Parasitic Capacitances, Inductive Coupling, and High-Frequency Behavior of AMR Sensors

Anisotropic Magneto-Resistive (AMR) sensors are popular in sensitive applications, especially in planar on-chip integrated form which implies the presence of several on-chip capacitive and inductive parasitic elements. However, there is a lack of equivalent circuit models capturing these parasitics and allowing the modeling of sensors' behavior at higher frequencies or in stability-sensitive closed-loop operation. This work fills this gap in the literature by introducing a high-frequency equivalent circuit model, for planar on-chip AMR sensors with integrated offset and Set/Reset coils, capturing the main capacitive and inductive parasitic elements. A general methodology for estimating the values of the parasitic elements via a set of measurements is presented and it is applied to a particular commercial AMR sensor. Using the introduced circuit model, the transfer function from the offset coil voltage to the output voltage of the sensor, via the parasitic capacitive coupling path, is derived. The transfer function from the S/R coil voltage to the offset coil voltage is also derived. Both are used to create a system-level model of the AMR sensor which can be used in system-level analysis to help the designer predict and optimize the open or the closed-loop sensor circuit system behavior. The introduced high-frequency equivalent circuit model and the two related transfer functions have been verified experimentally using the component values derived from the examined commercial AMR sensor.