AN4650 datasheet(24/102 Pages) STMICROELECTRONICS

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Operating modes

AN4650

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DocID027415 Rev 1

2.6.2

Accelerometer turn-on/off time

The accelerometer reading chain contains low-pass filtering to improve signal-to-noise

performance and to reduce aliasing effects. For this reason it is needed to take into account

the settling time of the filter when the accelerometer / gyroscope power mode is switched or

when the accelerometer / gyroscope ODR is changed.

The delay in order to switch accelerometer modes is shown in Table 11.

No anti-aliasing filter is used in Low-Power mode and in Normal mode: in these cases no

samples have to be discarded.

The anti-aliasing filter is bypassed also when the XL_BW_SCAL_ODR bit of the CTRL4_C

the number of samples to be discarded is equal to 2 and 4, respectively.

Table 12 clarifies how many accelerometer samples have to be discarded in High-

Performance mode depending on the internal filter bandwidth and output data rate

selection. Bandwidth value selection is described in Table 7.

Setting the DRDY_MASK bit of the CTRL4_C register to 1, the accelerometer and

gyroscope data-ready signals are masked until the settling of the sensor filters is completed:

this feature allows automatically ignoring the samples to be discarded.

Table 11. Accelerometer turn-on/off time

Starting mode

Target mode

Max turn-on/off time

Power Down

Low Power / Normal

First sample correct

Power Down

High Performance

see Table 12

Low Power / Normal

High Performance

see Table 12

Low Power / Normal /

High Performance

Power Down

1 μs

Table 12. Accelerometer number of samples to be discarded (High-Perf. mode)

Accelerometer

ODR [Hz]

BW = 400 Hz BW = 200 Hz BW = 100 Hz BW = 50 Hz

No filter

13 Hz (High Perf.)

26 Hz (High Perf.)

N.A.

52 Hz (High Perf.)

N.A.

104 Hz (High Perf.)

N.A.

208 Hz (High Perf.)

N.A.

416 Hz (High Perf.)

N.A.

833 Hz (High Perf.)

N.A.

1.66 kHz (High Perf.)

N.A.

3.33 kHz (High Perf.)

6.66 kHz (High Perf.)

112

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