| 摘要 |
PURPOSE:To attain automatic gain control to an amplitude level of an input analog signal by arranging a variable gain amplifier changing the gain of the input analog signal to the pre-stage of an A/D converter and supplying the converted analog signal to the variable gain circuit. CONSTITUTION:An analog signal coming to an input terminal Inp is amplified by a variable gain amplifier 11, converted into a digital signal by an A/D converter 6 and outputted to a signal processing circuit 7 and fed to a level detection circuit 12, where the amplitude level is detected. The difference of the detected signal with an output voltage, that is, a reference value of a reference level setting circuit 14 is amplified by a differential amplifier 13 and an output signal of the differential amplifier 13 is outputted to a D/A converter 15. An error signal converted into an analog signal is impressed to the variable gain amplifier 11 via an LPF 16. Thus, the amplification factor of the variable gain amplifier 11 is changed in response to the error signal and the gain control is attained so that the output of the level detection circuit 12 is equal to the reference value of the reference level setting circuit 14.
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