8 Rotary angle example with an AMR sensor

The AMR sensor comprises four magnetoresistance Wheatstone bridges, where the voltage differences of two bridges’ output terminals will reflect the external magnetic field magnitude.

Compared to Hall-effect sensors, AMR sensors have higher frequency operation and a higher signal-to-noise ratio (SNR). Compared to GMR and TMR sensors, AMR sensors have a relatively negligible orthogonality error. In applications such as servo drives that need a high-accuracy encoder, AMR sensors are often preferable given their higher magnetic field tolerance, yielding overall better immunity.

The TMAG6180-Q1 2D AMR angle sensor measures magnetic fields and produces two differential (or single-ended) voltage outputs proportional to those magnetic fields. The <2µs latency of the TMAG6180-Q1 also minimizes angle errors caused by high-speed movement. Integrated Hall-effect switches produce two digital quadrant outputs (Q0 and Q1), thus extending the angle detection range to 360 degrees. Together with the sine and cosine waveforms, the Q0 and Q1 digital outputs are enough to determine the absolute rotary angle. Figure 4 is a functional block diagram of the TMAG6180-Q1, while Figure 5 shows the output waveforms.

For better accuracy, the MCU should integrate a high-speed, high-ENOB analog-to-digital converter, be able to run a digital filter such as a finite impulse response filter to eliminate signal-chain noise, and have an additional compensation algorithm to eliminate errors caused by mechanical tolerances and the signal chain’s gain and offset mismatch. The High-Resolution, Low-Latency, Compact Absolute Angle Encoder Reference Design with AMR Sensor is a small-form-factor (3cm diameter) reference design with the TMAG6180-Q1 and MSPM0G3507 MCU, with integrated dual 12-bit ADCs up to 128X oversampling and a math accelerator to help improve efficiency and reduce system cost. The system achieves an angle measurement with a 94.7dB SNR equivalent to 15.4 ENOB and an angle error below 0.05°, as shown in Figure 6.