Isolating the USB in Medical Systems

Note: Technologies like those discussed in this article and others related to medical electronics design will be the central focus of the DesignMED Conference [7], February 8-10, 2011, in Anaheaim, Ca.

 

The widespread adoption of general-purpose personal computers (PCs) is reshaping how medical systems are built. At their core, these systems are PCs that have been configured with specialized software and features tailored to the application. This approach reduces costs and development time because many PC components are readily available at commodity prices. It also allows for greater interoperability with other systems (e.g., a service technician's laptop) and peripherals (e.g., printers, keyboards, and mouses). However, adoption can sometimes be hindered by the lack of a straightforward, cost-effective isolation option for standard PC interfaces.

The communications (RS-232) port, a PC interface that is easy to isolate, is steadily being phased out in favor of USB—a more robust, higher-speed interface for which considerably more peripherals are available. Unlike RS-232, however, USB is difficult to isolate because it is differential and bidirectional. Until recently, isolating USB required the use of multiple USB controllers, isolators, and other components, which added cost and delayed development. Now there is USB isolation technology that integrates all the functionality needed to isolate USB in medical devices without the need for additional components; it can be inserted directly in the USB signal path without modification of host or peripheral software.

Isolated Interfaces

Medical systems use isolation to protect the operator, patient, or system itself. Isolation also separates noise from one part of the system from another, more sensitive part of the system. Where required for safety, isolation devices are governed by standards from groups such as UL and IEC; the appropriate standards are determined by the application. For example, IEC 60601 dictates safety requirements for medical devices whereas IEC 60950 governs information technology equipment.

Within safety standards, there are certain terms related to the level or quality of isolation for medical systems, as follows.

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

Working Voltage. Working voltage refers to the continuous voltage applied across the isolation barrier. As with isolation rating, working voltage is typically specified as an ac voltage, but the isolation barrier is expected to withstand this voltage over its operational life. Typical values are about 400 V rms.

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

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

Clearance. Clearance is the shortest distance through the air between two conductors. The required creepage and clearance for a given application depend on a number of factors including the safety standard, type of isolation (basic/single versus reinforced/double), working voltage, etc.

Medical applications that relate to patient safety typically require reinforced isolation with a working voltage of 125 V rms or 250 V rms, and creepage and clearance of at least 8 mm.

The level of isolation is determined by how the system is partitioned. Figure 1 is a block diagram for a generic medical device with various interfaces noted to show where isolation could be implemented. The patient must be isolated from the main system, so isolation for patient safety is required at points B, C, or D. In many cases, D is not an option because a sensor or other device must be connected directly to the patient. In other cases, such as in ultrasound equipment, isolation at point D is provided by the plastic casing of the sensor head. The information at point C is still in the analog domain, so it is not cost-effective to isolate here while maintaining accuracy. Therefore, isolation in medical equipment is often implemented at point B. That leaves the operator and peripherals unprotected, so isolation may also be used at the other interfaces as well.

Medical safety standards allow for two types of isolation: means of patient protection (MOPP) and means of operator protection (MOOP). MOPP is governed by IEC 60601, whereas MOOP may be governed by less-stringent requirements such as IEC 60950. In the example above, the system may be partitioned so that interface B requires IEC 60601 certification whereas interfaces A, E, F, and G may require only IEC 60950.

Some medical systems ensure the highest level safety by complying with IEC 60601 at all interfaces, because these systems may allow for patients to come into contact with peripheral devices. In addition, the portion of the system connected to the patient may be considered a peripheral and connected to any one of the same ports as shown at interfaces E, F, and G. IEC 60601 also provides safety during the use of highly charged defibrillators. Without IEC 60601 certification, anything connected to a patient must be removed during defibrillation, precisely when there is no time to do so.

USB Adoption

Intrasystem interfaces, such as those at A, B, and C in Figure 1, are typically UART, SPI, and I2C depending on cost, performance, and size. In contrast, system architects also choose interfaces for external connections based on interoperability. In the past, PC-based systems have relied on serial communications through the RS-232 interface. But RS-232 ports on PCs, especially laptops, are becoming scarce, and the number of available peripherals has declined.

USB, by contrast, is on the rise due in part to its widespread availability and the large number of peripherals available. The plug-and-play nature of USB also reduces development overhead and the need for special dedicated software. In medical equipment, USB's use is not exclusive to trained operators; a patient can use a device at home to download data to a USB memory drive to bring to a hospital for diagnosis. USB can also be used to connect a sensor or other measurement device to a main system. One advantage of USB is that it allows for up to 127 devices to operate on a single bus, so multiple peripherals can be used even when there is only a single USB port. In contrast, the RS-232 serial communications port can handle only one device.

Isolating USB

Overall, USB has some considerable advantages over RS-232, including the following:

• Expandable to 127 peripherals.

• Plug-and-play operation.

• Hot-swap capability.

• High data rates (1.5 Mb/sec, 12 Mb/sec, and 480 Mb/sec).

• Industry-standard compatibility.

• Widespread use and availability on PCs.

Despite these advantages, adoption of USB in medical systems has not been as rapid as it has been in other consumer applications. What distinguishes the medical segment from other areas is the need for isolation. Despite the many advantages of USB over RS-232, it turns out that isolating USB interfaces is not as straightforward as isolating other interfaces.

USB is difficult to isolate because it is differential, bidirectional, and requires configuration (via pull-up and pull-down) resistors to indicate bus speed. The bidirectional nature alone presents a significant challenge because there must be some means to determine the direction of the data transmission. In an isolated USB interface, this information must be passed across the isolation barrier. Flow of control is determined by data structures rather than by control signals.

The USB interface comprises four lines:

• VDD

• D+

• D–

• VSS

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

• 1.5 Mb/sec (low speed).

• 12 Mb/sec (full speed).

• 480 Mb/sec (high speed).

The USB 2.0 standard supports all three data rates (USB 1.1 supports only low- and full-speed data rates). It is important to note that a device can be USB 2.0 compliant without supporting 480 Mb/sec.

Because standard optocouplers are unidirectional by nature, an isolated interface using optocouplers or other unidirectional isolators must first translate the USB signals into a set of unidirectional signals, as shown in Figure 2 on p. 21. (Not shown are the EEPROMS often required to store code used in the signal translation.) Here, the D+/D– lines from a microcontroller are translated into single-ended, unidirectional SPI signals. These signals are isolated and then translated back into USB signals using a USB serial interface engine, or USB controller. Instead of a simple, two-wire bus, this controller adds multiple components and increases the number of wires. The result is expensive, consumes considerable board space, and requires additional design time in part because the microcontroller requires software configuration. The complexity of such of an implementation is the primary reason that medical system architects have been slow to adopt USB.

Single-Package USB Isolation

A simpler, more cost- and area-
effective way to isolate USB is to use a dedicated USB isolator that can be inserted directly into the D+/D– USB signal path. Such isolation technology now exists, and it also provides reinforced isolation of up to 5 kV rms with support for low- and full-speed data rates.

Unlike optocouplers, which use LEDs and phototransistors to transmit data across an isolation barrier via light transmission, isolators based on more recent technologies can utilize planar transformers to transmit data across a 20-µm-thick polyimide insulation layer that can withstand up to 6 kV rms. Data are transmitted by induction from one coil to the other. Figure 3 shows the transformer structure; Figure 4 shows how rising and falling edges of a data stream are encoded as double or single 1-nanosecond pulses, respectively. These pulses are decoded on the receiver side to recreate the transmitted data.

Benefits of Isolated USB

Using a single-package, dedicated USB isolator has a number of benefits compared with optocouplers. The use of transformers allows data to be transmitted in either direction across the isolation barrier. Although this technology uses dedicated transformers for transmit and receive signals, all coils are identical and contained within one package. This cannot be accomplished with optocouplers; a similar setup with an optocoupler would require separate devices to handle each direction of communication.

Transformers are also inherently faster than the LED-phototransistor combination used in optocouplers. This allows isolators to support the higher data rates and shorter propagation delays required by USB. The isolators also consume less power, allowing them to meet USB's stringent standby power requirements.

The most critical advantage of this isolation technology is the ability to integrate additional functionality into isolator products. The space-saving benefits of such integration are shown in Figure 2, where the USB isolator consumes 75% less board space compared with a multi-IC configuration of USB transceivers and optocouplers.

With a cost- and area-effective isolated USB technology that is easy to implement, medical applications could start to take advantage of USB. In medical systems, for example, isolated USB ports on home patient monitors can enable real-time connectivity between at-home patients and doctors to provide better, more accurate care. With isolated USB, such a home patient monitor can be connected to a PC, allowing real-time data transfer to a hospital via the Internet. With IEC 60601 medical-grade safety approval, systems with isolated USB can even remain connected to patients during defibrillation.

Conclusion

The increasing use of USB presents a challenge to architects of medical systems who wish to take advantage of its benefits. It can be difficult and expensive to isolate USB in these systems, yet the functional performance and eventual cost benefits of using USB cannot be ignored. Fortunately, a new class of USB isolators is being developed to solve this problem. Such technology can directly isolate the differential, bidirectional D+ and D– USB signal lines.

David Krakauer is product line manager of iCoupler isolation products for Analog Devices Inc. (Norwood, MA). He can be reached via e-mail at david.krakauer@analog.com [8]. ■