| 摘要 |
PURPOSE:To prevent an FET from breaking down against a large in-phase noise by using an enhancement FET as the electronic switch of a scanner circuit, and holding the gate potential of the FET turned on invariably without reference to whether an input signal is positive or negative. CONSTITUTION:Two-wire bipolar analog input signal A of plural channels are inputted to a scanner circuit successively on time-division basis through insulating transformers T1, provided for respective CHs, and an electronic switch X to perform A/D conversion. The switch X uses an enhancement FET with normally off characteristics, and its gate electrode is connected to one terminal of the secondary winding of a pulse transformer T11 for turning-on pulse voltage appliation; and the parallel circuit of diodes D1 and D2 and resistances R1 and R2, and capacitors C1 and C2 is connected between the other terminal of the winding and an input circuit. Consequently, even when analog inputsIand II are inverted in polarity, no voltage greater than forward voltage drops across the D1 and D2 is applied to the gate of the FETX and even if an in-phase noise is applied to the analog inputs, no excessive voltage is applied among the source, drain and gate of the FET. |
