A Linear and High Sensitive Interfacing Scheme for Wireless Passive LC Sensors

This paper presents a simple and efficient interfacing scheme for passive LC sensors, connected to the measurement unit through a magnetically coupled readout and sensor coils. The proposed scheme always measures at or near the parallel resonant frequency f(p) of the system. The scheme first converts the capacitance of the sensor into a phase angle (phi) between the excitation voltage (V-in) and the current I-1 through the readout coil. The final output is given by tan(phi), but easily realized using a phase sensitive detector (PSD). The inputs to the PSD are Vin and a signal proportional to the I-1. This new approach gives the following advantages: 1) it realizes a simple, hardware optimized measurement system, which draws the minimum current from Vin as the measurement is at f(p); 2) the time taken for the measurement is much less, compared with the frequency sweep approach; 3) the output is linear; and 4) the output is insensitive to the variations in the coupling coefficient (k), for a wide range of k. In addition, a new, but simple method is employed to calibrate the system from the readout coil side. This enables the output to be less sensitive to the variation in the quality factor, coupling coefficient, and resistance of the coil. A prototype of the proposed readout system has been developed and tested for a range of +/- 5 pF. The output of the interfacing scheme showed a high sensitivity of 5.5 degrees/pF. The worst case linearity noted was 0.52% and the worst case error observed in the test was less than 0.67%.