| 摘要 |
PURPOSE:To enable the time of an A/D conversion of fall in a desired range by comparing a photometric output with a reference voltage and changing cover a measuring range depending on the result of a comparison. CONSTITUTION:A photoelectric transducer SPD converts its photometric quantity to electric signals. An operational amplifier OPA and a negative feed back capacitor C1 constitute an integrating circuit. A comparator COP compares the integrated output of the amplifier OPA and a reference voltage Vr with each other. At the same time, the integrating circuit conducts an integration for a prescribed time and, when the integrated output of the amplifier OPA exceeds the reference voltage Vr, outputs a signal to a changeover circuit X to make capacitors C1 and C2 parallel and change over a range to 'high'. When an integration time has elapsed, the capacitors C1 and C2 are discharged by a signal from a microcomputer K and by a constant current from a constant voltage source V5 and a comparator COP' detects that the integrated output of the amplifier OPA becomes zero. Thus, the discharge time of the integrating circuit are counted by clock pulses whereby a photometric value is calculated.
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