ADIS16445SLASHPCBZ datasheet(18/22 Pages) AD

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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

FACTORY

CALIBRATION

AND

FILTERING

xGYRO_OUT

xACCL_OUT

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 (216), 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.

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