In this work the design and characterization of an omnidirectional in-plane magnetic field sensor are presented. The sensor is based on the Giant Magneto Impedance (GMI) effect in glass-coated amorphous microwires of composition (Fe6Co94)72.5Si12.5B15. For the first time, a circular loop made with a microwire is used for giving omnidirectional response. In order to estimate the GMI response of the circular loop we have used a theoretical model of GMI, determining the GMI response as the sum of longitudinal sections with different angles of incidence. As a consequence of the circular loop, the GMI ratio of the sensor is reduced to 15% instead of 100% for the axial GMI response of a microwire. The sensor response has been experimentally verified and the GMI response of the circular loop has been studied as function of the magnetic field, driven current, and frequency. First, we have measured the GMI response of a longitudinal microwire for different angles of incidence, covering the full range between the tangential and perpendicular directions to the microwire axis. Then, using these results, we have experimentally verified the decomposition of a microwire with circular shape as longitudinal segments with different angles of incidence. Finally, we have designed a signal conditioning circuit for the omnidirectional magnetic field sensor. The response of the sensor has been studied as a function of the amplitude of the incident magnetic field.