An example of a capacitive sensor for detecting physical quantities (angular velocity, acceleration and the like) is a sensor that includes capacitance of a variable capacitance element for detection that varies depending on a physical quantity applied, and converts, with a capacitance-to-voltage conversion circuit, the amount of variation in the capacitance of the variable capacitance element into a voltage value (e.g., JP 2006-71498 A).
A certain capacitance-to-voltage conversion circuit included in the capacitive sensor uses a correlated double sampling to detect the amount of variation in the capacitance. In this type of capacitance-to-voltage conversion circuit, for example, one of input terminals of a differential amplifier is connected to a connecting point of two variable capacitance elements that are supplied alternately with voltages complementary with respect to a reference voltage, and a sample and hold circuit is connected subsequent to the differential amplifier. The capacitance-to-voltage conversion circuit once resets the two variable capacitance elements to the reference voltage, and after resetting, integrates a difference of the capacitances of the two variable capacitance elements as a difference in electric charge with the differential amplifier, to retrieve the result in the form of a voltage signal with the sample and hold circuit.
In the capacitance-to-voltage conversion circuit, it is known that a device noise occurs from various devices such as semiconductor devices and resistive elements that are disposed on the circuit. This device noise can include a white noise that is not dependent on a specific frequency and a 1/f noise whose noise intensity increases in a low-frequency bandwidth. Among those device noises, a device noise contained in the bandwidth of the differential amplifier of the capacitance-to-voltage conversion circuit undergoes a phenomenon (hereinafter, referred to as “noise returns”) in which it is processed in superposed manner for not only a device noise with a sample and hold sampling frequency, but also a device noise with a high-order frequency that can be sampled at a frequency (i.e., Nyquist frequency) that is half the sampling frequency, for example.
As a result, with the device noise processed in the superposed manner by the noise returns contained in an output signal of the capacitance-to-voltage conversion circuit, the output signal is outputted to a subsequent circuit (e.g., an analog-to-digital conversion circuit or a buffer amplifier).
In order to solve this, it may be possible to suppress the high-order frequency device noise by limiting the bandwidth in accordance with the sampling frequency. The band limitation can be performed, for example, by connecting a capacitance element for band limitation to an output terminal of the differential amplifier. However, this configuration may undesirably increase the processing time needed for the capacitance-to-voltage conversion and also increase power consumption.