The paper describes a differential output magnetic field sensor based on a current-splitting m.o.s.f.e.t. structure with two drain contacts. The sensor is integrated onto silicon with two n.m.o.s. voltage controlled oscillators and a temperature sensor, using the standard Southampton n.m.o.s. process. An input to the chip provides for offset compensation. The oscillators are driven by the sensor outputs, so that it is the difference in frequency which is proportional to magnetic field. Pulses from the oscillators may be counted directly into a register by a microcomputer providing a simple form of A to D conversion suitable for most applications. The microcomputer may also linearize the sensor output and provide temperature compensation using a separate input from the temperature sensor. In its simplest form the complete transducer may consist of two chips, the sensor and a single chip microcomputer, providing a serial digital output and mounted in a compact form on a thick film hybrid circuit.