  A detailed study about magnetic sensing techniques based on anisotropic
magnetoresistive sensors shows that the technology is suitable for
low-frequency space applications like the eLISA mission. Low noise magnetic
measurements at the sub-millihertz frequencies were taken by using different
electronic noise reduction techniques in the signal conditioning circuit. We
found that conventional modulation techniques reversing the sensor bridge
excitation do not reduce the potential $1/f$ noise of the magnetoresistors, so
alternative methods such as flipping and electro-magnetic feedback are
necessary. In addition, a low-frequency noise analysis of the signal
conditioning circuits has been performed in order to identify and minimize the
different main contributions from the overall noise. The results for chip-scale
magnetoresistances exhibit similar noise along the eLISA bandwidth ($0.1\,{\rm
mHz}-1\,{\rm Hz}$) to the noise measured by means of the voluminous fluxgate
magnetometers used in its precursor mission, known as LISA Pathfinder.
