ISM303DAC datasheet(32/80 Pages) STMICROELECTRONICS

32 / 80 page

Functionality

ISM303DAC

32/80

DocID030988 Rev 2

The selection of the signal (LPF or HPF) which is sent to the OUT registers is determined by

the FDS_SLOPE bit of CTRL2_A (21h) register. When it is logic ‘1’, the HPF signal is

selected, LPF otherwise.

The signal that is sent to the digital functions (wakeup, double-tap, activity/inactivity and 6D

orientation) is always the HPF signal. Anti-aliasing filtering is guaranteed by the ADC

sampling frequency and the digital LPF cutoff frequency. Anti-aliasing filtering is available in

HR/HF mode only. When the accelerometer is in LP mode, the circuitry is periodically turned

on/off (reducing power consumption) with a fixed on-time and a period that is a function of

the selected ODR. For this reason the LPF cutoff is fixed to 3.2 kHz, so the user must take

care to select the proper ODR value Vs application sampling frequency in order to avoid

aliasing (based on the noise characteristics of the system in use).

Accelerometer slope filter

As shown in Figure 8, the ISM303DAC device embeds a digital slope filter which is used for

wakeup and single/double-tap features. The slope filter output data is computed using the

following formula:

slope(tn) = [ acc(tn) - acc(tn-1) ] / 2

An example of a slope data signal is illustrated in Figure 8.

Figure 8. Accelerometer slope filter

Slope filter bandwidth is ~ ODR/4 and its data is available in the output registers and FIFO

by setting the FDS_SLOPE bit of CTRL2_A (21h) to '1'.

ISM303DAC Datasheet(PDF) 32 Page - STMicroelectronics