Constrained Design of Potentiometric RFID Sensors Based on Auto-Tuning ICs

Auto-tuning integrated circuits (ICs), interconnected with radio-frequency identification (RFID) antennas in the UHF band, can provide digital information about the changes in the local boundary conditions in the proximity of the device. This feature, in conjunction with a varactor diode, can be used to create a general-purpose framework for the controlled wireless sensing of chemical agents. The varactor works as a transducer converting the potentiometric output of a chemical sensor into a capacitance change, which in turn is detected by the IC. A constrained optimization method is here introduced to maximize the sensitivity of the above transduction mechanism by deriving closed-form expressions that relate the digital output of the IC to the chemical variation. In this way, said variation will be optimally mapped within the retuning dynamic range of the IC. The method is demonstrated with reference to a pH sensor and is capable of providing a sensitivity of 30 units per unitary pH variation with stable communication performance. Moreover, the application to an on-body plaster-like antenna, namely when there are not fully controllable boundary conditions, demonstrated that the sensing performance is practically unaffected by the placement region over the human body as well as by the inter-user variability.