A novel 915 MHz, RFID-based pressure sensor for glaucoma using an electrically small antenna

The design and development of a radio frequency identification based pressure-sensing system to increase the range of current intra-ocular pressure sensing systems is described. Most current systems use near-field inductive coupling for the transfer of energy and data, which limits the operational range to only a few centimeters and prevents continuous monitoring. Increasing the powering range of the telemetry system offers continuous monitoring capability since the reader can be attached to a waist belt or put on a night stand when sleeping. The system operates at ultra-high frequencies and uses the electromagnetic far field to transfer energy and data, which increases the potential range of operation and allows for the use of smaller antennas. The system uses a novel electrically small antenna (ESA) to receive the incident RF signal. A four stage Schottky diode circuit rectifies and multiplies the received RF signal and provides DC power to a Colpitts oscillator. The oscillator is connected to a pressure sensor and provides an output signal frequency that is proportional to the change in pressure. The system was fabricated using a mature, inexpensive 0.5 mu m CMOS process. The performance of the system compares well with current state of the art, but uses a smaller antenna and a less expensive fabrication process. The system was powered over a range of at least 6.4 cm when the ESA with a ka value of 0.096 was used as the receiving antenna for the pressure sensing system.