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Data Sheet - D e t a i l e d D e s c r i p t i o n
The angular displacement (
Θ) of the magnetic source with reference to the Hall sensor array may then be modelled by:
Θ = arctan
The ±0.5º angular error assumes a magnet optimally aligned over the center of the die and is a result of gain mismatch errors of the AS5140H.
Placement tolerances of the die within the package are ±0.235mm in X and Y direction, using a reference point of the edge of pin #1 (Figure 21).
In order to neglect the influence of external disturbing magnetic fields, a robust differential sampling and ratiometric calculation algorithm has
been implemented. The differential sampling of the sine and cosine vectors removes any common mode error due to DC components introduced
by the magnetic source itself or external disturbing magnetic fields. A ratiometric division of the sine and cosine vectors removes the need for an
accurate absolute magnitude of the magnetic field and thus accurate Z-axis alignment of the magnetic source.
The recommended differential input range of the magnetic field strength (B(X1-X2),B(Y1-Y2)) is ±75mT at the surface of the die. In addition to this
range, an additional offset of ±5mT, caused by unwanted external stray fields is allowed. The chip will continue to operate, but with degraded
output linearity, if the signal field strength is outside the recommended range. Too strong magnetic fields will introduce errors due to saturation
effects in the internal preamplifiers. Too weak magnetic fields will introduce errors due to noise becoming more dominant.
7.11 Failure Diagnostics
The AS5140H also offers several diagnostic and failure detection features, which are discussed in detail further in the document.
7.11.1 Magnetic Field Strength Diagnosis
By Software: The MagINCn and MagDECn status bits will both be high when the magnetic field is out of range.
By Hardware: Pins #1 (MagINCn) and #2 (MagDECn) are open-drain outputs and will both be turned on (= low with external pull-up resistor)
when the magnetic field is out of range. If only one of the outputs is low, the magnet is either moving towards the chip (MagINCn) or away from
the chip (MagDECn).
7.11.2 Power Supply Failure Detection
By Software: If the power supply to the AS5140H is interrupted, the digital data read by the SSI will be all “0”s. Data is only valid, when bit OCF
is high, hence a data stream with all “0”s is invalid. To ensure adequate low levels in the failure case, a pull-down resistor (~10k
Ω) should be
added between pin DO and VSS at the receiving side.
By Hardware: The MagINCn and MagDECn pins are open drain outputs and require external pull-up resistors. In normal operation, these pins
are high ohmic and the outputs are high (see Table 16). In a failure case, either when the magnetic field is out of range or the power supply is
missing, these outputs will become low. To ensure adequate low levels in case of a broken power supply to the AS5140H, the pull-up resistors
Ω) from each pin must be connected to the positive supply at pin 16 (VDD5V).
By Hardware - PWM Output: The PWM output is a constant stream of pulses with 1kHz repetition frequency. In case of power loss, these
pulses are missing.
By Hardware - Incremental Outputs: In normal operation, pins A(#3), B(#4) and Index (#6) will never be high at the same time, as Index is only
high when A=B=low. However, after a power-on-reset, if VDD is powered up or restarts after a power supply interruption, all three outputs will
remain in high state until pin CSn is pulled low. If CSn is already tied to VSS during power-up, the incremental outputs will all be high until the
internal offset compensation is finished (within tPwrUp).
7.12 Angular Output Tolerances
Accuracy is defined as the error between the measured angle and the actual angle. It is influenced by several factors:
The non-linearity of the analog-digital converters
Internal gain and mismatch errors
Non-linearity due to misalignment of the magnet
As a sum of all these errors, the accuracy with centered magnet = (Errmax – Errmin)/2 is specified as better than ±0.5 degrees @ 25ºC (see
Figure 23). Misalignment of the magnet further reduces the accuracy. Figure 22 shows an example of a 3D-graph displaying non-linearity over
XY-misalignment. The center of the square XY-area corresponds to a centered magnet (see dot in the center of the graph). The X- and Y- axis
extends to a misalignment of ±1mm in both directions. The total misalignment area of the graph covers a square of 2x2 mm (79x79mil) with a
step size of 100µm. For each misalignment step, the measurement as shown in Figure 23 is repeated and the accuracy (Errmax – Errmin)/2 (e.g.
0.25º in Figure 23) is entered as the Z-axis in the 3D-graph.