A Wide-Band Fully-Integrated CMOS Ring-Oscillator PLL-Based Complex Dielectric Spectroscopy System

A fully-integrated sensing system is proposed for wideband complex dielectric detection of materials under test (MUT). The system utilizes a ring oscillator-based phase-locked loop (PLL) for wide tuning range and precise control of the sensor´s excitation frequency. Characterization of both real and imaginary MUT permittivity is achieved by measuring the frequency difference between two voltage-controlled oscillators (VCOs): a sensing oscillator, with a frequency that varies with MUT-induced changes in capacitance and conductance of a delay-cells´ sensing capacitor loads, and a MUT-insensitive reference oscillator that is controlled by an amplitude-locked loop (ALL). The fully integrated system is fabricated in 0.18 μm CMOS, and occupies 6.25 mm<;sup>2<;/sup> area. When tested with common organic chemicals (ϵ´<;30), the system operates between 0.7-6 GHz and achieves 3.7% maximum permittivity error. Characterization is also performed with higher ε<;sub>r<;/sub>´ water-methanol mixtures and phosphate buffered saline (PBS) solutions, with 5.4% maximum permittivity error achieved over a 0.7-4.77 GHz range.