| 摘要 |
PURPOSE:To automatically calibrate the offset voltage of a photometry circuit by setting the reference value of an operational amplification means according to output of the amplification means during the charging of a capacitor for photometry through a switching circuit. CONSTITUTION:While photometering is conducted, the transistor as switching element in a photometry circuit is turned ON with a control circuit 8 and causes a capacitor 2 for accumulating charge corresponding to photometering to be discharged while making terminals 6b and 6c of an analog switch 6 non-conducting. Then, the offset voltage of the photometry circuit due to a transistor 3, amplifier 4 and the like as outputted from the amplifier 4 for amplifying the accumulated charge of the capacitor is fed to the circuit 8 through an operational amplifier 5. Consequently, the set reference value of the amplifier 5 changes according to the offset voltage with the circuit 8, a D/A converter 7 and the like and thus, the offset voltage of the photometry circuit is automatically calibrated not affected by the level of the offset voltage thereby accomplishing the photometry without offset errors. |
