Data Sheet - Freescale Semiconductor

3.5.4.4 Signal compensation
The device includes internal gain and compensation circuitry to compensate for sensor offset, sensitivity and non-linearity.
3.5.4.5 Offset cancellation
The device provides an optional offset cancellation circuit to remove internal offset error. A block diagram of the offset cancellation
is shown in Figure 23.
INPUT DATA
Input Data downsampled to 256 μs
OFFSET CANCELLATION
n n z
1 2
a
0
1 – + ( ⋅ )
d d z
1 2 1 –
LOW-PASS FILTER
⋅ ------------------------------------
+ ( ⋅ )
The transfer function for the offset LPF is:
0.5 Hz (Derived from fOSC)
OFFMON NEG
OFFMON POS
HIGH-PASS FILTER DATAPATH
OFFSET RATE LIMITED DATAPATH
2 kHz (Derived from fOSC)
OFFSET RATE LIMITING
INC/DEC
UP/DOWN
COUNTER
Figure 23. Offset cancellation block diagram
Response parameters are specified in Section 2 and the offset LPF coefficients are specified in Table 36.
During start up, two phases of the offset LPF are used to allow for fast convergence of the internal offset error during initialization.
The timing for the startup phases is shown in Table 35.
The offset low-pass filter used in normal operation is selected by the OC_FILT[2:0] bits in the ACC_CFG register. Output rate
limiting can be applied to the output of the offset low-pass filter. Rate limiting is also enabled by the OC_FILT[2:0] bits. If rate
limiting is enabled, the offset cancellation output is updated by OFF Step LSB every t OffRate seconds.
The offset cancellation circuit output value is frozen when self-test is active, even if the offset cancellation circuit is in a startup
phase. The timers controlling the startup phase times listed in Table 35 are not frozen. To ensure proper offset cancellation
startup, prior to activating self-test, the user should verify that the offset cancellation initialization is complete by monitoring the
OC_INIT bit in the DEVSTAT register.
Eqn. 6
MMA27XXW
Sensors
Freescale Semiconductor, Inc. 49
CLK
OFFSET MONITOR
no
1
no
2
z
Hz ( ) ao
0
1 – + ( ⋅ )
do
1
do
2
z 1 –
=
⋅ -------------------------------------------
+ ( ⋅ )
OUT
INC/DEC
UP/DOWN
COUNTER
CLK
OC_FILT[0]
OUT
OFFMON CNTLIMIT
To Interpolation
OFF_ERR