3.8.3
Low Pass Filter
n0 + ( n1 ? z – 1 ) + ( n2 ? z – 2 ) + ( n3 ? z – 3 ) + ( n4 ? z – 4 )
Data from the Sinc filter is processed by an infinite impulse response (IIR) low pass filter.
H ( z ) = -----------------------------------------------------------------------------------------------------------------------------------------
d0 + ( d1 ? z – 1 ) + ( d2 ? z – 2 ) + ( d3 ? z – 3 ) + ( d4 ? z – 4 )
MMA685x provides the option for one of twelve low-pass filters. The filter is selected with the LPF[3:0] bits in the DEVCFG_X
register. The filter selection options are listed in Section 3.1.7.3 , Table 11 . Response parameters for the low-pass filter are spec-
ified in Section 2.4 . Filter characteristics are illustrated in Figures 14 , 15 , 16 , 17 , 18 and 19 .
Table 24. Low Pass Filter Coefficients
Description
Sample Time ( μ s)
Filter Coefficients
Group Delay
n 0
2.08729034056887e-10
d 0
1
50 Hz LPF
100 Hz LPF
150 Hz LPF
300 Hz LPF
200 Hz LPF
400 Hz LPF
200 Hz LPF
3-pole
400 Hz LPF
3-pole
400 Hz LPF
800 Hz LPF
500 Hz LPF
1000 Hz LPF
16
8
16
8
16
8
16
8
16
8
16
8
n 1
n 2
n 3
n 4
n 0
n 1
n 2
n 3
n 4
n 0
n 1
n 2
n 3
n 4
n 0
n 1
n 2
n 3
n 4
n 0
n 1
n 2
n 3
n 4
n 0
n 1
n 2
n 3
n 4
8.349134489240434e-10
1.25237777794924e-09
8.349103355433541e-10
2.087307211059861e-10
1.639127731323242e-08
6.556510925292969e-08
9.834768482194806e-08
6.556510372902331e-08
1.639128257923422e-08
5.124509334564209e-08
2.049803733825684e-07
3.074705789151505e-07
2.049803958150164e-07
5.124510693742625e-08
2.720393240451813e-06
8.161179721355438e-06
8.161180123840722e-06
2.720393634345496e-06
0
7.822513580322266e-07
3.129005432128906e-06
4.693508163398543e-06
3.129005428784364e-06
7.822513604678875e-07
1.865386962890625e-06
7.4615478515625e-06
1.119232176112846e-05
7.4615478515625e-06
1.865386966264658e-06
d 1
d 2
d 3
d 4
d 0
d 1
d 2
d 3
d 4
d 0
d 1
d 2
d 3
d 4
d 0
d 1
d 2
d 3
d 4
d 0
d 1
d 2
d 3
d 4
d 0
d 1
d 2
d 3
d 4
-3.976249694824219
5.929003009577855
-3.929255528257727
0.9765022168437554
1
-3.928921222686768
5.789028996785419
-3.791257019240902
0.9311495074496179
1
-3.905343055725098
5.72004239520561
-3.723967810019985
0.9092692903507213
1
-2.931681632995605
2.865296718275204
-0.9335933215174919
0
1
-3.811614513397217
5.450666051045118
-3.465805771100349
0.8267667478030489
1
-3.765105724334717
5.319861050818872
-3.34309015036024
0.7883646729233078
26816/f osc
9024/f osc
6784/f osc
5632/f osc
3392/f osc
2688/f osc
Note: Low Pass Filter Figures do not include g-cell frequency response.
MMA685x
Sensors
26
Freescale Semiconductor, Inc.
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