Analysis on the Zero Offset of a Microampere DC Current Sensor Using the Open-Loop Magnetic Modulator and the Suppression

In a wide range of electrical applications, the requirement for noncontact measurement of weak direct currents is continuously increasing. Among all types of dc current sensors, the magnetic modulator (fluxgate) is more suitable for measuring weak dc currents due to its low noise and high stability. As the most common type, the open-loop magnetic modulator has made possible a current down to 1 mA to be measured, while the further development could be hardly made due to the zero offset. In this article, it is revealed that the difference in two-cores’ magnetic characteristics is the main root cause for the zero offset. With the subtraction loop used to describe this difference, keeping this loop symmetric could greatly weaken the zero offset. On this basis, a controlled-greed pairing strategy is proposed to control the zero offset at a rather low level by screening out two cores with similar magnetic offsets to form one magnetic modulator. Furthermore, the pairing experiment of 10 magnetic cores and the application case of 30 magnetic modulators are conducted. The results show that the pairing scheme obtained with the proposed strategy is better than the random pairing scheme in 98.863% of cases, and the average of zero offset decreases from 131.27 to $19.50 ~\mu \text{A}$ in mass production.