ADXL375 datasheet(29/32 Pages) AD

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Data Sheet

ADXL375

Rev. 0 | Page 29 of 32

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 non-self-test

related shift corrupts the test.

Proper configuration of the ADXL375 is necessary for an

accurate self-test measurement. To configure the part for self-

test, follow this procedure.

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. For accurate self-test measurements, configure the part for

normal power operation by clearing the LOW_POWER bit

(Bit D4) in the BW_RATE register (Address 0x2C).

3. After the part is configured for accurate self-test measure-

ment, retrieve samples of x-, y-, and z-axis acceleration

data from the sensor and average them together.

The number of samples averaged is selected by the system

designer, but a recommended starting point is 0.1 sec worth

of data for data rates of 100 Hz or greater—that is, 10 samples

at the 100 Hz data rate.

4. Store the averaged values and label them appropriately

as the values with self-test disabled, that is, XST_OFF,

YST_OFF, and ZST_OFF.

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

in the DATA_FORMAT register (Address 0x31).

The output requires some time (approximately four samples)

to settle after self-test is enabled.

6. After allowing the output to settle, retrieve samples of x-, y-,

and z-axis acceleration data and average them together.

It is recommended that the same number of samples be taken

for the self-test average as was done for the non-self-test

average.

7. Store the averaged values and label them appropriately

as the values with self-test enabled, that is, XST_ON,

YST_ON, and ZST_ON.

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

in the DATA_FORMAT register (Address 0x31).

With the stored values for self-test enabled and disabled, the

self-test change is as follows:

XST = XST_ON − XST_OFF

YST = YST_ON − YST_OFF

ZST = ZST_ON − ZST_OFF

Because the measured output for each axis is expressed in LSBs,

XST, YST, and ZST are also expressed in LSBs. These values can be

converted to acceleration (g) by multiplying each value by the

49 mg/LSB scale factor.

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

successful. Generally, a part is considered to pass if the minimum

magnitude of change is achieved. However, a part that changes by

more than the maximum magnitude is not necessarily a failure.

The self-test response in the x- and y-axes exhibits bimodal

behavior and, therefore, is not always a reliable indicator of

sensor health or potential shift in device sensitivity. For this

reason, perform the self-test check in the z-axis.

Another effective method for using the self-test to verify accel-

erometer functionality is to toggle the self-test at a certain rate

and then perform an FFT on the output. The FFT should 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.

ADXL375 Datasheet(PDF) 29 Page - Analog Devices

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