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Abstract

An apparatus and method are provided for reducing the effects that common mode currents and/or voltages have on the measurements made by instrumentation that senses physical phenomena. The apparatus includes a cell having a first sensor, a second sensor and a passageway for containing the fluid to be measured. The cell, sensors and passageway are symmetrically disposed about an axis. Due to this symmetry, a ground can be located at the axis of symmetry. The apparatus also includes first and second measuring circuits associated, respectively, with the first and second sensors. The first measuring circuit produces a signal respresentative of the measured parameter between the ground and the first sensor. The second measuring circuit provides a like function for the second sensor. The signals produced by the first and second measuring circuits are summed by a potentiometer. If a common mode voltage or current is present, it enters the circuitry through the ground located at the axis of symmetry and affects the signals generated by both the first and second measuring circuits. Due to the complementary nature of the apparatus, the common mode signal affecting the first measuring circuit is of an opposite polarity from that affecting the second measuring circuit. Due to this opposite polarity, the common mode signal can be reduced when the signals are summed at the potentiometer.

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