Detector for capacitive sensing devices
First Claim
1. A detector for producing an output indicative of the state of a variable capacitor having first and second plates and minimum and maximum capacitance values, comprising:
- a reference capacitor having first and second plates and a capacitance value between the minimum and maximum capacitance values of the variable capacitor;
a sense node adapted to connect the first plates of the variable capacitor and said reference capacitor;
a comparator having an input terminal, and an output terminal at which is produced an output signal indicative of whether the voltage at the input terminal is smaller or larger than a precharge voltage;
means connecting the input terminal of said comparator to said sense node;
precharging means connected to the input terminal of said comparator and said sense node, said precharging means being operable to impress the precharge voltage on the input terminal of said comparator and the first plates of the variable capacitor and said reference capacitor during a first time period, and, during a second time period, to allow the voltage on said sense node and connected input terminal of said comparator to assume a voltage different from the precharge voltage; and
a drive signal generator connected to the second plate of said reference capacitor and having a drive terminal adapted to be connected to the second terminal of the variable capacitor, said drive signal generator being operable to impress first and second predetermined voltages of different values on the second plates of said reference and variable capacitors respectively during the first time period and to impress the second and first predetermined voltages on the second plates of said reference and variable capacitors respectively during the second time period, whereby, during the second time period, the capacitance value of the variable capacitor determines whether the voltage on the input terminal of said comparator is smaller or larger than the precharge voltage.
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Accused Products
Abstract
A method and circuit are disclosed for detecting the state of a variable capacitor by comparator means whose threshold voltage is impressed on first plates of the variable capacitor and a reference capacitor having a capacitance between the minimum and maximum capacitances of the variable capacitor, and charging and discharging the second plates of the variable and reference capacitors by equal amounts in opposite senses. The voltage on the first plates of the capacitors then changes in a sense dependent on the relative capacitance values of the capacitors and is compared with the threshold voltage by the comparator means.
55 Citations
21 Claims
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1. A detector for producing an output indicative of the state of a variable capacitor having first and second plates and minimum and maximum capacitance values, comprising:
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a reference capacitor having first and second plates and a capacitance value between the minimum and maximum capacitance values of the variable capacitor; a sense node adapted to connect the first plates of the variable capacitor and said reference capacitor; a comparator having an input terminal, and an output terminal at which is produced an output signal indicative of whether the voltage at the input terminal is smaller or larger than a precharge voltage; means connecting the input terminal of said comparator to said sense node; precharging means connected to the input terminal of said comparator and said sense node, said precharging means being operable to impress the precharge voltage on the input terminal of said comparator and the first plates of the variable capacitor and said reference capacitor during a first time period, and, during a second time period, to allow the voltage on said sense node and connected input terminal of said comparator to assume a voltage different from the precharge voltage; and a drive signal generator connected to the second plate of said reference capacitor and having a drive terminal adapted to be connected to the second terminal of the variable capacitor, said drive signal generator being operable to impress first and second predetermined voltages of different values on the second plates of said reference and variable capacitors respectively during the first time period and to impress the second and first predetermined voltages on the second plates of said reference and variable capacitors respectively during the second time period, whereby, during the second time period, the capacitance value of the variable capacitor determines whether the voltage on the input terminal of said comparator is smaller or larger than the precharge voltage.
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2. A detector for producing an output indicative of the state of a variable capacitor having first and second plates and minimum and maximum capacitance values, comprising:
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comparator means having an input terminal adapted to be connected to the first plate of the variable capacitor and an output terminal at which is produced an output signal indicative of whether the voltage at the input terminal is smaller or larger than a threshold voltage; first switch means connected to the input and output terminals of said comparator means, said comparator means and said first switch means, when enabled, being operable to cause the voltage at the input terminal of said comparator means to be substantially equal to the threshold voltage; a reference capacitor having first and second plates, the first plate being connected to the input terminal of said comparator means, said reference capacitor having a capacitance between the minimum and maximum capacitance values of the variable capacitor; and reset means adapted to receive a reset signal of first or second states and operable when receiving a reset signal of the first state to enable said first switch means and charge the second plate of said reference capacitor to a first predetermined voltage, said reset means further being operable when receiving a reset signal of the second state to disable said first switch means and discharge the second plate of said reference capacitor to a second predetermined voltage, whereby if the second plate of the variable capacitor is charged in response to the reset signal of the second state so that its voltage increases by an amount equal to the difference between the first and second predetermined voltages, a voltage is produced at the input terminal of said comparator means which is smaller or larger than the threshold voltage depending on whether the capacitance of the variable capacitor is smaller or larger than the capacitance of the reference capacitor. - View Dependent Claims (3, 4, 5, 6, 7, 8)
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9. Capacitive keyboard apparatus comprising:
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an array of capacitive key switches each including a capacitor having first and second plates and minimum and maximum capacitance values; a clock generator for generating first and second alternating clock signals; scanning means connected to said clock generator and operable in response to the first clock signals to sequentially charge the first plates of the capacitors in said array from a first predetermined voltage to a second predetermined voltage and to provide for sequential sensing of the states of the capacitors in said array; comparator means having an input terminal connected to said scanning means for sequentially receiving signals corresponding to the voltages on the second plates of individual capacitors in said array, a reset terminal connected to said clock generator, and an output terminal at which is produced an output signal indicative of whether the voltage at the input terminal is less or greater than a threshold voltage, said comparator means being operable in response to the second clock signals to cause the voltage at the input teminal and the second plate of the capacitor whose state is being sensed to be substantially equal to the threshold voltage; a reference capacitor having first and second plates of which the first plate is connected to the input terminal of said comparator means, said reference capacitor having a capacitance between minimum and maximum capacitance values of the capacitors in said array; and reset means connected to said clock generator and operable in response to the second clock signals to charge the second plate of said reference capacitor so that its voltage increases by an amount equal to the difference between the first and second predetermined voltages, said reset means further being operable in response to the first clock signals to discharge the second plate of said reference capacitor so that its voltage decreases by said difference, whereby upon occurrence of the first clock signals said comparator means produces an output signal sequentially indicative of whether the capacitances in said array are smaller or larger than that of the reference capacitor. - View Dependent Claims (10, 11, 12, 13, 14, 15, 16)
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17. A method for detecting the state of a variable capacitor by means of circuitry including a reference capacitor having a capacitance between the minimum and maximum capacitance values of the variable capacitor and comparator means which changes output states at an input threshold voltage, the method comprising the steps of:
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providing an inverter having input and output terminals, the input terminal being connected to first plates of the variable and reference capacitors; during a first interval, connecting the output terminal of the inverter to its input terminal so that the first plates of the variable and reference capacitors are charged to the threshold voltage of the inverter; during a subsequent interval, disconnecting the output terminal of the inverter from its input terminal and charging second plates of the variable and reference capacitors so that the voltages on the second plates change by equal amounts in opposite senses, whereby the voltage on the first plates of the variable and fixed capacitors is caused to be less or greater than the threshold voltage if the capacitance of the variable capacitor is smaller or greater respectively than the capacitance of the reference capacitor; and utilizing the output signal of the inverter during the subsequent interval to indicate the state of the variable capacitor. - View Dependent Claims (18, 19, 20, 21)
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Specification