Magnetic flow meter

Claim 1

In a magnetic flow meter for measuring the rate of flow of a liquid flowing through a conduit, wherein a magnetic alternating flux is generated almost normal to the direction of flow of the liquid through the conduit, and a voltage is measured over two measurement electrodes which are located on an axis almost normal to the direction of the flux and to the direction of the flow of the liquid, an electronic circuit which comprises a preamplifier with high common mode rejection ratio and high input impedance, comprising two identically connected operational amplifiers having different inputs, each feeding one of the differential inputs of a third operational amplifier, two identical impedances connected in series between the inverting inputs of the first two operational amplifiers, a fourth operational amplifier connected as a voltage follower with its input connected to the node of the said two impedances and its output connected through two pairs of series resistors to the corresponding non-inverting inputs of the two first mentioned operational amplifiers, the nodes of said pairs of series resistors being connected through further resistors to the output of the corresponding operational amplifiers, the input terminals of the pre-amplifier being formed by the said non-inverting inputs of the two first mentioned operational amplifiers and the output of the pre-amplifier being formed by the output of the third operational amplifier.