ADIS16133

Rev. 0 | Page 12 of 20

CALIBRATION

The ADIS16133 factory calibration produces correction formulas

for the gyroscope and programs them into the flash memory.

Table 19 contains a list of user control registers that provide

opportunity for user optimization after installation. Figure 19

illustrates the summing function of the sensor’s offset correction

register.

Table 19. Registers for User Calibration

Register

Address

Description

GYRO_OFF2

0x08

Gyro bias correction, lower 16 bits

GYRO_OFF

0x0A

Gyro bias correction, upper 16 bits

GLOB_CMD

0x28

Bias correction command

MEMS

GYRO

FACTORY

CALIBRATION

AND

FILTERING

ADC

GYRO_OUT GYRO_OUT2

GYRO_OFF GYRO_OFF2

Figure 19. Gyroscope Bias Calibration User Controls

The factory calibration addresses initial and temperature depen-

dent bias errors in the gyroscopes, but some environmental

conditions, such as temperature cycling and mechanical stress

on the package, can cause bias shifts in MEMS gyroscope struc-

tures. For systems that value absolute bias accuracy, there are

two options for optimizing absolute bias accuracy: autonull and

manual correction.

Automatic Bias Correction

Set GLOB_CMD[0] = 1 (DIN = 0xA801) to start the automatic

bias correction (ABC) function, which uses the following internal

sequence to calibrate each gyroscope for bias error:

1. Wait for a complete output data cycle to complete, which

includes the entire average and decimation time in the

DEC_RATE register.

2. Read the output registers of the gyroscope.

3. Multiply the measurement by −1 to change its polarity.

4. Write the final value into the offset registers.

5. Update the flash memory.

The Allan variance curve shown in Figure 7 provides a trade-off

between bias accuracy and averaging time. The DEC_RATE

register provides a user control for averaging time when using

the ABC function. Set DEC_RATE[7:0] = 0x10 (DIN = 0xA210),

averaging time of 64 seconds (65,536 ÷ 1024 SPS) for this function.

Then, set GLOB_CMD[0] = 1 (DIN = 0xA801), and keep the

platform stable for at least 65 seconds while the gyroscope bias

data accumulates.

After this completes, the ADIS16133 automatically updates the

flash memory. The SPI interface is inactive during the entire

time it takes the ABC function to complete. The only way to

interrupt the ABC function is to remove power or initiate a

hardware reset using the RST pin. When using DEC_RATE =

0x0010, the 1 σ accuracy for this correction is approximately

0.0025°/sec for the gyroscope correction factor. For further

optimization, use the manual bias correction function, with a

100 sec average for the bias estimate. See Table 29 for more

information on the GLOB_CMD register.

Manual Bias Correction

The GYRO_OFF and GYRO_OFF2 registers (see Table 20 and

Table 21) provide a bias adjustment function for the output of

each sensor. GYRO_OFF uses the same format as GYRO_OUT,

and GYRO_OFF2 uses the same format as GYRO_OUT2.

Table 20. GYRO_OFF Bit Descriptions

Bits

Description (Default = 0x0000)

[15:0]

Gyroscope offset correction; twos complement,

0.05°/sec per LSB, 0x0000 = 0°/sec

Table 21. GYRO_OFF2 Bit Descriptions

Bits

Description (Default = 0x0000)

[15:0]

Gyroscope offset correction, finer resolution; uses the

same format as GYRO_OUT2 (see Table 11)

Restoring Factory Calibration

Set GLOB_CMD[1] = 1 (DIN = 0xA802) to execute the factory

calibration restore function. The restore function resets each

user calibration register to 0x0000, resets all sensor data to 0,

and automatically updates the flash memory within 72 ms. See

Table 29 for more information on GLOB_CMD.