Designing MEMS driver ASIC for contact lens sensor

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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 0.35 μm process, AnSem was able to incorporate power capture and conditioning, RF signaling, 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-percent 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 minimize losses at low power levels, while a current shunt safety circuit avoided overvoltage within the device when high power levels were received. As power levels were in the μW 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 34ºC (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 reduced corner set. AnSem will now manage the complete ASIC life cycle for Sensimed, including industrialization 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 realizable specification.


Courtesy of EETimes Europe