PURPOSE: To perform AD conversion without being affected by the drift of a midpoint potential and the drift of the reference voltage of an AD converter by providing a signal processing circuit which outputs an input analog signal as a positive and a negative signal with the midpoint potential as a reference and the AD converter, and using the midpoint potential as the reference potential of the AD converter.
CONSTITUTION: A resistance bridge circuit is formed by using heat wires and reference resistors as its respective sides, the signal processing circuit 2 distributes a bridge output VO to the positive and negative sides with the midpoint potential VL as a reference, and the AD converter 3 inputs the output signal of the signal processing circuit 2 as a sensor signal VS and performs AD conversion. Then the midpoint potential VL of the signal processing circuit 2 which outputs he bridge output VO as the positive and negative signals is amplified by an operational amplifier, whose voltage is supplied as the reference voltage Vref of the AD converter 3. Even if the drift component of the midpoint potential is added to the sensor signal VS, the reference voltage Vref varies with it, so the AD-converted digital signal does not vary.
NISHIO TOMOYUKI
UNO TAKESHI
