| 摘要 |
PURPOSE:To output correct signals by reducing the changing quantity of the leak current of a switch which tuns on and turns off the feedback resistance of an operational amplifier. CONSTITUTION:In a photoelectric conversion circuit having two measurement ranges, resistances R21 and R22 dor feedback are connected between one input terminal and output terminal of an operational amplifier OP21 and a 1st switch SW21 is connected with the resistance R22. Then a 2nd switch SW22 is provided and one end of the switch SW22 is connected with a connecting point (a). The potential at the other end of the switch SW22 is made equal to that at the other input terminal of the amplifier OP21. Control signals are directly supplied to the SW22 and supplied to the SW21 through an inverter IC21 is that the switch SW22 can be turned off when the switch SW21 is turned on and vice versa. When such constitution is used, the switch SW22 is actuated and makes the voltage across both ends of the switch SW21 almost zero irrespective of the output of the amplifier OP21 at the time of turning off the switch SW21 and cutting off the resistance R22 from the amplifier OP21. Therefore, accurate signals can be outputted, because the changing quantity of the leak current of the SW21 is reduced and the zero point correction is not deviated. |
