INDUSTRY
WATCH Medical Devices
Transitioning from Analog to Digital in Medical Designs The move from analog to digital design in medical devices enables smaller size, lower power, greater noise immunity and lower parts count for powerful, portable solutions in the health care sector.
by Joseph Sankman, Microchip Technology
T
he burgeoning medical device industry stands to make significant advances with a new generation of microcontrollers that boasts high performance and low power consumption. These microcontrollers are integrated with a full complement of peripheral devices that meets the noise and accuracy requirements of medical devices. Traditionally, medical designs have relied on discrete analog circuit blocks, but digital microcontrollers are now powerful enough to assume the functions of their analog counterparts; sacrificing nothing in terms of speed and accuracy, while gaining reliability and smaller system volume. By moving to predominantly digital designs, cost minimization, design flexibility and time-to-market are improved, since software alterations are trivial in comparison to hardware redesigns. Additionally, there is a wide spectrum of available microcontrollers to meet the needs of next-generation, digital medical devices.
An Example Application
A pulse oximeter is an excellent application to demonstrate the shift to digital
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design. This noninvasive medical device shines red and infrared (IR) light through a patient’s finger or ear, and measures the absorption at each wavelength to determine blood-oxygen saturation. In addition, the pulsation of a patient’s heart is detectable, allowing the heart rate to be calculated (see “What is Pulse Oximetry?” p.xx). Portable oximeters have several critical requirements: low power dissipation for maximum battery life, small size that does not encumber the user, and high accuracy and repeatability. The last requirement is particularly important, since incorrect bloodoxygen saturation readings could endanger the health of the user. As with any analog-based product, a gamut of factors affects the performance and design. Semiconductor products, such as operational amplifiers (op amps), are sensitive to temperature variations. Specifications, such as offset voltage and input offset current, drift with temperature and lead to measurement variations. 1/f and broadband noise also play a part in corrupting the accuracy of measurements. Another issue to consider is system size. The dramatic, ongoing reductions in the sur-
face area of digital integrated circuits have not been matched by analog chips, giving digital-based devices an advantage in reducing system volume. Designers usually need many discrete components for analog designs, which could introduce problems with reliability and increased cost. The transition to digital alleviates many of the problems that analog implementations incur. Generally speaking, digital designs are inherently smaller than analog designs. For example, analog signal processing, which commonly involves one or more op amps and a number of passive components, can be completely translated into software routines. Not only does this transition reduce the number of parts and system volume, it also frees more design time, since revisions only require code alternations. Digital circuitry has much better noise immunity compared to analog circuits and is not as susceptible to temperature drift. For wireless medical devices, the superior noise immunity of digital circuitry also improves the rejection of EMI noise. Microchip’s dsPIC33F digital signal controller (DSC) is well suited to realize a