Analog signal processing in an AC electromagnetic flowmeter

AC electromagnetic flowmeters yield a low-frequency signal whose amplitude is proportional to the average fluid velocity. Typically, the input signal magnitude is below 1 mV and suffers a strong electric and magnetic interference from the coils that create the driving magnetic field. This paper provides a novel circuit design to amplify and demodulate the voltage picked up by the electrodes and to minimize interference. The amplifier includes a fully differential front end. The demodulation consists of synchronous rectification and sampling with zero-order hold followed by low-pass filtering. The result is a 0.25-mV/(m/s)/mT voltage, insensitive to DC level shifts.