Dielectric and inductive sensing using fringing electromagnetic fields from temperature-stabilized LC oscillators

A sensing platform to determine material permittivity and metal proximity using self-referenced and temperature-compensated CMOS LC oscillators (LCOs) is presented. Electromagnetic fields fringe from the planar surface of the integrated LCO and are modulated when a material is placed in close proximity to the surface of the die. The interaction of the material and fields modulates the self-resonant frequency of the LCO and causes the temperature coefficient of frequency (TCf). Thus the circuit functions as a single chip sensor and frequency read-out circuit. Changes in the TC_f enable the temperature coefficients of the permittivity of the material-under-test to be determined. Frequency drift induced by a conductive material-under-test, placed up to 1mm from the surface of the die can be used to determine material proximity. The paper will demonstrate these sensing principles and discuss the limitations of using the proposed platform.