Data Sheet - Freescale Semiconductor

3.5.4.1 Decimation Sinc filter
The serial data stream produced by the ΣΔ converter is decimated and converted to parallel values by a 3rd order sinc filter with
a decimation factor of 16.
Hz ( )
1 z
Eqn. 4
16 – –
16 1 z 1 –
---------------------------------
× ( – )
3
=
Figure 16. Sinc-filter response
3.5.4.2 Digital gain
The DSP applies a selectable one or two times digital gain to the output of the sinc filter. The gain applied is dependent on the
output range of the device.
3.5.4.3 Low-pass filter
Data from the Sinc filter is processed by an infinite impulse response (IIR) low-pass filter.
Hz ( )
n
11
z
a
0
0 ( ⋅ ) n
12
z 1 – ( ⋅ ) n
13
z 2 –
+ + ( ⋅ )
d
11
z0 ( ⋅ ) d
12
z 1 – ( ⋅ ) d
13
z 2 –
n
21
z
--------------------------------------------------------------------------------------------
+ + ( ⋅ )
0 ( ⋅ ) n
22
z 1 – ( ⋅ ) n
23
z 2 –
+ + ( ⋅ )
d
21
z0 ( ⋅ ) d
22
z 1 – ( ⋅ ) d
23
z 2 –
=
⋅ ⋅ --------------------------------------------------------------------------------------------
+ + ( ⋅ )
Eqn. 5
The device provides the option for one of six low-pass filters. The filter is selected with the LPF[3:0] bits in the ACC_CFG register.
The filter selection options are listed in Section 3.1.25.1. Response parameters for the low-pass filter are specified in Section 2.7.
Filter characteristics are illustrated in the following figures.
MMA27XXW
Sensors
Freescale Semiconductor, Inc. 41