A 0.62–10 GHz Complex Dielectric Spectroscopy System in CMOS

This paper presents an integrated sensing system for complex dielectric spectroscopy in the 0.62-10 GHz frequency range. A capacitive sensor exposed to the material under test (MUT) shows variations in its admittance according to the complex permittivity of MUT. The sensing capacitor along with a fixed capacitor forms a voltage divider circuit and is excited by an RF signal at the sensing frequency. The magnitude and phase of the voltages across the two capacitors which depend on the sensor admittance are measured using a quadrature downconversion architecture to find the real and imaginary parts of the MUT's permittivity. At the lower frequency end, the system is configured as a direct-conversion architecture with third and fifth harmonic-rejection to alleviate the problem of harmonic mixing and improve the sensitivity. On the other hand, at the higher frequency end, the system works as a dual-downconversion topology and employs a sub-harmonic mixing technique to reduce the required input clock frequency span. As a proof of concept, the spectroscopy system is used for complex permittivity detection of pure organic chemicals and shows an rms permittivity error of less than 1% over the frequency range of 0.62-10 GHz. The fabricated chip in 0.18-μm CMOS occupies an active area of 2.3 mm
²
and consumes 65-72 mW from a 1.8 V supply.