Abstract
We report on the design and characterization of two inductive proximity sensors based on LC oscillators operating at 500 MHz. The distance from the target is obtained by measuring the oscillation frequency variation. Both sensors contain an off-chip LC resonator consisting of a two turns 4 mm diameter planar coil and a ceramic capacitor. The LC resonator is connected to integrated CMOS electronics. The first sensor output stage consists of a buffer whereas the second sensor output stage includes a co-integrated 21 bits digital frequency counter. The first sensor has frequency noise floor of 0.085 Hz/Hz above 3 kHz. At a distance of 200 μm from the coil, the resolution is 0.2 pm/Hz and the integrated resolution from 1 mHz to 1 kHz is 22 pmrms. The second sensor reaches a noise floor of 2 Hz/Hz corresponding to a resolution of 5 pm/Hz and an integrated resolution of 900 pmrms.
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