Abstract: Gain switching is performed by a DC amplifying circuit. The DC amplifying circuit is provided with individual gain generating circuits and a gain selecting circuit, and saturation preventing circuits are provided in earlier stages than the individual gain generating circuits. The individual gain generating circuit generates a gain G1, the individual gain generating circuit generates a gain G2 (where G2>G1), and the individual gain generating circuit generates a gain G3 (where G3>G2). The gain selecting circuit selects, as a used gain generating circuit, one of the individual gain generating circuits, and sends the output thereof to an A/D converting circuit in a later stage.

Abstract: A noise eliminating circuit is provided between a sample hold portion and an A/D converting portion. In the noise eliminating circuit, a DC flow rate signal is sent from the sample hold portion through a buffer to one input terminal of an operational amplifier as a first input signal, and through a high-pass filter and a buffer to the other input terminal of the operational amplifier as a second input signal. The high-pass filter passes, as a noise component, a signal of higher than a specific frequency included in the DC flow rate signal. The operational amplifier calculates the difference between the first input signal (the flow rate signal component+the noise component) and the second input signal (the noise component), and outputs this difference (the flow rate signal component) to the A/D converting portion as the DC flow rate signal.

Abstract: A noise eliminating circuit is provided between a sample hold portion and an A/D converting portion. In the noise eliminating circuit, a DC flow rate signal is sent from the sample hold portion through a buffer to one input terminal of an operational amplifier as a first input signal, and through a high-pass filter and a buffer to the other input terminal of the operational amplifier as a second input signal. The high-pass filter passes, as a noise component, a signal of higher than a specific frequency included in the DC flow rate signal. The operational amplifier calculates the difference between the first input signal (the flow rate signal component+the noise component) and the second input signal (the noise component), and outputs this difference (the flow rate signal component) to the A/D converting portion as the DC flow rate signal.

Abstract: Gain switching is performed by a DC amplifying circuit. The DC amplifying circuit is provided with individual gain generating circuits and a gain selecting circuit, and saturation preventing circuits are provided in earlier stages than the individual gain generating circuits. The individual gain generating circuit generates a gain G1, the individual gain generating circuit generates a gain G2 (where G2>G1), and the individual gain generating circuit generates a gain G3 (where G3>G2). The gain selecting circuit selects, as a used gain generating circuit, one of the individual gain generating circuits, and sends the output thereof to an A/D converting circuit in a later stage.