MMA685X datasheet(41/52 Pages) FREESCALE

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Sensors

Freescale Semiconductor, Inc.

MMA685x

3.11

Overload Response

3.11.1 Overload Performance

MMA685x is designed to operate within a specified range. Acceleration beyond that range (overload) impacts the output of

the sensor. Acceleration beyond the range of the device can generate a DC shift at the output of the device that is dependent

upon the overload frequency and amplitude. The MMA685x g-cell is overdamped, providing the optimal design for overload per-

formance. However, the performance of the device during an overload condition is affected by many other parameters, including:

• g-cell damping

• Non-linearity

• Clipping limits

• Symmetry

Figure 32 shows the g-cell, ADC and output clipping of MMA685x over frequency. The relevant parameters are specified in

Section 2.1, and Section 2.6.

Figure 32. Output Clipping Vs. Frequency

3.11.2 Sigma Delta Over Range Response

Over range conditions exist when the signal level is beyond the full-scale range of the device but within the computational limits

of the DSP. The

ΣΔ converter can saturate at levels above those specified in Section 2.1 (G

ADC_CLIP). The DSP operates pre-

dictably under all cases of over range, although the signal may include residual high frequency components for some time after

returning to the normal range of operation due to non-linear effects of the sensor.

5kHz

fg-Cell

fLPF

gADC_Clip

gg-cell_Clip

Determined by g-cell

10kHz

g-cellRolloff

Acceleration (g)

Frequency (kHz)

LPFRolloff

Reg

ion C

lipp

ed b

y g-

cell

Reg

ion C

lipp

ed b

y A

Reg

ion o

f Si

gna

l Di

stor

tion

due

Asy

etry

d N

on-

Line

arity

Region of No Signal Distortion Beyond

Specification

Region of Interest

roll-off and ADC clipping

gRange_Norm

Determined by g-cell

roll-off and full scale range

Region Clipped

by Output

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