iC-TW8 16-BIT SIN/COS INTERPOLATOR
Serial Configuration Mode
Jan 9, 2013 Page SC35/36
1 Input Cycle
the unused harmonic. The design tool shows the re-
sultant angular error due to the distortion and the
residual error left after correction. The residual an-
gular error is due to the finite (8 bit) resolution of
the LUT.
SIN
COS
For example, if FFT analysis of a particular sensor
showed 2% second harmonic distortion at 0° phase
shift and 1% third order harmonic distortion at 180°
phase shift, the following angular error results.
360°
Sensor
Input
Angle
LUT
Output
Angle
0°
127
LUT
0
Values
–127
Figure 39: Constant LUT
A constant LUT is useful in single turn absolute ap-
plications such as synchronous AC motor commuta-
tion where the feedback signal must be aligned to
the motor poles. For example, to shift the interpola-
tor 0° position 3.8° to the right, the constant LUT
value is calculated as
Figure 40: Sensor Distortion
3.8 16384
The design tool shows that this sensor distortion
produces peak angular error of 1.88 input cycle de-
grees. After correction by the LUT, 0.04° (15.76 arc
minutes) of angular error remain.
43
360
4
More complex LUTs require careful FFT measure-
ment of sensor distortion. When sensor distortion
harmonics are known, the design tool can be used to
calculate a LUT that corrects for up to two sensor
harmonics which are identical on both Sin and Cos
input channels.
The GUI LUT tab in the design tool shown the cal-
culated LUT as a graph and LUT values. Save the
GUI LUT tab as a .csv file for direct import into the
GUI software. The LUT entries are also copied into
the GUI Hex tab of the design tool.
In the LUT Configuration tab of the design tool,
enter up to two sensor distortion harmonic to be cor-
rected. The harmonic number, amplitude in percent,
and phase in degrees are required. To correct for
only one distortion harmonic enter 0 amplitude for
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