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Scientific instruments for challenging and cost-optimized space missions have to reduce their resource requirements while keeping the high performance levels of conventional instruments. In this context the development of an instrument front-end ASIC (0.35 mum CMOS from austriamicrosystems) for magnetic field sensors based on the fluxgate principle was undertaken. It is based on the combination of the conventional readout electronics of a fluxgate magnetometer with the control loop of a sigma-delta modulator for a direct digitization of the magnetic field. The analogue part is based on a modified 2-2 cascaded sigma-delta modulator. The digital part includes a primary (128 Hz output) and secondary decimation filter (2, 4, 8,..., 64 Hz output) as well as a serial synchronous interface. The chip area is 20 mm2 and the total power consumption is 60 mW. It has been demonstrated that the overall functionality and performance of the magnetometer front-end ASIC (MFA) is sufficient for scientific applications in space. Noise performance (SNR of 89 dB with a bandwidth of 30 Hz) and offset stability ( < 5 pT deg C-1 MFA temperature, < +/-0.2 nT within 250 h) are very satisfying and the linear gain drift of 60 ppm deg C-1 is acceptable. Only a cross-tone phenomenon must be avoided in future designs even though it is possible to mitigate the effect to a level that is tolerable. The MFA stays within its parameters up to 170 krad of total ionizing dose and it keeps full functionality up to more than 300 krad. The threshold for latch-ups is 14 MeV cm2 mg-1.