| 摘要 |
PURPOSE:To reduce the influence of the DC offset voltage error of an A/D converter upon focusing operation by controlling DC bias potential by an offset value detected in a light shielded state, and supplying the offset value of the controlled DC bias potential to a bias control means by a closed loop. CONSTITUTION:A microcomputer 50 is turned into an offset voltage detection mode immediately after power supply is applied, and controls an iris motor drive circuit 62, and turns a diaphragm mechanism 60 into a perfect closed state, and changes PWM output from the microcomputer 50 so that the offset value comes into a prescribed range by making a focusing evaluation value obtained from an evaluation value generation circuit 45 the offset value. The PWM output is held by a voltage hold circuit 44 through a detection circuit 41, and simultaneously, the bias potential is changed by a subtracter 22. Namely, a system forms the closed loop until the offset value after PWM modulation becomes within the prescribed range, and the PWM output is adjusted automatically. After adjustment is finished, the system turns into a usual state, and the PWM output comes not to be outputted. |
