ADIS16445 Data Sheet
Rev. F | Page 18 of 22
CALIBRATION
The mechanical structure and assembly process of the ADIS16445
provide excellent position and alignment stability for each sensor,
even after subjected to temperature cycles, shock, vibration, and
other environmental conditions. The factory calibration includes a
dynamic characterization of each gyroscope and accelerometer over
temperature and generates sensor specific correction formulas.
GYROSCOPES
The XGYRO_OFF (see Table 30), YGYRO_OFF (see Table 31),
and ZGYRO_OFF (see Table 32) registers provide user-
programmable bias adjustment function for the x-, y-, and
z-axis gyroscopes, respectively. Figure 20 illustrates that they con-
tain bias correction factors that adjust to the sensor data imme-
diately before it loads into the output register.
xGYRO_OFF
xACCL_OFF
MEMS
SENSOR
ADC
FAC
T
ORY
CALIBRA
TION
AND
FIL
TERING
xGYRO_OUT
xACCL_OUT
1
1051-020
Figure 20. User Calibration, Gyroscopes, and Accelerometers
Gyroscope Bias Error Estimation
Any system level calibration function must start with an estimate
of the bias errors, which typically comes from a sample of gyro-
scope output data, when the device is not in motion. The sample
size of data depends on the accuracy goals. Figure 7 provides a
trade-off relationship between averaging time and the expected
accuracy of a bias measurement. Vibration, thermal gradients,
and power supply instability can influence the accuracy of this
process.
Table 30. XGYRO_OFF (Base Address = 0x1A), Read/Write
Bits Description (Default = 0x0000)
[15:0] X-axis, gyroscope offset correction factor,
twos complement, 0.0025°/sec/LSB, 0°/sec = 0x0000
Table 31. YGYRO_OFF (Base Address = 0x1C), Read/Write
Bits Description (Default = 0x0000)
[15:0] Y-axis, gyroscope offset correction factor,
twos complement, 0.0025°/sec/LSB, 0°/sec = 0x0000
Table 32. ZGYRO_OFF (Base Address = 0x1E), Read/Write
Bits Description (Default = 0x0000)
[15:0] Z-axis, gyroscope offset correction factor,
twos complement, 0.0025°/sec/LSB, 0°/sec = 0x0000
Gyroscope Bias Correction Factors
When the bias estimate is complete, multiply the estimate by
−1 to change its polarity, convert it into digital format for the
offset correction registers (see Table 30, Table 31, and Table 32),
and write the correction factors to the correction registers. For
example, lower the x-axis bias by 10 LSB (0.025°/sec) by setting
XGYRO_OFF = 0x1FF6 (DIN = 0x9B1F, 0x9AF6).
Single Command Bias Correction
GLOB_CMD[0] (see Table 19) loads the xGYRO_OFF registers
with the values that are the opposite of the values that are in
xGYRO_OUT, at the time of initiation. Use this command, to-
gether with the decimation filter (SMPL_PRD[12:8], see Table 28),
to automatically average the gyroscope data and improve the
accuracy of this function, as follows:
1. Set SENS_AVG[10:8] = 001 (DIN = 0xB901) to optimize
the xGYRO_OUT sensitivity to 0.0025°/sec/LSB.
2. Set SMPL_PRD[12:8] = 0x10 (DIN = 0xB710) to set the
decimation rate to 65,536 (2
16
), which provides an averaging
time of 80 seconds (65,536 ÷ 819.2 SPS).
3. Wait for 80 seconds while keeping the device motionless.
4. Set GLOB_CMD[0] = 1 (DIN = 0xBE01) and wait for the
time it takes to perform the flash memory backup.
ACCELEROMETERS
The XACCL_OFF (see Table 33), YACCL _OFF (see Table 34),
and ZACCL_OFF (see Table 35) registers provide user pro-
grammable bias adjustment function for the x-, y-, and
z-axis accelerometers, respectively. These registers adjust the
accelerometer data in the same manner as XGYRO_OFF in
Figure 20.
Table 33. XACCL_OFF (Base Address = 0x20), Read/Write
Bits Description (Default = 0x0000)
[15:0] X-axis, accelerometer offset correction factor,
twos complement, 0.25 mg/LSB, 0 g = 0x0000
Table 34. YACCL_OFF (Base Address = 0x22), Read/Write
Bits Description (Default = 0x0000)
[15:14] Not used
[13:0] Y-axis, accelerometer offset correction factor,
twos complement, 0.25 mg/LSB, 0 g = 0x0000
Table 35. ZACCL_OFF (Base Address = 0x24), Read/Write
Bits Description (Default = 0x0000)
[15:14] Not used
[13:0] Z-axis, accelerometer offset correction factor,
twos complement, 0.25 mg/LSB, 0 g = 0x0000
Accelerometer Bias Error Estimation
Under static conditions, orient each accelerometer in positions
where the response to gravity is predictable. A common approach
is to measure the response of each accelerometer when they are
oriented in peak response positions, that is, where ±1 g is the
ideal measurement position. Next, average the +1 g and −1 g
accelerometer measurements together to estimate the residual
bias error. Using more points in the rotation can improve the
accuracy of the response.
