Direct Interface for Magnetoresistive Sensors

AMR and GMR magnetoresistive sensors are typically connected to form a Wheatstone bridge with four or two active sensing elements. The resistance change of each sensing element is proportional to the external magnetic field over a limited field range. The bridge output voltage is then linear for bridges with four active sensors and nonlinear for bridges with only two active sensors. This paper proposes an alternative method to directly connect magnetoresistive sensors to microcontrollers without placing any active component between them. The sensor bridge is considered to be a resistive network with three inputs and one output Using one input in turn to measure the discharge time of a capacitor connected to the bridge output, we obtain three different discharge times. We propose two equations that combine those three discharge times to obtain a digital output proportional to the fractional resistance change; one equation is used for sensors with four active sensing elements and the other equation is used for sensors with only two active sensing elements. Measurements on an emulated sensor bridge yield a nonlinearity error below 0.3%FSR and a resolution of 12 bit Measurements on commercial AMR and GMR sensors yield nonlinearity errors below 0.6%FSR and 2.5%FSR respectively, which are smaller than the specified nonlinearity error for each sensor alone.