PURPOSE: To reduce the power consumption and to reduce cost by making a reference voltage applied to a 1-bit D/A converter variable, thereby changing reference voltage in accordance with an input voltage.

CONSTITUTION: An analog input X is connected to one input of a subtractor 4. An output of the subtractor 4 connects to one input of a subtractor 4. An output of the subtractor 6 is connected to a 1-bit D/A converter 9 via an integration device 7. Furthermore, an output of the D/A converter 9 is respectively connected to the other input of the subtractors 4, 6. An output voltage of a variable reference voltage generating circuit 10a being reference voltages -Vref, +Vref are multiplied by 2XFs/Vref and the result is fed to the D/A converter 9, then the reference voltage of the D/A converter becomes -2XFS and +2XFS. Since the input level range is doubled, an optimum input level range is obtained.