Common-mode rejection amplifying system

Claim 1

An amplifying system responsive to a compound signal having a signal component and a common-mode noise component, said compound signal being derived from the electrodes of an electromagnetic flowmeter having an A-C energized electromagnet to establish an electromagnetic field across a flow tube whereby said compound signal is induced in said electrodes as A function of flow rate, said system comprising: a. first and second differential amplifier circuits each having first and second inputs and a single output, said first and second amplifier circuits each including a negative feedback impedance connected between the output and the second input thereof, b. means including a coupling impedance to connect the output of the second circuit to the second input of the first circuit, c. means to apply the compound signal to the first inputs of the first and second circuits to cause the signal component to appear at the input of the first circuit in summing relationship and the noise component to appear in subtractive relationship, said first and second circuits having impedance parameters to effect amplification of the signal component and to effect rejection of the noise component in the first circuit, and d. means coupled to the single output of said first circuit and independent of the single output of said second circuit to derive therefrom the amplified signal component substantially free of said noise component.