Sensors

26

Freescale Semiconductor, Inc.

MMA685x

3.8.3

Low Pass Filter

Data from the Sinc filter is processed by an infinite impulse response (IIR) low pass filter.

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-

Note: Low Pass Filter Figures do not include g-cell frequency response.

Table 24. Low Pass Filter Coefficients

Description

Sample Time (

μs)

Filter Coefficients

Group Delay

50 Hz LPF

16

2.08729034056887e-10

1

8.349134489240434e-10

-3.976249694824219

1.25237777794924e-09

5.929003009577855

100 Hz LPF

8

8.349103355433541e-10

-3.929255528257727

2.087307211059861e-10

0.9765022168437554

150 Hz LPF

16

1.639127731323242e-08

1

6.556510925292969e-08

-3.928921222686768

9.834768482194806e-08

5.789028996785419

300 Hz LPF

8

6.556510372902331e-08

-3.791257019240902

1.639128257923422e-08

0.9311495074496179

200 Hz LPF

16

5.124509334564209e-08

1

2.049803733825684e-07

-3.905343055725098

3.074705789151505e-07

5.72004239520561

400 Hz LPF

8

2.049803958150164e-07

-3.723967810019985

5.124510693742625e-08

0.9092692903507213

200 Hz LPF

3-pole

16

2.720393240451813e-06

1

8.161179721355438e-06

-2.931681632995605

8.161180123840722e-06

2.865296718275204

400 Hz LPF

3-pole

8

2.720393634345496e-06

-0.9335933215174919

0

400 Hz LPF

16

7.822513580322266e-07

1

3.129005432128906e-06

-3.811614513397217

4.693508163398543e-06

5.450666051045118

800 Hz LPF

8

3.129005428784364e-06

-3.465805771100349

7.822513604678875e-07

0.8267667478030489

500 Hz LPF

16

1.865386962890625e-06

1

7.4615478515625e-06

-3.765105724334717

1.119232176112846e-05

5.319861050818872

1000 Hz LPF

8

7.4615478515625e-06

-3.34309015036024

1.865386966264658e-06

0.7883646729233078

Hz

()

n

0

n

1

z 1

–

⋅

()

n

2

z 2

–

⋅

()

n

3

z 3

–

⋅

()

n

4

z 4

–

⋅

()

+

+++

d

0

d

1

z 1

–

⋅

()

d

2

z 2

–

⋅

()

d

3

z 3

–

⋅

()

d

4

z 4

–

⋅

()

+

+++

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