In this paper, we present a low-power, two-axis fluxgate magnetometer. The planar sensor is integrated in a standard CMOS process, which provides metal layers for the coils and electronics for the signal extraction and processing. The ferromagnetic core is placed diagonally above the four excitation coils by a compatible photolithographic post process, performed on a whole wafer. The sensor works using the single-core principle, with a modulation technique to lower the noise and the offset at the output. In contrast to traditional fluxgate approaches, the sensor features a high degree of integration and minimal power consumption at 2.5 V of supply voltage that makes it suitable for portable applications. A novel digital feedback principle is integrated to linearize the sensor characteristics and to extend the linear working range.