Circuit for suppressing common-mode voltages and electrode-disturbing DC voltages in connection with the evaluation of measuring voltages supplied by measuring electrodes of an inductive flow meter

摘要

1. A circuit arrangement for suppressing inphase voltages and electrode interference DC voltages which are emitted, in addition to measured value voltages of changing polarity, by measuring electrodes (11, 12) of an inductive flow meter operating with a magnetic field of periodically changing direction, in which each measuring electrode (11, 12) is connected to a non-inverting input of a respective operational amplifier (15, 16), the output of which is fedback to its other, inverting input via a resistor (18, 19), characterised in that the inverting inputs of the two operational amplifiers (15, 16) are connected to one another by a series connection (17, 24) of a resistor (17) and a capacitor (24), in that the feedback resistors (18, 19) have equal values (R1 = R'1 ), in that the resistor (17) in the series connection (17, 24) has a value R2 which is small in relation to the value (R1 , R'1 ) of the feedback resistors (18, 19), in that the outputs of the two operational amplifiers (15, 16) are connected to a voltage storing means (23) during periods (t1 , t2 ) in which the flux of the magnetic field between the electrodes (11, 12) is constant with changing direction of the magnetic field, and in that successively stored voltages are subtracted from one another in the voltage storing means (23) and the results of the substraction further processed to form the measured value signals.