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This paper presents a CMOS integrated circuit (IC) for an orthogonal fluxgate sensor that is suitable for weak magnetic field measurement. The IC chip includes both driving and readout circuits. The driving circuit is able to deliver a pulse current up to 80 mA to drive the wires in the sensor core. The readout circuit employs a flux lock loop to accurately set the operating point of the sensor. A low-pass notch filter determines the bandwidth of the sensing system and rejects the 50-Hz interference. The measured total noise in the pass-band of 0.5-16 Hz is about 2.2 μV. A minimum detectable magnetic field of 1.7×10-9 T without using the averaging has been achieved in the unshielded environment.