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.