Eddy-Current Sensor Interface for Advanced Industrial Applications

In this paper, a novel integrated eddy-current sensor interface is presented. The main targeted application is displacement measurement in an industrial environment, with resolution in the submicrometer range. A high excitation frequency of about 22 MHz is applied to minimize the skin effect of the generated eddy currents, when thin targets are used. The price to be paid is to process high-frequency signals. This is very challenging when high performance has to be achieved with respect to resolution and stability at minimum power consumption. To ensure high immunity of the interface to electromagnetic interferences, a second-order oscillator with a steep bandpass resonator is utilized as a front-end stage. The noise performance of the front-end stage is analyzed. To reduce the effect of this noise source on the resolution, a ratiometric measurement principle is proposed. In order to extract the displacement information, a novel amplitude-demodulation approach, including an offset cancellation technique, is introduced. The proposed circuit has been designed and implemented in a 0.35-μm BiCMOS process. In this design, the full-scale displacement range is 1.5 mm. The noise level allows a dynamic range of 75 dB with a measurement signal bandwidth of 1 kHz and only 9.5-mW power dissipation. A comparison with state-of-the-art eddy-current sensor interfaces shows an improved figure of merit, which confirms the high performance of the proposed interface.