2 AMR Angle Sensing

The variation shown in Figure 1-3 is particularly helpful if the sensor is setup in the format of a Wheatstone bridge, shown in Figure 2-1. This particular configuration produces two parallel resistor dividers with elements physically orthogonal to each other. The variation in resistance results from the incident magnetic field producing a sinusoidal response as the field travels through a full rotation, balancing the structure with AMR elements with equally sized resistances along each axis.

Figure 2-1 Wheatstone Bridge Configuration

What is particularly noteworthy about the response of the sensing element is that the output varies with cos²Ɵ. As a result, a typical AMR sensor produces two output cycles per revolution and a single Wheatstone bridge only directly resolves a physical rotation of 90°.

Given the response of the sensor to the rotating magnetic field, creating an electrically orthogonal set of sinusoidal outputs by integrating a second Wheatstone bridge rotated 45° is possible.

Figure 2-2 Sine and Cosine Wheatstone Arrangement

Together, the outputs represent sine and cosine, and can then be used to calculate an arctangent with a full 360° response for every 180° of mechanical rotation.

TMAG6180-Q1 and TMAG6181-Q1 take advantage of this sensing technology by combining a fast, two-dimension, Hall-effect latch to create an AMR sensor capable of detecting quadrant and extending the sensing range from 180° to the full 360°. TMAG6180-Q1 generates Q0 and Q1 outputs which can calculate absolute angle. TMAG6181-Q1 provides a turns counter that can increment or decrement at each quadrant change. This counter tracks the relative angle from power-on and absolute angle is possible if the turns counter is always initialized within the same 180° range.

Figure 2-3 Simulated TMAG6180 Output Response

Automatic Gain Control (AGC) in both devices is set to produce outputs with a peak to peak amplitude of 0.6 × V_cc with the output centered at V_cc/2, irrespective of the input magnetic field strength. The sensor adjusts the applied gain to maintain a constant output level following Equation 2 when changes in the magnetic field produce vector magnitude drift.

The calculated signal typically swings between ±0.6 × Vcc when measured differentially. For example, when Vcc = 3.3 V, each output signal can vary between 0.66 V and 2.64 V, and when taken differentially the resulting signal ranges between ±1.98 V.