Feature Article

The ubiquitous USB

It is no exaggeration to say that today the personal computer (PC) is a necessary tool of daily life. Huge sales volumes and fierce competition between vendors in the PC market have sharply driven down component prices. This means that other sectors such as medical equipment can take advantage of affordable technologies originally developed or popularised for PCs.

For connecting medical equipment, the familiar universal serial bus (USB) port has started to replace the RS-232 converter as the interface of choice. USB is faster and more robust than RS-232 and a wider range of cost-effective peripherals is available. The “plug and play” nature of USB also reduces development overheads and the need for special dedicated software.

However, USB has one disadvantage: it is more difficult to isolate electrically because it is differential, bidirectional and requires configuration to indicate bus speed and bus control. Until recently, solutions for isolating USB required the use of multiple USB controllers and isolators, with the disadvantage that this approach required additional board space and increased overall cost.

Electrical isolation

Galvanic isolation in medical systems protects the patient, the operator and the system itself from harmful surges or spikes in the electrical signal. In isolated systems, sensitive electronic circuitry is also protected against electrical noise or spikes that may cause damage. Where required for safety, isolation devices are governed by standards from groups such as Underwriters Laboratories and the International Electrotechnical Commission. These standards set out the terms and definitions related to the level or quality of isolation, as follows:

• Isolation rating, which is the transient over voltage that the isolator can withstand; a typical value for one minute is 2.5-kV rms, but this is higher in the medical device industry where isolation standards go up to 5 kV rms for one minute.
• Working voltage, which refers to the continuous voltage applied across the isolation barrier; typical values are approximately 250 V rms.
• Single insulation refers to a device that has just one isolation barrier; this contrasts with double insulation, which refers to a device that has two independent isolation barriers.
• Basic isolation is similar to single insulation and reinforced isolation is similar to double insulation.
• Creepage is the shortest distance along the surface of the package between two conductors on either side of the isolation barrier; clearance is the shortest distance through the air between two conductors.

Depending on the medical application and how it relates to patient safety, a certain continuous, reinforced insulation and creepage and clearance distance is required. 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 60601,¹ whereas MOOP is governed by less stringent requirements such as IEC 60950.²

Some medical systems ensure the highest safety level by complying with IEC 60601-1 at all interfaces. Those systems allow patients to come into contact with peripheral devices (including the portion of the system connected to the patient).

IEC 60601 also provides safety against the use of highly charged defibrillators. Without IEC 60601 certification, anything connected to a patient must be removed during defibrillation, at just the moment when there is no time to do so.

Existing approaches to isolation

The typical approach to isolation is to use optocouplers. The signal is transferred across the isolation barrier using light so that there is no direct electrical connection across the device. Standard optocouplers are unidirectional, thus an isolated interface using optocouplers or other unidirectional isolators must first translate the bidirectional USB signals into a set of unidirectional signals. Also, optocouplers must be driven with rather high currents, which can be milli-amperes or multiples depending on the data rate. The USB isolator is using an inductive isolation barrier, whereby a transformer is employed to provide the isolation between the primary and secondary side. The input currents are in the low micro-ampere range, thus they can be driven directly from a processor or other driver chip on the printed circuit board. The use of optocouplers, therefore, requires multiple components, which increases the required boardspace and the number of wires compared with the USB isolator. There are also reliability issues because of multiple components and the ageing effects of an optocoupler.

The result is expensive and takes additional design time to implement the microcontroller and its required software configuration. The complexity of an implementation such as this is the primary reason industrial, medical and instrumentation system architects have been slow to adopt USB.

Isolation in a single package

Figure 1: Representation of the digital isolation planar transformer. The coils are separated by a 20-μm polyimide insulating layer that is capable of isolation ratings up to 6-kV rms for 1 minute.

A simpler, more cost-effective and space efficient way to isolate USB is to use a dedicated USB isolator that can be inserted directly into the USB signal path. One product of this type is available in a single package. It provides reinforced isolation of up to 5 kV rms with support for low- and full-speed data rates up to 12 Mbps and is compliant with the USB 2.0 standard (www.usb.org). The device incorporates proprietary digital isolation technology, which uses planar transformers to transmit data across a 20-μm thick polyimide insulation layer that can withstand up to 6-kV rms. Data is transmitted by induction from one coil to the other. Figure 1 shows the structure of the transformer. The benefits of this digital isolation technology over optocouplers include

• bidirectional data transfer across the isolation barrier
• single package isolation; optocouplers would require separate devices to handle each direction of communication
• higher data transfer rates
• lower input current drive.

Its most critical advantage is its ability to integrate additional functionality into isolator products. This means it uses 75% less board space compared with a multi-IC configuration of USB transceivers and opto-couplers.

Jan-Hein Broeders is Healthcare Business Development Manager Europe at Analog Devices, Wilhelm Wagenfeld Strasse 6, D-80807 Munich, Germany, tel. 00800 266 822 823 (toll-free) e-mail: cic@analog.com, www.analog.com