Novel switched capacitor direct digital converters for variable reluctance sensors

Novel, simple and accurate Direct Digital Converters suitable for variable reluctance sensors are presented in this paper. These sensors have two inductors that vary in a push-pull manner with-respect-to the measurand. It is known that a small linear range can be obtained by taking the difference between the values of the sensor inductances. The proposed digital readout circuits accept the sensor inductances having non-linear characteristics, and provide a linear digital output directly proportional to the measurand, which is linear for the entire sensor range. The converters work on the principle of voltage-to-time based conversion and possess many advantages. Prototypes have been built and tested with a typical differential inductive sensor arrangement with a nominal value of 10mH. Output was linear for the full range of input and the worst-case error was less than 0.2%.