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Simplify USB Isolation in Medical Applications

Wed, 03/31/2010 - 5:10am
Mark Cantrell, Analog Devices, Inc., www.analog.com

Mark CantrellThe personal computer (PC), currently the standard information-processing device for office and home use, communicates with most peripherals using the universal serial bus (USB). Standardization, cost, and the availability of software and development tools have made the PC very attractive as a host-processor platform for medical and industrial applications, but the safety and reliability requirements of these growing markets—especially regarding electrical isolation—are very different from the office environment that has historically driven the design of the personal computer.

USB has come to replace RS-232 as the standard communications port in personal computers and their peripherals and has features that are far superior to the older serial port in nearly every respect. It enables on-the-fly connection of hardware and drivers, and up to 127 devices can exist on the same hub-and-spoke style network.

But it has been difficult and costly to provide the necessary isolation for medical and industrial applications, so in these areas USB has been principally used for diagnostic ports and temporary connections.

This article discusses various ways of implementing isolation with USB. In particular, a new option, which allows simple, inexpensive isolation of peripheral devices—especially including the D+ and D– lines—increasing the usefulness of USB in medical and industrial applications. (1)

About the Universal Serial Bus
The USB physical layer consists of only four wires: two provide 5V power and ground to the peripheral device; the other two, D+ and D–, form a twisted pair that can carry differential data (Figure 1). These lines can also carry single-ended data, as well as idle states that are implemented with passive resistors. When a device is attached to the bus, the passive resistor configuration establishes an initial bus speed as well as a non-driven idle state. The data is organized into data frames or packets. Each frame can contain bits for clock synchronization, data type identifier, device address, data payload, and an end-of-packet sequence. 

Figure 1 Standard elements of USB

Control of this complex data structure is handled at each end of the cable by a serial interface engine (SIE). The SIE may be built into a microprocessor, providing only the D+ and D– lines to the peripheral. Isolating this bus presents several challenges:

1. Isolators are nearly always unidirectional devices, while the D+ and D– lines are bidirectional.
2. The SIE does not provide an external means to determine data transmission direction.
3. Isolators must be compatible with the pull-up and pull-down functions of passive resistors, making them match across the barrier.

Typical approaches to isolate the USB largely seek to sidestep the above challenges by isolating the entire SIE. This is particularly difficult if the SIE is built into a micro, but non trivial in all cases.

A new approach: The holy grail of USB isolation is to provide a way to isolate existing USB enabled devices. We can now insert the isolation directly into the D+ and D– lines (Figure 2). This allows D+/D– isolation to be added to existing USB applications without rewriting drivers or adding a redundant SIE, a significant advantage over the other approaches. Isolating the D+ and D– lines has several challenges for unidirectional isolators, as the device must be able to handle flow of control like an SIE, as well as permit application of pull-up resistors and determine bus speed across its isolation barrier based only on the information available from interface. It should also operate without requiring for the overhead of additional device drivers. 

Figure 2 Isolating the Dplus Dminus lines

These challenges have been met with the ADuM4160 USB isolator (Figure 3), the industry’s first chip-scale device that supports direct isolation of low- and full-speed USB D+ and D– lines. 

Figure 3 ADuM4160 block diagram

The primary challenges in developing a USB isolator are properly determining the direction of data transmission—and knowing when to disable output drivers to allow an idle bus state. The packet-oriented nature of USB data allows a simple method of determining data direction without the overhead of a complete SIE. When the bus is idle, pull-up and pull-down resistors hold the USB in an idle state with no buffers driving the bus.

Analog Devices’ iCoupler-based ADuM4160 USB isolator monitors the upstream and downstream segments of the bus, waiting for a transition from either direction. When a transition is detected, it is encoded and transmitted across the barrier. The data is decoded, and the output drivers are enabled to transmit on the other cable segment. From this first transition, the direction of data flow is identified, and the reverse-direction isolation channels are disabled. The isolator continues to transmit data in the same direction as long as data continues to be received. When the USB packet is complete, special data, the end-of-packet (EOP) sequence, is transmitted. The EOP contains an unbalanced signal that should not be included in any other data structure. The isolator can distinguish an EOP marker from valid data. This signals that the bus should be returned to the idle state. The output drivers are disabled, and the isolator begins to monitor its upstream and downstream inputs for the next transition—which will set the next direction for data transmission.

The USB isolator will likely be used in one of three ways: 

• It can be installed in a peripheral to isolate its upstream port. The ADuM4160 was designed with this configuration as the base application. It leads to the simplest power and control configurations (Figure 4). 

• It can be used in an isolated cable configuration (Figure 5). 

• It could also be used to isolate a hub and therefore all of the peripherals downstream of the hub (not covered here, but fully supported) The following illustrations show how the USB isolator will be connected in each of these applications. 

In the peripheral application (Figure 4), where the peripheral has its own source of power, almost no power is required from the USB cable—about 10 mW to run the isolator’s upstream side and the pull-up resistor. Since the peripheral operates at a single speed, the isolator is hardwired for the desired speed setting, either full speed or low speed. If the peripheral port happens to be high-speed-capable, then it sends a high-speed “chirp” pattern during enumeration. This would normally initiate negotiations for high-speed operation, but the isolator blocks the chirp signal and automatically forces the high-speed peripheral to operate at full speed. For low-power peripherals that don’t have their own supply, an isolated dc-to-dc converter can be used to supply the peripheral and the ADuM4160, drawing power from the USB cable. 

Figure 4 Isolated peripheral port

Driving an isolated USB cable requires use of a dc-to-dc converter to supply power to the downstream port and cable. To satisfy the requirements of the USB specification, the downstream segment of the cable must provide 5V power to the pull-up of the peripheral device. An isolated dc-to-dc converter can provide this power with enough left over to provide power for downstream devices with low power requirements.

Figure 5 Isolated cable interface including isoPower

Medical Market Applications
Isolated USB has much to offer to both the medical system designer and the health care professional. Leveraging this technology, medical device design manufacturers can cut design time in half, lower costs, and reduce time-to-market of smaller, lighter, more energy-efficient USB-enabled devices, such as laptops, cameras and storage devices used in medical environments.

For healthcare equipment companies, this means that designers can create commercially available, cost-efficient devices for patient monitoring, disease management, health and wellness, and drug delivery systems that can connect across the internet in real time.

For healthcare professionals, this means that nurses can perform remote access and control from their stations; equipment is protected from making corrupt readings; and devices support medical regulatory requirements.

Isolated ECG module
Everyone who has had an ECG taken at their physician’s office is familiar with this procedure. The nurse wheels up a machine that includes a multi-pen strip-chart recorder, a cable that branches to about a dozen leads and a complicated looking set of controls to operate it. The nurse places the leads and records a strip of paper that is torn off and folded into your chart. If further analysis needs to be done, or the information must be shared among several doctors, both the chart and the paper strip must be copied and mailed or faxed.

Availability of high-speed isolated communication between an ECG front-end sensor unit and the analysis, storage and display capability of an inexpensive commercial computer completely changes how this type of diagnostic test is performed and how the information is handled. As shown in Figure 10, the simplicity of isolating USB allows a designer to focus on the data acquisition module. Data is acquired in the module and transferred to a laptop where it is displayed and stored along with comments from the doctor. Enough power to operate the front-end electronics can be derived from the cable through a dc-to-dc converter such as the ADuM5000. The ECG module is configured as a USB peripheral as described above. When the module is plugged in to the USB host, it opens its own acquisition program. The data can be saved in the patient’s database or easily forwarded to other healthcare professionals as needed. 

Figure 6  Isolated USB cable powered ECG

Home healthcare, glucose monitor
Another application that realizes great advantages from isolated USB communications is home healthcare. Let’s examine glucose meters as an illustrative application as shown in Figure 11. Some high-end glucose testers for diabetes management already have USB interfaces. They are used for two proposes: to download data from the meter into a database where trends can be observed graphically, and to recharge the unit’s batteries from the USB power. Rechargeable batteries significantly expand the functionality of glucose meters as they support much higher current applications such as color displays or RF links to insulin pumps. These USB interfaces are not currently isolated, which can be problematic as the devices cannot be used while they are connected to a power supply or a USB host. If the battery in the meter runs down, the patient must wait until it has recharged before using it again. Isolation of the USB interface will allow real time connection to a host computer to supply operating and recharge power to the unit. It will also allow the user to update device parameters and annotate data in real time without disconnecting to take a glucose measurement. 

Figure 7 Home Glucose Meter

Many home health monitors for blood pressure, respiration, temperature, heart rate, etc., can be implemented in a similar way. Currently these devices must be returned to a hospital or doctor’s office periodically for data to be downloaded and power to be replenished. The availability of a simple on-the-fly isolated connection to the home computer turns all of these devices into network enabled remote sensors. Data can be downloaded periodically or it can be monitored in real time by a remote healthcare provider.

Conclusion
USB isolation products, such as the ADuM4160 USB isolator, allow simple and inexpensive isolation of peripheral devices in which USB can add greater functionality. This, in turn, will increase the penetration of USB into the medical application space well beyond diagnostic ports and temporary connections. The digital isolator’s focus on providing isolation in the D+/D– lines makes implementation extremely simple. Support for both full-speed and low-speed operation provides sufficient bandwidth for a wide range of applications. The needs of the medical community have been addressed up front with support for 5000Vrms isolation and support of the IEC60601 medical standard built in.

References

1Information on all ADI components can be found at www.analog.com.

Mark Cantrell is an applications engineer for the iCoupler product line at Analog Devices, Inc. (ADI). Prior to joining ADI, Mark spent six years at California Eastern Laboratories, where he was responsible for applications support for NEC’s optocoupler and solid-state-relay product lines. Mark’s experience also includes 17 years at Lockheed Martin Missiles and Space, where his job as a radiation-effects test engineer included work on the Gravity Probe B satellite program. Mark received his MS in physics from Indiana University. He can be reached via email at mark.cantrell@analog.com. For more information, contact Analog Devices, Inc., 831 Woburn St., Wilmington, MA 01887-4601; 800-262-5645; www.analog.com.

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