A Non-Invasive Material Sensing System and Its Integrated Interface Circuits

In this paper, a resonator-based non-invasive material sensor and its sensor interface circuits are presented. The sensing interface circuits detect the resonant frequency and resonator loss caused by the material under test. The readouts of resonance frequency and resonator loss correspond to the real and the imaginary parts of the sample relative permittivity respectively. The resonant amplitude is maintained constant by injecting different amounts of tail current to the resonator from a current mode digital-to-analog converter (I-DAC). The control bits of this I-DAC indicates the loss of the resonator. The interface circuit employs an integration-and-count approach to digitize the resonance frequency directly. The signal-to-noise ratio increases with the integration interval without being limited by the resolution of a dedicatedly designed analog-to-digital converter. A prototyped chip has been designed and fabricated in a 0.18 mu m CMOS process. The preliminary measurement results show that the proposed sensor system can distinguish air, deionized water, and different concentrations of ethanol and methanol.