3.8.7
Data Interpolation
MMA68xx includes 2 to 1 data interpolation to minimize the system sample jitter. Each result produced by the digital signal
processing chain is delayed one half of a sample time, and the interpolated value of successive samples is provided between
sample times. This operation is illustrated in Figure 21 .
Internal Sample Rate
S n-3
t s
S n-2
t s
S n-1
t s
S n
t
S n – 3 + S n – 2
------------------------------- 1 -
------------------------ n -
S n-3
--------------------------------
2
S n-2
S n – 2 + S n –
2
S n-1
S n – 1 + S
2
Output Sample Rate
t
Response to SPI acceleration request occurring in this window receives interpolated sample
Response to SPI acceleration request occurring in this window receives true sample.
Figure 21. Data Interpolation Timing
The effect of this interpolation at the system level is a 50% reduction in sample jitter. Figure 22 shows the resulting output data
for an input signal.
80
75
Internally
Sampled Values
70
65
60
55
Earliest Transmission
Point of Interpolated
Values
Earliest Transmission
Fixed Latency:
t S / 2
Point of Internally
Sampled Values
50
45
40
Window of
Transmission for
Interpolated Values
(Maximum: t S / 2)
Window of
Transmission for
= Signal Jitter = Sampled Values
(Maximum: t S / 2)
0
5
10
15
20
25
30
35
40
Input Signal
Time
Internally Sampled Signal
Interpolated Samples
Figure 22. Data Interpolation Example
MMA68xx
Sensors
Freescale Semiconductor, Inc.
35
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