Medical Device Link.

Originally Published MEM Fall 2003

ADVANCED IC TECHNOLOGY

The latest system-on-chip technology can help designers of portable and implantable medical equipment address requirements for performance, power consumption, and size.

Jonas Weiland

The market for medical electronics is growing rapidly as OEMs in the medical sector turn to ever-more-sophisticated solutions for the identification and treatment of illnesses and the ongoing delivery of patient care. One emerging trend is the move toward more portable equipment and implantable devices.

Such equipment includes blood glucose monitoring systems, insulin pumps, and body temperature sensors. Others range from defibrillators to neurological stimulators, and pacemakers and hearing aids. These products not only simplify the testing, monitoring, and treatment processes, but can also help to improve the quality of life for the patient by minimizing time spent in hospitals and often providing automatic, continuous treatment of chronic conditions.

The significance of the market for portable and implantable medical devices is reflected in the fact that the worldwide cardiac rhythm management (CRM) market (i.e., the market for pacemakers, defibrillators, and congestive heart failure devices) is currently worth $5.1 billion annually and is growing about 10% a year. At the same time, U.S. sales of hearing aids, while experiencing a slower rate of growth, now exceed $2.6 billion, according to Frost & Sullivan's report, "U.S. Medical Development Market Outlook for 2003."

Application Requirements

System-on-chip solutions minimize size and reduce power consumption.

Applications such as those have a number of fundamental design requirements. In the case of implantable devices, for example, the key is developing devices that have a long and relatively maintenance-free life. This means that battery life must be as long as possible (typically more than 10 years).

At the same time, size is critical, as well as the method with which a medical team can program, communicate with, or control the device. This functionality increasingly means the use of low-power, low-data-rate wireless communications technology rather than more-conventional, lower-performance, and more-intrusive inductive-sensing techniques.

There is also growing demand for devices to offer high levels of built-in intelligence and memory storage. Another demand is for devices to deliver improved sensing functionality and enhanced configurability. In CRM applications, control output stages are likely to involve high voltages, ranging from 20V (pacemakers) to almost 1000 V (defibrillator control). Operating frequencies can range from 0.1 to 1000 Hz.

Hearing aids, too, demand low-power operation (a typical device is powered from a 0.9–1.5 V zinc-air battery). High levels of functionality, including advanced digital signal processing (DSP) capabilities, are important. Wireless communications is again an emerging trend (this time to allow the doctor and the user to adjust the medical device remotely and unobtrusively).

For portable equipment such as glucose meters and blood pressure monitors, the pressure is on driving down power, cost, and size. For this market segment, battery-operated supplies typically deliver between 0.9 and 5 V. A common feature of such portable equipment is the need for low-frequency, high-precision analog-to-digital conversion alongside embedded microprocessors and memory. True portability also demands that only a minimal number of cables should be plugged into the unit. Such requirements also fuel the trend toward low-power wireless communications.

Mixed-Signal Solutions

To address requirements for performance, power consumption, and size, medical OEMs are moving away from discrete designs. The aim now is to incorporate as much functionality as possible into a single integrated circuit (IC). As a result, there is demand for complex system-on-chip (SoC) and integrated-sensor interface implementations.

It is clear that such SoCs need to integrate both analog and digital capabilities and, in many cases, deliver short-range, low-data-rate wireless communications functionality. Furthermore, some applications may also require that high-voltage output stages be integrated into the same device. A variety of semiconductor technologies, intellectual property (IP) blocks, and support tools can help to significantly simplify the implementation of SoCs for implantable and portable medical devices.

Low-Data-Rate Wireless

A good example of a low-power, low-data-rate wireless technology that can be employed in medical SoC designs is an application-specific transmit and receive IC mixed-signal radio-frequency (RF) technology platform. This platform can be used to rapidly develop a stand-alone transceiver IC. It can also be incorporated alongside other functional blocks into a highly integrated medical SoC solution.

Based around a proven and highly integrated 0.35-µm complementary metal oxide semiconductor (CMOS) process that features up to five metal layers, the SoC designs allow developers to make use of many analog and digital IP building blocks to reduce both design complexity and development time. These blocks include solutions for clock management and power management analog front ends, as well as microcontroller options and embedded memory solutions.

Figure 1. A functional block diagram of the ASTRX2 transceiver integrated circuit. (click to enlarge)

Figure 1 shows a functional block diagram of a transceiver IC, a device developed using the CMOS SoC technology. In this application, the transceiver is designed to provide a 433.92 MHz narrowband RF link. Using amplitude shift keying (ASK) modulation, this transceiver operates in half-duplex mode with user-selectable data rates of between 1.0 and 19.2 Kb/sec. The transceiver IC incorporates a low-power oscillator and has very low current consumption. Transmit output power and receiver sensitivity are rated at +6 and –103 dBm, respectively.

A power-conservation mode makes the technology ideal for implantable medical devices. The technology enables the device to wake up from a low-power state, poll for the presence of an RF signal, and then shut down again if no RF source is detected. This entire process can be done in less than 100 microseconds. The fast sniff cycle in combination with appropriate duty cycling make it possible to achieve very low average power consumption when monitoring the RF channel.

Figure 2. This oscilloscope trace illustrates sniff mode, which reduces the on-time duty cycle of the receiver.

The sniff mode, which is illustrated by the oscilloscope trace in Figure 2, ensures significant power savings by reducing the on-time duty cycle of the receiver. Time between the sniff signal acquisition polls can be preset from 0.5 to 16 seconds. Receiver sensitivity during operation is –90 dBm. Receive power varies proportionally with the receiver on time (e.g., a 0.1% duty cycle would result in an average receive current consumption of just 7.5 µA.

Sensor Interface ASICs

Figure 3. A sample of elements that can be integrated into a smart-sensor interface ASIC. (click to enlarge)

The nature of medical applications demands high-performance sensor solutions that deliver accuracy and functionality in the smallest possible form factors with the lowest power consumption. As a result, low-power mixed-signal application-specific integrated circuit (ASIC) technologies are increasingly being chosen to implement smart-sensor SoCs. Figure 3 shows an example of all of the elements that can be integrated into a smart-sensor interface ASIC. As the diagram indicates, the ASIC includes solutions for signal conditioning, conversion, and processing of the signal received from the sensor element. Available functionality can also cover capabilities such as temperature sensing, calibration, diagnostics, and memory components. In addition, a variety of methods for output of the data to the user (including low-data-rate wireless communications) can also be integrated into a single smart sensor IC.

Higher-Voltage Requirements

Hearing aids demand low-power operation, small size, and long battery life.

Many medical applications now require a single IC that can combine analog and digital capabilities with higher-voltage output stages. For example, the AIMS I3T80, a 0.35-µm process with high-voltage transistors, can be used to deliver high-density mixed-signal ASICs capable of handling voltages up to 80 V.

This technology can handle voltages up to 50 V and can reduce the size of sensor interface ICs by up to 60%. A deep-trench isolation technique dramatically reduces isolation distances between an ASIC's high-voltage devices.

The new 50-V technology is built around a low-voltage 0.35-µm CMOS process that features metal-metal capacitors and well-matched high-ohmic resistors. The technology is available with a full library of high-voltage diffusion MOS (DMOS) and bipolar devices, including high-performance floating vertical natural DMOS (nDMOS) transistors. These transistors have a drain-to-drain on resistance (R^_DS (^_ON)) of below 50 m(omega)*mm² at a breakdown voltage in excess of 50 V. Electrostatic discharge withstand capability for the technology is rated at 4.5 kV HBM (human body model) and 750 V CDM (charged device model).

Conclusion

Highly integrated, mixed-signal SoC solutions are ideal for medical OEMs looking to increase the functionality of implantable and portable medical applications. In addition, these SoC solutions minimize size and reduce power consumption. The availability of the latest mixed-signal processes, including smart sensor interfaces, will play an important role in decreasing the development time of such solutions.

A particularly strong emerging trend is the move toward SoCs that can provide low-data-rate, short-range wireless communications. Many hearing-aid users, for example, wear a unit in each ear. In addition to volume adjustment, wireless technology could be used to adjust the pickup of the microphone in each unit to achieve a better sound balance. Wireless functionality may also allow hearing-aid users to receive direct audio signals in theaters, from TVs, or in classroom settings. Building wireless capabilities into implantable medical devices and portable diagnostic equipment also allows doctors and patients to capture and record data without the intrusions associated with conventional techniques. Improved and simplified access to such data enables caregivers to provide increased independence for the patients themselves.

PHOTO BY INSONUS MEDICAL INC./COURTESY OF VALTRONIC SA

Jonas Weiland is strategic marketing manager for AMI Semiconductor (Pocatello, ID). He can be reached at 760-602-7214.

Copyright ©2003 Medical Electronics Manufacturing