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Developing MEMS driver ASIC for contact lens sensor

Posted: 28 Sep 2012     Print Version  Bookmark and Share

Keywords:Glaucoma management  ASIC  ADC 

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 components in the system. The bumped die is attached directly onto copper traces within the lens during manufacture, connecting it to the antenna and MEMS array.

Being powered from an RF source meant that the device had to first bridge rectify the AC signal coming from the antenna. The obvious rectification method using schottky diodes would heavily constrain the process choice. Normal PN diodes on the other hand would exhibit too much power loss. Instead, AnSem designed a patent pending RF speed, voltage multiplying, low dropout active rectifier with superior dropout performance even than schottky diodes and that could be implemented on virtually any mainstream 5v tolerant process. Power supply decoupling had to be achieved using only on-chip capacitance. This capacitance consumed a significant fraction of the silicon area. Behind this capacitance multiple voltage domains were established using internal LDOs, again using only internal capacitance to ensure stability.

By choosing ON Semiconductor's I3T50 0.35µm process, AnSem was able to incorporate power capture and conditioning, RF signalling, high linearity ADC and digital control functionality on a single die within the meager power budget and at low cost. Sensimed also benefits from the security of supply afforded by the automotive qualified process, which is so important to companies operating in the medical electronics field. As the lenses are single-use devices, cost was also an important factor. The ASIC digitizes the MEMS sensor reading and transmits the measurements back to the recorder via the same RF link used to power the device using load modulation techniques. For maximum RF power coupling an internal tuning capacitor had to be matched to the antenna inductance.

The +5-per cent tolerance of this capacitor was an important parameter when selecting the process. The I3T50 process is well-controlled, and we were able to work closely with On Semiconductor to meet this requirement.

The actual coupling between the lens antenna and the patch antenna can vary by a factor of 3 or more depending upon the relative orientation of the two coils. This wide variation in input power meant that the circuit needed to operate reliably and safely over a large input power range. The low dropout of the active rectifier helped minimise losses at low power levels, while a current shunt safety circuit avoided over-voltage within the device when high power levels were received. As power levels were in theµW range there was no danger of any self-heating related discomfort in the eye.

The signals coming from the strain gauge are very small such that the LSB of the ADC is at the microvolt level. Keeping the switching noise of the rectifier away from the ADC was a problem both at circuit level and at layout level. Noise coupling through the substrate had to be mitigated through clever use of the high voltage pockets available in the I3T50 process.

In this application, the die is mounted directly onto a transparent lens structure. Care had to be taken with light sensitivity of the circuitry. The back die had to be metal coated to prevent light penetration. Meeting specifications over temperature is usually a large part of our design work. It is well known that transistor parameters vary widely over temperature. Thankfully in this application the patient's eye maintains the device at around 34�C (temperature of the surface of the eye).

This saved both in design time and simulation time as we were able to run simulations with a much minimized corner set. AnSem will now manage the complete ASIC life cycle for Sensimed, including industrialisation and supply chain for volume production, leaving Sensimed to concentrate on serving their market. For the medical market the most important thing is to translate the customer's requirements into a complete and physically realisable specification.

About the author
Stephen Ellwood is the Vice President for Engineering at AnSem.

To download the PDF version of this article, click here.

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