An Interface for Eddy-Current Displacement Sensors With 15-bit Resolution and 20 MHz Excitation

This paper presents an integrated interface for eddy-current sensors (ECSs) for displacement measurement. The employed architecture helps bridging the performance gap between the requirements of demanding and precision industrial applications and the performance of existing ECS interfaces. The interface operates with a sensor excitation frequency of 20 MHz, which is more than one order of magnitude higher than typical values. This high excitation frequency limits the eddy-current penetration depth in the target down to a few tens of micrometers, thus enabling the use of thin targets required in precision applications. The proposed interface consists of a low-power front-end oscillator that incorporates the sensor, and a two-channel offset-compensated synchronous demodulator. A ratio-metric measurement approach along with offset and 1/f noise reduction techniques is applied to improve the system stability. The interface has been realized in a 0.35-μm 3.3 V BiCMOS technology and consumes 18 mW. Measurement results obtained using two flat sensing coils show a full-range non-linearity of the sensor interface of only 0.4%, and a resolution of 15.5 bits (65 nm on a 3 mm measurement range), with 1 kHz signal bandwidth. This translates into 1.5 pico-Henry inductance-measurement resolution, which is comparable with the performance of the most advanced LCR meters. Using the proposed solution, a long-term instability below 20 ppm (for 17 hours) and a thermal drift of 30 ppm/°C are obtained without any temperature compensation. Compared to the state-of-the-art, the proposed interface achieves a considerably better trade-off between power consumption, resolution, bandwidth, and excitation frequency.

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