| 摘要 |
PURPOSE:To detect simply and assuredly a fault of the filter characteristics of a signal selecting circuit which is caused by the frequency change of a clock signal or a sampling signal, by supplying the check signal of a fixed frequency produced by a system different from that for the clock signal for signal selecting circuit to a sampling circuit together with the signal to be selected and then monitoring the output level of the check signal which is outputted from the signal selecting circuit. CONSTITUTION:The selection outputs corresponding to input signals are obtained from signal selecting parts 51-5n in accordance with the frequency and the level of the input signal supplied to a terminal 1 as long as each circuit is nondefective. When the frequency of a clock signal CP or a sampling signal SP has a change, it is impossible for a signal selector to select accurately the input signal to the terminal 1 owing to variation of the filter characteristics of a signal selecting circuit 5. Thus the output level of a check signal selecting part 5x is reduced. Then a level monitor circuit 9 detects a fault of said filter characteristics.
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