PURPOSE: To reduce loopbacked noise detoured from a signal earth circuit by connecting other terminal of a capacitor whose one terminal connects to a noninverting input terminal of an operational amplifier to a power supply circuit of the operational amplifier.

CONSTITUTION: Resistors R1, R2 in the circuit comprising an operational amplifier 1 are connected in series between an input terminal and a noninverting input terminal of the amplifier 1 and a capacitor C1 is connected between the connecting point of the resistors R1, R2 and an output terminal of the amplifier 1. One terminal of a capacitor C2 connects to a noninverting input terminal of the amplifier 1, and the other terminal connects to a power supply ground terminal of the amplifier 1, not to a signal ground terminal. Thus, the noise generated in a sampling system circuit detoured from the signal ground terminal is prevented and the loopbacked noise as a pre-stage low pass filter is reduced.