IC-TW8_16 Datasheet, PDF (56/67 Pages) IC-Haus GmbH – 16-BIT SIN/COS INTERPOLATOR

iC-TW8 16-BIT SIN/COS INTERPOLATOR
Serial Configuration Mode
Aug 16, 2016 Page SC25/36
Auto Calibration
Once the iC-TW8 has been configured, the signal
path must be calibrated 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. Al-
ternatively, initial parameter values may be written
to the parameter registers using the serial interface
and allowed to adapt. The adapted parameter values
must then be written to the EEPROM for use after
subsequent startups.
Auto Calibration Using the GUI
The iC-TW8 can also be auto calibrated using the
GUI software instead of the CALIB input. With the
GUI running and connected to the iC-TW8, make
sure the Enable Write Immediately checkbox (in the
lower left corner of the GUI screen) is checked so
that changes made in the GUI are immediately writ-
ten to the chip. In the Monitor tab, make sure the
ADC Quality Monitor Enable and Adapt Quality
Monitor Enable checkboxes are unchecked. This
disables the quality monitors so they do not interfere
with initial calibration.
Auto calibration is initiated by pulling CALIB (pin
5) to DVSS. A pushbutton switch connected be-
tween CALIB and circuit ground (as shown in Fig-
ure 8 on page 10) 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, analog offset
correction, digital offset correction, digital gain
match, and digital phase correction parameters to
provide lowest error and jitter in the interpolated AB
outputs.
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 by a few
revolutions; a linear encoder by moving the sensor
back and forth on the scale by a few centimeters.
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 for subsequent
startups (see page 15).
In the Signal Path tab, make sure the Enable Con-
tinuous Parameter Read checkbox in the Signal Path
pane is checked so that the parameter values can be
observed during auto calibration. In the Auto Adap-
tion pane, set the Adaption Gain Tolerance and
Adaption Offset Tolerance to their lowest values,
1.0005 (0x00) and 0.49 mV (0x00) respectively, to
ensure the most accurate calibration. Finally, check
the OFF/ON checkbox at the top of the Auto Adap-
tion pane to enable auto adaption of all parameters.
Now, 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. The sensor input used for auto-
calibration does not need to be at a constant fre-
quency nor must it be unidirectional. A rotary en-
coder can be calibrated by moving the disc or wheel
back and forth by a few revolutions; a linear encod-
er by moving the sensor back and forth on the scale
by a few centimeters.
After the parameters in the Signal Path pane have
converged, click the Write EEPROM button in the
lower right corner of the GUI screen to store all the
calibrated parameter values in the external
EEPROM. These auto-calibrated values are then
used for subsequent startups (see page 15).
After calibration, the auto adaption enable and toler-
ance parameters in the Auto Adaption pane of the
Signal Path tab may be set back to their desired op-
erational values. The quality monitors may also be
enabled and the thresholds and limits set as desired
in the Monitor tab.
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