INTEGRATED CIRCUITS
The surge in the incidence of diabetes and heart disease has produced a growing need for accurate and portable home patient-monitoring devices, including better, more-convenient portable devices for monitoring cholesterol, blood glucose levels, and blood pressure. Meanwhile, developing technology is making it possible for the users of such units to track their important test results over time. Through the creation of fairly uniform standards and form factors, advance in ultraportable technology have set the stage for developing handheld products that deliver high availability and ease of use to the medical technology consumer.
An example is the evolution of the analog switch for use in cell phones, which illustrates the way design trends emerge in all portable devices. However, designers must consider numerous questions when it comes to streamlining and adapting such technology for patient-monitoring equipment.
This article discusses those design considerations and examines the latest integrated-circuit (IC) devices available to aid in this process of adaptation. Specific topics include device ruggedness and protection against electrostatic discharge (ESD), the use of analog switches to increase the functionality of field-programmable gate arrays (FPGAs) and application-specific integrated circuits (ASICs), input/output (I/O) standards, and the advantages of designing a universal serial bus (USB) port into medical devices.
Equipment Safety Precautions
As ASICs and FPGAs increase in popularity, the decision to use them as the core of a portable monitoring unit can appear to be straightforward. For instance, the initial cost-benefits are fairly easy to calculate. So are the space savings resulting from these devices' integrated features and compact pin sets. However, it is important to consider the various components that have to be added around an all-in-one ASIC or FPGA device.
The core operating voltage for portable designs is often 2.5 V or lower, for example, making possible a dense gate count for the IC and providing for low-current operation. This low voltage is very helpful when the device operates off a battery, since it enables a simple boost-per-buck power-management device to maintain efficiency with minimal voltage differential. However, the technology that allows this low core voltage, especially combined with the copious gate count within the ASIC, does not come without a trade-off.
Even though ASICs and FPGAs contain minimal current, these devices are easily affected by the large ESD voltage spikes. Typical ESD ratings for ASICs are roughly 2 kV HBM (for human-body model), when such a specification is even listed in the data sheet at all. This rating is not a major issue for internal interfaces within the portable medical device, but ESD can lead to device failure for any silicon tied to an external connector. These external connectors are exposed to the outside world and can be subjected to harsh conditions.
Though it sounds harmless, simply walking across a carpeted floor with a portable electronic device in a low-humidity environment can push ESD protection to its limits. End-user-inflicted ESD events are not a major concern for the electronics of the medical unit; however, on the other hand, some care must be taken to provide ESD protection during manufacture of the device. A cable that is put together on an assembly line by hand, for instance, may need protection at the end of the process when driving less-robust silicon.
Analog Switch Implementation
Analog switches are perfectly suited for situations in which ESD protection is needed. They offer a range of connector interfaces, from video to a data I/O port and even an audio headset. An analog switch is available to fit any application.
New analog switches easily support 8 kV HBM of protection and can even exceed 10 kV. This specification is even more important for the medical applications that must be rugged.
Bandwidths provided by these switches range from a few kilohertz up to those of the latest selection of video switches, which provide transfer rates above 1 GHz. Not only do they protect the ASIC, the switches can also add functionality. And they do so in a near-chip-scale package to offer significant space savings and thermal advantages. The number of switches in a single package can range from one to four. They support both single-ended and differential interfaces. In addition, to facilitate implementation, their functionality nomenclature is that of a standard switch: SPST corresponds to a single-pole, single-throw switch, and SPDT would be a double-throw switch.
) |
Figure 1. An analog switch specifically designed for headset ports can toggle between two sound generators or provide undershoot protection. (click to enlarge) |
Audio Functions. Creativity in analog switch design can lead to a more diverse ASIC. For instance, if a medical device has two separate sensors, such as two blood analysis sensors in a diabetes monitor, either for the purpose of redundancy or to maximize accuracy, an SPDT switch can be used to toggle between the ASIC and each sensor. Analog switches are available specifically for headset ports, either to toggle between two sound generators or to provide undershoot protection to eliminate the slight pop heard when a headset is inserted into the jack (see Figure 1).
Portable medical devices historically have not had much audio communication capability, primarily due to the cost and large amount of disk space required to store speech. But nowadays, with multiple gigabytes being stored in package sizes of less than a square inch, entire speech recognition and response programs can be implemented in even the smallest portable applications. End-users with poor eyesight would benefit greatly from applications that allow them simply to press a Help button to have the entire system menu read aloud. Through use of an analog switch, this audio function could be implemented over a speaker phone or through a simple headset jack.
Memory Functions. An important feature on any portable device is the ability to add and remove memory as additional features become available. In medical devices, this memory could be used to store historical data, such as blood pressure readings recorded over several months, and later could be removed and plugged into a computer with charting capabilities.
Media slots also allow increased functionality. For instance, the SD Association, organizer of the secure digital (SD) form factor media slot, has already defined a fingerprint-reading SD input/output (SDIO) card that enables any device with an SD port to be activity-restricted to only the intended user.
There are now numerous media stick standards, a few of the most common being SD, MicroSD, and XD (extreme digital). A design's interoperability need not be limited to the use of just one media stick. Through the use of an analog switch, it is possible to add two or more slots. Since only one card is accessed at a time, a dynamic hub-type switch is not needed. In addition, advances in the SD card have allowed multiple applications to be integrated into the actual memory card. Medical applications already include fingerprint scanners and Bluetooth, Wi-Fi, and global positioning systems, and may continue to proliferate. The addition of a media card to any portable design can pay off in end-user benefits as new applications are developed.
Multiplexing Functions. Analog switches can also be used when there is a need to multiplex between various video sources. Multiplexing allows two separate sources to drive one common point without signal interference. This is currently an uncommon requirement for a portable medical device; however, a video-out connector is appearing more often on all types of portable devices, so it could become more prevalent in medical equipment.
For example, cell phones have already begun to add a multiplex feature, primarily for viewing pictures taken via the popular camera option. This kind of video-out will eventually make its way into portable medical applications, too. A composite video output on a medical unit would allow complete review of past recordings, or could perhaps be used to play back a brief instruction video to full-size display. The graphics process is often an option available through ASIC vendors. However, a video driver also would be needed to drive the cable. Integrated, ultrasmall video filter and line drivers are now offered that provide ESD protection as well.
The USB Port
Whether or not it has a video port, a portable instrument must include a means of importing and exporting data. And imagine that this port was also the primary means of charging the battery. The USB is the I/O port of choice today, and it can do exactly that. The widely used USB standard has nearly replaced the earlier serial connection in some areas. The change is under way in the medical field.
More options are emerging for USB as this trend continues to mature. One is USB OTG, for on the go. USB OTG holds promise for medical devices because it does not require the use of a personal computer (PC) to negotiate the communication link and it would conveniently allow a device to be plugged directly into a printer. Wireless USB adds even more convenience. The key decision here is whether to integrate the USB transceiver in the ASIC or use a stand-alone PHY-level (physical-layer-level) transceiver.
The advantages associated with a discrete-transceiver approach are similar to those of the analog switch. A stand-alone transceiver frees up the pins of the ASIC for more-complicated I/Os, and also provides an ESD buffer for the more costly core device. Some USB transceivers offer ESD protection of 15 kV. These USB ports in a PC are also capable of sourcing 5 V at 500 mA via the Vbus pin. This capability can be exploited to power the device while in synchronization mode, or even to charge the battery. Designers should consult the USB standards to ensure compliance with power specifications. They may find it necessary to place a current-limiting device on the port, as well.
) |
Figure 2. Diagram of the IntelliMAX load switch from Fairchild Semiconductor Corp. (South Portland, ME), an example of an analog switch whose chief purpose is to limit inrush current in portable electronic devices. (click to enlarge) |
New load switches entering the semiconductor market are focused exclusively on limiting inrush current in portable devices (see Figure 2). Because today's advanced batteries are very good sources of current and are capable of sourcing several amps of current if given the opportunity, these load switches with current-limiting capability offer an easy solution. They even add ruggedness to a device through overcurrent protection, thermal shutdown, and undervoltage lockout. I/O Standards
It is worth noting that the advances in I/O standards for ultraportable applications, such as those seen in cell phone designs, can easily be leveraged for medical device design. Gone are the days of 5-V transistor-transistor logic (TTL) and power-hungry single-ended signaling. Data signals that require high throughput and are continuously active are now sent over differential pairs. Low-voltage differential signaling (LVDS) has been around for some time, but recent advances in this technology have resulted in low-power LVDS (LpLVDS), which exhibits smaller voltage swings. These LVDS lines are ideal for running wide data buses, as well as for simple low-skew clock distribution.
Numerous types of sensors are used to collect patient data—temperature, blood pressure, blood cell count, whatever else it might be. These medical device sensors are under very tight calibration; therefore, data signals must be transmitted over traces that will remain low in noise. Differential signals must be run side by side over equal distances so that equal noise susceptibility for each trace in the differential pair will result. As both differential lines move up or down with the noise—which could be generated by a source as ordinary as a cell phone—the delta between the lines remains the same, and the sensor data are accurate.
A variety of LVDS devices are available from several vendors. These range from TTL to LVDS transmitters and include efficient LVDS repeaters for driving signals greater distances. LVDS and LpLVDS devices are also very proficient at driving twisted-pair cables. This would be an advantage in a medical device application that has the capability to accept multiple sensor inputs via cables and connectors.
Conclusion
Medical device end-users are looking for ease of use, ruggedness, and accuracy in portable electronic instruments. They have seen the potential of multitasking portable technology demonstrated by recent advances in cell phone designs and would like to have to manage as few portable medical units as possible. The medical technology industry must follow the lead of consumer electronics by offering such advanced devices as an all-in-one blood analyzer, thermometer, and blood pressure measurement unit.
New power-management switches can be adapted for use in these applications, enabling designers to create safer, more-convenient, and more-advanced medical devices. For instance, this technology would allow a medical instrument to be coupled with a PC via a USB port, giving the end-user or home patient ready access to accurate test results and thorough tracking reports. Designers who are aware of what cutting-edge ICs are capable of achieving in the small-signal arena will be able to envision and develop better end products for the medical monitoring applications.
