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

Abstract

This paper presents an integrated interface for eddy- current sensors (ECSs) for displacement measurement. The em- ployed architecture helps bridging the performance gap between the requirements of demanding an d precision industrial applica- tions and the performance of existing ECS interfaces. The inter- face operates with a sensor excitation frequency of 20 MHz, which is more than one order of magnit ude higher than typical values. This high excitation frequency lim its the eddy-current penetration depth in the target down to a few tens of micrometers, thus en- abling the use of thin targets required in precision applications. The proposed interface consists of a low-power front-end oscil- lator that incorporates the sensor , and a two-channel offset-com- pensated synchronous demodulat or. A ratio-metric measurement approach along with offset and 1/f noise reduction techniques is ap- plied to improve the system stability. The interface has been real- ized 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 sen sor 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 transl ates into 1.5 pico-Henry induc- tance-measurement resolution, which is comparable with the per- formance of the most advanced L CR meters. Using the proposed solution, a long-term instability below 20 ppm (for 17 hours) and a thermal