HIGH FREQUENCY MEASURING CIRCUIT WITH INHERENT NOISE REDUCTION FOR RESONATING CHEMICAL SENSORS

The concentration of an analyte in a fluid is measured by a quartz crystal microbalance sensor device (10) in the fluid containing a concentration of t he analyte. The sensor device exhibits a resonant frequency representative of t he concentration of the analyte in the fluid. A quartz crystal microbalance reference device (16) has a resonant frequency lower, by a predetermined amount, than the initial resonant frequency of the sensor device before the sensor device is exposed to the analyte. A signal is derived based on the difference of the resonant frequencies of the sensor and the reference devic e, and a first counter (20) counts a predetermined number of cycles of the difference signal frequency to derive a sample time period. A second counter (24) counts cycles of a clock signal frequency during the sample time period to derive a count representative of the difference signal frequency.