iC-TW8 16-BIT SIN/COS INTERPOLATOR
Serial Configuration Mode
Jan 9, 2013 Page SC6/36
Digital Phase Correction
Sensor Distortion LUT
Lastly, phase correction is applied to the Sin and
Cos channels to guarantee 90° phase shift between
the two signals. The actual digital phase correction,
dphase, is determined by the DPH parameter in the
PAR block in internal memory. A range of ±53° in
steps of 0.056° is available.
An optional 64-byte lookup table (LUT) is provided
to allow correction of any remaining sensor distor-
tion (see Using the LUT on page 34). The LUT can
be bypassed if not required.
Filter
A filter is provided in the AB signal path that adds
two bits of resolution to the angle calculation (giv-
ing 16-bit angle resolution), reduces noise and jitter
in the AB outputs, and can reduce angle lag at con-
stant speed due to interpolator latency. The filter is
implemented as a PI servo loop with feedback path
delay as shown below.
Angle Calculation (Arctan)
The angle within an input cycle indicated by the
conditioned digital Sin and Cos signals is next cal-
culated as arctan(Sin/Cos) with a resolution of 14
bits using a CORDIC algorithm.
pgain
igain
1
s
From LUT
To Hysteresis
14
16
1
s
-
Delay
delay
Figure 4: AB Signal Path Filter
The filter parameters pgain, igain, and delay are
determined by variables in the MAIN_FLTR regis-
ter in the CFG block in internal memory. The
MAIN_FLTR register value necessary to realize a
given filter configuration is called its “instance”.
P Mode is the simplest filter mode and uses only
pgain (igain = delay = 0). In P mode, the signal path
filter configuration reduces to
pgain
Because of the performance tradeoffs related to the
selection of the filter parameters, it is helpful to de-
fine three filter modes to make parameter selection
easier. Experimentation may be necessary to deter-
mine optimal filter parameters.
1
s
-
Figure 5: P Mode Filter Configuration
iC-TW8 Filter Modes
Mode
Description
P
Low-pass filter with lag.
Critically-damped low-pass filter with
no filter lag.
Under-damped low-pass filter with no
filter lag and reduced latency lag.
In P mode, the pgain value determines the noise and
jitter bandwidth of the filter. Position lag at constant
sensor input velocity is directly proportional to the
velocity and inversely proportional to pgain. There
is no position or speed overshoot in P mode.
PI
Lag
Recovery
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