iC-TW8 16-BIT SIN/COS INTERPOLATOR
Pin Configuration Mode
Jan 9, 2013 Page PC12/15
Auto Calibration
iC-TW8 Rotary Encoder Design Tool
Once the iC-TW8 has been configured using the
configuration inputs, the signal path must be cali-
brated to determine proper values for gain, offset
correction, gain match, and phase correction. This is
most easily done using the Auto Calibration feature
of the iC-TW8 to automatically determine optimum
values for these parameters.
A design tool in the form of an Excel spreadsheet is
available to simplify configuration of the iC-TW8 in
rotary encoder applications. Only 10 values (in en-
gineering units) are required to completely specify
the desired iC-TW8 configuration. The required
configuration resistors values and performance lim-
its and graphs are then available for implementation
and characterization of the encoder.
Auto Calibration is initiated by pulling CALIB (pin
5) low. A pushbutton switch connected to CALIB
(as shown in Figure 2 on page 3) is an easy way to
achieve this in series production. With CALIB held
low, provide sensor input of a few hundred Sin/Cos
cycles and the iC-TW8 “tunes” the analog gain, ana-
log offset correction, digital offset correction, digital
gain match, and digital phase correction parameters
to provide lowest error and jitter in the interpolated
AB outputs.
In the design tool spreadsheet, cells with a blue
background are used for user entry. Start by entering
the operating voltage (Vdd) as 5.0 or 3.3 and the
frequency of the crystal you wish to use. The operat-
ing voltage and crystal frequency affect the current
drawn by the iC-TW8, which is calculated and
shown for reference. The crystal frequency also af-
fects the maximum speed and the filter response. In
general, use the highest possible frequency unless
there is a need to minimize current.
The sensor input used for auto calibration does not
need to be at a constant frequency nor must it be
unidirectional. A rotary encoder can be calibrated by
moving the disc or wheel back and forth a few revo-
lutions; a linear encoder by moving the sensor back
and forth on the scale by a few centimeters.
Continue by entering the resolution of the magnetic
wheel or optical disc. This value is the number of
Sin/Cos cycles per revolution of the wheel or disc
produced by the sensor connected to the iC-TW8.
Next enter the desired output resolution of the en-
coder as the number of AB cycles per revolution
(CPR). This is also sometimes called PPR (pulses
per revolution) and is equivalent to the number of
“lines” in a non-interpolated optical encoder.
After providing sufficient input signals, release the
CALIB input (it is pulled high by an internal pull-up
resistor) and the iC-TW8 immediately stores all cal-
ibration parameters in the external EEPROM. These
auto-calibrated values are then used whenever the
iC-TW8 is powered up.
With these values entered, the design tool calculates
and displays the required interpolation factor. Inter-
polation factors not available with the iC-TW8 (or
not selectable in pin-configuration mode) are
flagged by a warning in red “Output resolution not
possible with given input resolution!” In general, the
output resolution in AB edges per revolution divid-
ed by the input resolution must be an integer.
Next, enter the desired maximum speed (speed lim-
it) for the encoder in RPM. The spreadsheet calcu-
lates and displays the closest speed limit that can be
implemented using the iC-TW8 in pin-configuration
mode. The calculated actual speed limit is shown in
RPM, AB channel frequency, AB edge frequency,
and as minimum time between edges. Exceeding the
actual speed limit activates the iC-TW8’s FAULT
output (pin 20).
For an enclosed encoder the maximum speed is usu-
ally dictated by the bearings used. For a modular
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