Electrical Engineering Department
BS in Electrical Engineering
The Active Contact Lens measures the cornea-scleral radius of the wearer’s eye, which correlates to intraocular pressure (IOP), Glaucoma’s primary indicator. IOP varies throughout the day, and is drastically different from person to person, so constantly measuring it over a period of a few days can provide individualized tracking of the disease’s development and will help doctors develop personalized treatment schedules to treat the disease more precisely.
The Active Contact Lens sensor measures strain, based on the cornea-scleral radius, and reports the results wirelessly, to allow monitoring of an individual’s IOP over time. The lens is powered similarly to a passive RFID tag, so the device can operate long-term. The contact lens itself is clear, and biologically safe for the user. This allows the user to wear the device when asked by their doctor with no medical repercussions.
This document proposes part of the sensor-to-antenna integration circuitry. The whole design consists of three stages. The first stage is a Wheatstone bridge which converts the sensor’s varying resistance into an analog voltage. The second stage is a biasing circuit which receives the analog voltage, amplifies it, and sets it within the determined input range for the third stage. The third and final stage, which this document focuses on, is a current-starved voltage-controlled ring oscillator (CSVCRO) that translates the voltage to frequency. The oscillator consists of 21 current starved CMOS inverters controlled by a current mirror biasing circuit. The final design has a frequency range of 736Hz to 38.58MHz, and an average power dissipation of 784.7 𝜇𝑊 at center frequency 23.4MHz. All circuitry was designed and simulated using 180nm CMOS technology.