Hardware
9
SLYU048 – July 2018
Copyright © 2018, Texas Instruments Incorporated
DRV5055-ANGLE-EVM
3.2
Hall Sensor(s) and Analog MUX
The magnetic flux density signals generated by the rotating magnet are measured at four locations around
the magnet by the DRV5055 linear Hall effect sensors. The PCB includes both package variants of the
DRV5055. Each sensor measures the flux density at the sensor location, and outputs a proportional
voltage signal that swings between 600 mV and 3.1 V. The Hall effect sensor outputs are routed to a dual
2:1 low-distortion analog MUX (
). The MUX that selects individual or pairs of sensors for
single-sensor or dual-sensor modes of operation, respectively. Dual-sensor mode requires sin/cos
encoding for angle sensing over the full 360° range. Select any pair of sensors located on the same circle
or on the same radial line in order to generate the required sin/cos signal pairs.
3.3
Simultaneous Sampling ADCs
Data conversion of the Hall effect sensor outputs is accomplished by a pair of
SAR ADCs
configured for simultaneous sampling. Simultaneous sampling is important to prevent any additional phase
errors caused by skewed sampling instants that disrupt the sin/cos relationship of the dual-sensor outputs.
SAR ADCs generally have extremely low aperture delay (a few nanoseconds at most) because SAR
ADCs are specifically designed to convert instantaneous samples of the input signal, rather than window-
averaged values . As a result, SAR ADCs are an excellent choice for simultaneous-sampling applications.
The ADS7042 normally supports a maximum throughput of 1 MSPS. However, the settling characteristics
of the ADC input drive circuit used on this EVM limit the max throughput to approximately 300 kSPS. The
maximum sampling rate is still sufficient for noise reduction through oversampling and averaging in most
applications. The maximum sampling rate is also sufficient for acquiring relatively high-speed input signals
of up to 20 kHz; the approximate bandwidth of the DRV5055.
3.4
MCU
The MCU captures the ADC outputs and post-processes the digital data (that is, applies calibration,
normalization, and the decode function) to estimate the shaft angle. The EVM uses the
,
an ultra-low-power MCU. The MSP430FR4133 natively supports a 7-segment LCD drive, and includes two
SPI interfaces to control the dual ADCs. The MPS430FR4133 also includes an optimized floating point
math library (MSPMATHLIB) to efficiently evaluate inverse trigonometric functions. ASIN is used for
single-sensor decoding, and ATAN2 is used for dual-sensor decoding. The MSP430 programming
interface is also accessible for experimentation.
