Capacitive sensor circuit
First Claim
1. A capacitive sensor circuit comprising:
- a sensor electrode (16) forming a first plate of a sensing capacitor (14);
a first inverting amplifier (2), having an input (4) connected to the sensor electrode (16) and having an output (6);
a second amplifier (26), having an input (24) coupled to the output (6) of the first inverting amplifier (2) and having an output (32);
a feedback resistor (8) connected between the output (6) and the input (4) of the first inverting amplifier (2), wherein a series resistor (22) is connected between the output (6) of the first inverting amplifier (2) and the input (24) of the second amplifier (26), which series resistor (22) has a resistance value substantially equal to the resistance value of the feedback resistor (8);
a reference capacitor (30) is connected between the input (24) of the second amplifier (26) and a reference terminal (10), which reference capacitor (30) has a capacitance value substantially equal to a predetermined value of the capacitance of the capacitive sensor electrode (16) relative to the reference terminal (10); and
wherein the first inverting amplifier (2) and the second amplifier (26) are constructed as Schmitt triggers having substantially the same low and high input threshold voltages.
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Accused Products
Abstract
A capacitive sensor circuit is provided which comprises: a sensor electrode (16) forming a first plate of a sensing capacitor (14) relative to a reference terminal (10), a first (2) and a second (26) inverting Schmitt trigger having the same threshold voltages. A feedback resistor (8) provides feedback from the output (6) to the input (4) of the first trigger (2), which together with the sensing capacitor (14) forms an oscillator whose frequency depends on the magnitude of the sensing capacitor (14). The input (24) of the second trigger (26) is connected to the output of the first trigger (2) via a series resistor (22) and to the reference terminal (10) via a reference capacitor (30). The resistance values of the feedback resistor (8) and the series resistor (22) are equal. A d.c. signal will appear on the output (32) of the second trigger if the capacitance of the sensing capacitor (14) is smaller than the capacitance of the reference capacitor (30) and an a.c. signal in the opposite case.
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Citations
10 Claims
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1. A capacitive sensor circuit comprising:
- a sensor electrode (16) forming a first plate of a sensing capacitor (14);
a first inverting amplifier (2), having an input (4) connected to the sensor electrode (16) and having an output (6);
a second amplifier (26), having an input (24) coupled to the output (6) of the first inverting amplifier (2) and having an output (32);
a feedback resistor (8) connected between the output (6) and the input (4) of the first inverting amplifier (2), wherein a series resistor (22) is connected between the output (6) of the first inverting amplifier (2) and the input (24) of the second amplifier (26), which series resistor (22) has a resistance value substantially equal to the resistance value of the feedback resistor (8);
a reference capacitor (30) is connected between the input (24) of the second amplifier (26) and a reference terminal (10), which reference capacitor (30) has a capacitance value substantially equal to a predetermined value of the capacitance of the capacitive sensor electrode (16) relative to the reference terminal (10); and
wherein the first inverting amplifier (2) and the second amplifier (26) are constructed as Schmitt triggers having substantially the same low and high input threshold voltages. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10)
- a sensor electrode (16) forming a first plate of a sensing capacitor (14);
Specification