| 摘要 |
PROBLEM TO BE SOLVED: To strongly inhibit common mode signals relatively simply and advantageously in terms of costs, and strongly inhibit low frequency components lower than a measurement frequency in a differential signal, by constituting a signal-processing circuit for a difference voltage in which a preamplifier is a differential preamplifier. SOLUTION: Asymmetry is generated between two signal paths because of an avoidable tolerance of two input coupling capacitors 5 and 6. The asymmetry forms a differential signal by a common mode signal. When the signal- processing circuit is used in relation to a magnetic induction type flowmeter, high common mode noises are always expected and an effect leading to the formation of the differential signal by the common mode signal should be inhibitted as much as possible. In other words, a preamplifier 1 has two operational amplifiers 7 and 8, and moreover includes a resistance circuit network 13 among input sides 9 and 10 of the operational amplifier 7, input sides 11 and 12 of the operational amplifier 8 and, input coupling capacitors 5 and 6, with two bootstrapped main resistors R1 and R1' belonging to the resistance circuit network 13, whereby the effect is eliminated.
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