ADIS16448SLASHPCBZ datasheet(19/24 Pages) AD

19 / 24 page

Data Sheet

ADIS16448

Rev. B | Page 19 of 24

CALIBRATION

The mechanical structure and assembly process of the ADIS16448

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 37), YGYRO_OFF (see Table 38),

and ZGYRO_OFF (see Table 39) registers provide user-

programmable bias adjustment function for the X-, Y-, and

Z-axis gyroscopes, respectively. Figure 21 illustrates that they

contain bias correction factors that adjust to the sensor data

immediately before it loads into the output register.

xGYRO_OFF

xACCL_OFF

MEMS

SENSOR

ADC

FACTORY

CALIBRATION

AND

FILTERING

xGYRO_OUT

xACCL_OUT

Figure 21. 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 37. XGYRO_OFF (Base Address = 0x1A), Read/Write

Bits

Description (Default = 0x0000)

[15:0]

X-axis, gyroscope offset correction factor,

twos complement, 0.01°/sec/LSB, 0°/sec = 0x0000

Table 38. YGYRO_OFF (Base Address = 0x1C), Read/Write

Bits

Description (Default = 0x0000)

[15:0]

Y-axis, gyroscope offset correction factor,

twos complement, 0.01°/sec/LSB, 0°/sec = 0x0000

Table 39. ZGYRO_OFF (Base Address = 0x1E), Read/Write

Bits

Description (Default = 0x0000)

[15:0]

Z-axis, gyroscope offset correction factor,

twos complement, 0.01°/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 37, Table 38, and Table 39), and

write the correction factors to the correction registers. For

example, lower the X-axis bias by 10 LSB (0.1°/sec) by setting

XGYRO_OFF = 0xFFF6 (DIN = 0x9BFF, 0x9AF6).

Single Command Bias Correction

GLOB_CMD[0] (see Table 25) 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,

together with the decimation filter (SMPL_PRD[12:8], see

Table 34), 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 0.01°/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 40), YACCL_OFF (see Table 41),

and ZACCL_OFF (see Table 42) registers provide user

programmable 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 21.

Table 40. XACCL_OFF (Base Address = 0x20), Read/Write

Bits

Description (Default = 0x0000)

[15:0]

X-axis, accelerometer offset correction factor,

twos complement, 1/1200 g/LSB, 0 g = 0x0000

Table 41. 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, 1/1200 g/LSB, 0 g = 0x0000

Table 42. 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, 1/1200 g/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

to this 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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