Temperature compensation of an integrated low power inductive proximity microsensor

A simple temperature compensation method for inductive proximity microsenso rs based on the differential relaxation oscillator has been developed and s uccessfully tested. With this compensation and for the temperature range fr om - 20 degrees C to + 80 degrees C, an accuracy better than + 10 mu m at 1 mm distance to an aluminum target has been measured. The microsensor has b een integrated with a 3-V, 1-mu m CMOS read-out circuit using a gold bumpin g layer to form a 3.8-mm side flat coil. The power consumption of the whole compensated microsystem is lower than 10 mW. To achieve this, the temperat ure behaviors of the whole microsensor and of its building elements, namely the sensing coil (nearby a target) and the read-out circuit, have been stu died and a compensation method has been developed. The inductance of the in tegrated coil is temperature-independent in the frequency range up to 12 MH z, whereas its resistance depends mainly on the temperature coefficient of the conductor resistivity. The resonance frequency of the coil is not affec ted by temperature. In its principle, the electronic circuit has a temperat ure-dependent drift in the sensing distance range. This drift can, however, be compensated using a negative temperature coefficient resistor. Analytic al derivations and simulation tools have been used for the choice of the op timal coefficient for a specific sensing distance range. (C) 2000 Elsevier Science S.A. All rights reserved.