A capacitive sensor includes a variable capacitor with a capacitance value that changes in response to external stimuli. One embodiment of a capacitive sensor is a pressure sensor in which a capacitance of a variable capacitor, which is also referred to as a transducer, changes with reference to a level of pressure that is applied to the transducer. For example, a microelectromechanical (MEM) transducer can include two or more electrically conductive plates arranged in parallel to one another and separated by an electrically insulating dielectric, which in an absolute pressure sensor is a vacuum, between the plates. The distance between the plates and the amount of dielectric, and corresponding capacitance of the transducer, changes as the level of pressure applied to the transducer increases and decreases. An analog or digital controller can identify the pressure applied to the transducer by measuring the capacitance of the transducer. Other forms of capacitive sensor include humidity sensors, position sensors, proximity sensors, material thickness sensors, and any device configured to use a variable capacitor for sensing applications.
Many devices and systems that employ capacitive sensors identify a level of capacitance in the variable capacitor and convert the capacitance level to a digital value for processing with digital logic devices. For example, existing capacitive sensors use a switched capacitor circuit to generate a voltage signal corresponding to the capacitance of the variable capacitor, and then generate a digital value corresponding to the voltage signal using an analog to digital converter (ADC). One drawback of using a switched-capacitor voltage detector circuit and ADC for identifying the capacitance of the variable capacitor is the cost and complexity of the analog switching circuitry and ADC.
Another sensor configuration that generates a digital output is a resonance sensor. In a variable frequency oscillator, an oscillator amplifier circuit is coupled to the variable capacitor and the resonance frequency of the oscillator changes as the capacitance of the variable capacitors changes. A digital frequency counter measures the oscillator signal frequency to identify a digital value corresponding to the capacitance of the transducer in the sensor. One or more digital logic devices, including microcontrollers, microprocessors, application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), and the like, perform additional processing with the digital value.
One drawback of using a resonance sensor is that the sensor requires a precise frequency reference to identify the frequency of the variable oscillator with appropriate accuracy. Precision clock sources, such as crystal oscillators, which are suitable for use with the resonance sensors, also add to the expense of the sensor. Additionally, the variable oscillator typically includes an inductive coil. The inductance of the inductive coil and the corresponding resonant frequency of the oscillator change in response to changes in the relative permeability of the inductive coil's surroundings. These changes to the inductance of the coil reduce the accuracy of the sensor. In light of the drawbacks with existing capacitive sensors, improved devices and methods for capacitive sensing would be beneficial.
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