USB. Isolating USB in these systems can be difficult and expensive without compromising the increased functionality and ultimate cost benefits of using USB. Fortunately, a new class of USB isolators has been developed to address this problem. This technology can directly isolate the differential, bidirectional D+/D- USB signal lines.