Conventionally, an analog signal collecting device is used, which utilizes an analog insulation multiplexer that insulates respective analog signal points from a ground for eliminating influences of the ground of a signal source and for collecting analog signals at low cost.
FIG. 6 is a structural diagram of an analog signal collecting device equipped with a conventional analog insulation multiplexer. Further, FIG. 7 is a circuit structural diagram of the conventional analog insulation multiplexer. In the analog insulation multiplexer shown in FIG. 7, it is necessary to apply an analog signal chopped by a FET switch FET2 (corresponding to second switching element) to a primary side of an analog signal insulation transformer T2 for inputting the analog signal insulated from the ground.
Therefore, in order to drive the FET switch FET2, a method is adopted in which a primary side of a drive insulation transformer T1 is controlled by another FET switch FET1 (corresponding to first switching element) concerning on and off of current (see Patent Document 1, for instance). Patent Document 1 describes various types of structures of the FET switch FET2 on the primary side of the analog signal insulation transformer T2, and it also describes that the drive insulation transformer T1 is driven by pulses.
In addition, as a circuit for driving the drive insulation transformer T1, a method is proposed in which a diode and a coil are combined so that drive time is decreased in accordance with data for reducing sag (see Patent Document 2, for instance).    Patent Document 1: JP 60-10449 B    Patent Document 2: JP 63-158911 A