Oscillator-Less Direct-Digital Front-End Realizing Ratiometric Linearization Schemes for TMR-Based Angle Sensor

Abstract

A simple oscillator-less direct-digital front end (ODF) for tunneling magnetoresistance (TMR) angle sensor is presented in this paper. The TMR sensor outputs vary as sine and cosine functions of the shaft angle to be measured. To these outputs, the ODF applies ratiometric-based techniques to obtain a linear output for the full-circle range. The first ratiometric linearization scheme (RLS-A) provides a linear output which is insensitive to the variation of sensor transformation constant. Alternatively, the second ratiometric linearization scheme (RLS-B) renders output with minimal nonlinearity (0.009%). The ODF does not require any sinusoidal oscillator for its operation, thereby eradicating one major error source. Furthermore, the proposed methodology uses dc excitation for the sensor. This makes the ODF insensitive to the sensor parasitic capacitances. The operation of the ODF and two linearizing schemes are explained in detail. Mathematical evaluation of the schemes is conducted to determine the effects of different sensor and circuit nonidealities. The static and dynamic performances of the ODF are studied using emulated TMR sensor outputs. The results establish a linear transfer-characteristic over 360° range. Tests are also conducted to quantify the effect of nonidealities like phase imbalance in sensor outputs and deviation in sensor transformation-constant. A close degree of agreement was observed between the emulation results and the mathematical analysis performed. Later, the ODF is interfaced and tested with a prototype TMR-based shaft-angle measurement unit. The interfacing results underline the fact that the developed ODF acts as an efficient linearizer for TMR angle sensors.