Capacitance sensor circuit and method for measuring capacitance and small changes in capacitance
AssigneeSentech Corporation
InventorMark E. Spencer
Life of this patent
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Abstract
An improved oscillator circuit and method for measuring capacitance and small changes in capacitance is described. The circuit provides a digital output signal that has a frequency inversely proportional to the capacitance being measured, is substantially immune to stray capacitance, has low sensitivity to noise and temperature variations, is relatively simple and inexpensive to implement and provides a high precision measurement. One embodiment of the oscillator includes a capacitor to be measured, an integrator, an inverting amplifier, a comparator, a summing amplifier and a digital output circuit. A second embodiment of the oscillator further provides compensation for the non-linear response of a parallel plate capacitive transducer.
Claims38
An oscillator circuit for measuring capacitance and small changes in capacitance comprising, in combination:
- a capacitor having a capacitance to be measured;
- means for generating an output signal that is proportional to the integral over time of an exponential current flowing through said capacitor;
- means coupled to said generating means for inverting said output signal;
- means coupled to said inverting means for switching a bistable signal at a frequency that is inversely proportional to said capacitance to thereby provide an indication of the value of said capacitance;
- and
- means coupled to said switching means, coupled to said inverting means and coupled to said generating means for summing an output of said switching means and an output of said inverting means.
A method for measuring capacitance and small changes in capacitance comprising the steps of:
- providing a capacitor having a capacitance to be measured;
- providing means for generating an output signal that is proportional to the integral over time of an exponential current flowing through said capacitor;
- providing means coupled to said generating means for inverting said output signal;
- providing means coupled to said inverting means for switching a bistable signal at a frequency that is inversely proportional to said capacitance to thereby provide an indication of the value of said capacitance;
- and
- providing means coupled to said switching means, coupled to said inverting means and coupled to said generating means for summing an output of said switching means and an output of said inverting means.
Dependencies are inferred from the claim text.