ADXL314 datasheet(24/27 Pages) AD

24 / 27 page

Data Sheet

ADXL314

APPLICATIONS INFORMATION

analog.com

Rev. A | 24 of 27

OFFSET COMPENSATION

Accelerometers are mechanical structures containing elements that

are free to move. These moving parts can be sensitive to mechani-

cal stresses, much more so than solid state electronics.

The 0 g bias, or offset, is an important accelerometer metric be-

cause it defines the baseline for measuring acceleration. Additional

stresses can be applied during assembly of a system containing

an accelerometer. These stresses can come from, but are not

limited to, component soldering, board stress during mounting, and

application of any compounds on or over the component. If offset

compensation is deemed necessary, it is recommended that it be

performed after system assembly.

USING SELF TEST

The self test change is defined as the difference between the

acceleration output of an axis with self test enabled and the

acceleration output of the same axis with self test disabled. Due

to device filtering, the output reaches its final value after 4 × τ

when enabling or disabling self test, where τ = 1/(data rate). This

definition assumes that the sensor does not move between these

two measurements; if the sensor moves, a nonself test related shift

corrupts the test.

The self test response in all axes exhibits bimodal behavior. Howev-

er, performing the self test check in the z-axis is a reliable way to

corroborate the integrity of the sensor.

Proper configuration of the ADXL314 is necessary for an accurate

self test measurement. To configure the device for self test, take the

following steps:

1. Set the data rate from 100 Hz to 800 Hz, or set the data

rate to 3200 Hz by writing to the rate bits (Bits[D3:D0]) in the

BW_RATE register (Address 0x2C). Write a value from 0x0A to

0x0D, or write 0x0F to the BW_RATE register.

2. Configure the device for normal power operation by clearing the

LOW_POWER bit (Bit D4) in the BW_RATE register (Address

0x2C).

3. Retrieve samples of z-axis acceleration data from the sensor

and average them together. Averaging at least 10 samples is

recommended.

4. Store the averaged value and label it as ZST_OFF.

5. Enable self test by setting the SELF_TEST bit (Bit D7) in the

DATA_FORMAT register (Address 0x31). Note that the output

requires approximately four samples to settle after self test is

enabled.

6. Retrieve samples of z-axis acceleration data and average them

together. Averaging at least 10 samples is recommended.

7. Store the averaged value and label it as ZST_ON.

8. Disable self test by clearing the SELF_TEST bit (Bit D7) in the

DATA_FORMAT register (Address 0x31).

With the stored value for self test enabled and disabled, the self test

change is as follows:

ZST = ZST_ON − ZST_OFF

This value can be converted to g by multiplying by the

48.83 mg/LSB scale factor.

If the self test change is within the valid range, the test is consid-

ered successful. Generally, a device is considered to pass if the

minimum magnitude of change is achieved. However, a device that

changes by more than the maximum magnitude is not necessarily a

failure.

Another effective method for using the self test to verify that

accelerometer functionality is to toggle the self test at a certain

rate and then perform an FFT on the output. The FFT must have

a corresponding tone at the frequency where the self test was

toggled. Using an FFT in this way removes the dependency of the

test on supply voltage and self test magnitude, which can vary

within a rather wide range.

ASYNCHRONOUS DATA READINGS

Asynchronous readings of acceleration data can lead to accessing

the acceleration data registers while they are being updated. To

avoid this, it is recommended to either enable FIFO stream mode or

to synchronize a SPI/I2C transaction to the DATA_READY interrupt

functionality, so that the host processor samples immediately after

the DATA_READY interrupt goes high.

DATA FORMATTING AT OUTPUT DATA RATE

OF 3200 HZ AND 1600 HZ

When using the 3200 Hz or 1600 Hz output data rate, the LSB of

the output data-word is always 0. When the data is right justified,

the LSB corresponds to Bit D0 of the DATAx0 register, and when

the data is left justified, the LSB corresponds to Bit D3 of the

DATAx0 register.

ADXL314 Datasheet(PDF) 24 Page - Analog Devices

ADXL314 Datasheet(HTML) 24 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

Datasheet Download

Link URL