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EETE JULAUG 2012

DESIGN & PRODUCTS MEDICal ElECTRONICS Designing a MEMS driver ASIC for a novel contact lens sensor By Stephen Ellwood anSEm iS in thE BuSinESS of translating real world prob- components in the system. lems into aSiC solutions by matching up Rf, analog and digital the bumped die is at- processing elements to transform real world inputs into the tached directly onto copper required outputs. medical applications often have particular traces within the lens during needs for ultra-low power consumption, high integration, small manufacture, connecting it size and novel powering techniques. the choice of process to the antenna and mEmS is often make or break for such a project which is why anSem array. works with a number of leading fabs in order to support its Being powered from customers. an Rf source meant that Sensimed aG, a Swiss company specialized in design, the device had to first development and com- bridge rectify the aC signal mercialization of integrated coming from the antenna. micro-systems for medical The obvious rectification method using schottky diodes would devices has developed a soft heavily constrain the process choice. normal Pn diodes on the silicone single use contact lens other hand would exhibit too much power loss. Instead AnSem called SENSIMED Triggerfish designed a patent pending Rf speed, voltage multiplying, low for Glaucoma management dropout active rectifier with superior dropout performance even and treatment – see figure 1. than schottky diodes and that could be implemented on virtually Glaucoma is an eye disease any mainstream 5v tolerant process. affecting about 4% of the pop- Power supply decoupling had to be achieved using only Fig. 1: The SENSIMED ulation over 40 years of age on-chip capacitance. This capacitance consumed a significant Triggerfish is a soft silicone which can lead to blindness fraction of the silicon area. Behind this capacitance multiple single-use contact lens for unless treated early. one of the voltage domains were established using internal LDos, again symptoms of glaucoma is an using only internal capacitance to ensure stability. Glaucoma management and increased intraocular pressure. By choosing ON Semiconductor’s I3T50 0.35 µm process, treatment. it has long been recognized anSem was able to incorporate power capture and condi- that, since intraocular pressure varies during the day, continu- tioning, Rf signaling, high linearity aDC and digital control ous monitoring of glaucoma patients is needed. the static functionality on a single die within the meager power budget measurements currently performed by ophthalmologists during and at low cost. Sensimed also benefits from the security of office hours are not able to detect pressure peak variations. supply afforded by the automotive qualified process, which is Passive and active strain gauges embedded in the SEn- so important to companies operating in the medical electron- SIMED Triggerfish silicone lens monitor fluctuations in intra- ics field. As the lenses are single-use devices, cost was also an ocular pressure through variations in diameter of the eye. the important factor. the aSiC digitizes the mEmS sensor reading patient wears the Sensimed Triggerfish for up to 24 hours and and transmits the measurements back to the recorder via the undertakes normal activities including sleep periods. When the same Rf link used to power the device using load modula- patient returns to his doctor, the data is transferred from the tion techniques. For maximum RF power coupling an internal recorder to the practitioner’s computer via Bluetooth technology tuning capacitor had to be matched to the antenna inductance. for immediate analysis – see figure 2. anSem was contracted to design the tiny data acquisition aSiC embedded within the lens. the aDC at the heart of the aSiC had to so sensitive as to detect the change in eye pres- sure caused by the patients’ heart beat (ocular pulsation). Since there is no room for a battery within the structure of the lens, the system must power itself entirely from a localized magnetic field emitted by an RF antenna worn around the eye and con- nected to a portable data recorder through a thin flexible Data Cable – see figures 3 and 4. The RF field provides both power to the sensor and the data back channel to the recorder. the aSiC was designed to mount directly within the contact lens. Other than the RF antenna; a coil of wire with a specified inductance, and the strain gauge, there are no other electrical Fig. 2: The lens monitors the fluctuations in intraocular Stephen Ellwood is VP of Engineering at anSem - pressure through variations in diameter of the eye. www.ansem.com – he can be reached at business@ansem.com 32 Electronic Engineering Times Europe July/August 2012 www.electronics-eetimes.com


EETE JULAUG 2012
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