This thesis describes the design of a CMOS magnetic field sensor system for measuring electronic currents. A hybrid system, using Hall plates and a pick-up loop is chosen to obtain improved performance compared to conventional Hall based magnetic field sensors. The pick up loop enables to reduce the current consumption and increases the bandwidth of the system. The design of the pick-up loop and readout amplifier are described in detail. Simulation results of the pick-up loop signal path offer a factor 10 increase in bandwidth using 24% less current compared to a system consisting of two Hall plate channels.