A capacitive sensor has a pair of sense capacitors with capacitances, the difference between which changes according to a physical quantity such as acceleration or pressure. One ends of the sense capacitors are connected together to provide a common terminal. Another ends of the sense capacitors are configured to provide separate detection terminals. The detection circuit applies a drive signal varying between two levels to the common terminal, converts the difference between the capacitances of the sense capacitors to a voltage, and outputs an electrical signal having a voltage value according to the physical quantity.
A detection circuit with a fully differential switched capacitor amplifier (sense amplifier) has been proposed as an example of this type of detection circuit (refer to a non-patent document 1). This detection circuit has a great effect on suppression of common-mode noise, is capable of reducing errors such as charge injection and clock feedthrough, and is capable of increasing amplitude of an output signal by using differential output.
Even when a fully differential amplifier is simply used in a detection circuit, a voltage at an input terminal varies largely according to a drive signal. A sense amplifier needs to have a wide input range for a variation in an input common-mode voltage. Further, influence of a mismatch of a parasitic capacitance at an input terminal of a C-V conversion circuit appears, and influence of an offset of a sense amplifier depending on an input common-mode voltage appears. Therefore, in the detection circuit disclosed in the non-patent document 1, a common-mode feedback loop is added to stabilize an input common-mode voltage to a desired constant voltage.
In another configuration to stabilize an input common-mode voltage, one end of a compensation capacitor is connected to a detection terminal, and an inversion signal opposite in phase to a drive signal is applied to the other end (refer to a patent document 1). In practice, even in this configuration, a common-mode feedback loop is necessary to prepare for a difference in capacitance between a sense capacitor and a compensation capacitor due to, for example, manufacturing variation, aging degradation, and temperature drift.
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