ADXL180
Rev. 0 | Page 39 of 56
SIGNAL RANGE AND FILTERING
TRANSFER FUNCTION OVERVIEW
The three-pole, low-pass Bessel filter has a selectable −3 dB
corner (f
LP
). The corner can be set to 100 Hz, 200 Hz, 400 Hz, or
800 Hz by programming the filter corner (FC) bits in the
configuration registers. In the pass band between f
HP
and f
LP
, the
response of the ADXL180 is flat with the nominal scale factor
defined by the settings of the range (RG) bits in the
configuration registers (see Figure 29). The auto-zero function
creates a first-order high-pass filter with a −3 dB corner at f
LP
.
Note that the output of this filter is slew rate limited. The auto-
zero function can be disabled by setting the appropriate bit in
the configuration registers. See the Specifications section for
more information.
–60dB/DECADE
BESSEL FILTER
FREQUENCY
f
LP
–3dB
NOMINAL
SENSITIVITY
LSB/
g
+20dB/DECADE
AUTO-ZERO FILTER
f
HP
07
54
4-
0
50
Figure 29. Bode Plot of ADXL180 Transfer Function
RANGE
Table 37. RG[2:0] Sensor Range Select Codes
RG2 RG1 RG0 Range
0 0 0 ±50
g
0 0 1 ±100
g
0 1 0 ±250
g
0 1 1 ±150
g
1 0 0 ±200
g
1 0 1 ±350
g
1 1 0 ±500
g
1 1 1 Not
used
The ADXL180 is configurable into the
g
-ranges shown in Table 37.
Adjusting the device
g
-range alters the
g
/LSB scale factor.
Selecting the 50
g
range offers increased data resolution of
0.125
g
/LSB; however, input signals above 50
g
appear clipped
on the output of the device. Selecting a higher
g
-rating decreases
the resolution of data; however, it allows for a wider full-scale
range of observable signals.
THREE-POLE BESSEL FILTER
Table 38. FC Low-Pass Filter Bandwidth Frequency Select
Codes
FC1
FC0
−3 dB LP Frequency
0 0 400
Hz
0 1 200
Hz
1 0 100
Hz
1 1 800
Hz
By configuring the FC1 and FC0 bits as shown in Table 38, the
output filter on the ADXL 180 can be set. This adjusts the −3 dB
frequency of the output filter to the desired bandwidth. The
ADXL180 low-pass filter is a third-order, low-pass Bessel filter
with a −60 dB per decade roll-off. See the Specifications table
for more information on the tolerances of the low-pass filter
bandwidth.
AUTO-ZERO OPERATION
The auto-zero function is enabled by setting the appropriate bit
in the configuration registers, see Table 44. This function helps
reduce slow offset drifts due to aging, temperature, and so forth.
The acceleration signal offset is determined by passing the
acceleration signal through a one-pole digital low-pass filter.
The output of this filter is then slew rate limited. The slew rate
limited offset value is then subtracted from the acceleration
data. This forms a slew rate limited high-pass filter as shown in
Figure 30.
If auto-zero mode is enabled, a fast offset compensation is
performed during start up of Phase 4 (fast auto-zero mode).
The filter output is set to the last zero reading average
performed by the self-test (Phase 3). The −3 dB frequency of
the digital low-pass filter is approximately 0.08 Hz, and the slew
rate limiter output (and therefore the offset correction) is
updated every 0.5 seconds. The fast update mode (Phase 4) is
15 seconds long in asynchronous mode and 65,535 × t
PS
in
synchronous mode (see the Phase 4: Auto-Zero Initialization
section).
If auto-zero mode is enabled, an offset compensation is
performed during normal operation (Phase 5). This offset
compensation is performed at a slower rate than during the
auto-zero initialization (Phase 4). The −3 dB frequency of the
digital low-pass filter is approximately 0.01 Hz and the slew rate
limiter output (and therefore the offset correction) is updated
every five seconds. The slow update mode persists until power
down. See the Phase 5: Normal Operation section.
The range of the offset corrected output is reduced compared to
when the auto-zero is disabled. This is the function of the
limiter block in Figure 30. This range reduction is shown in
Table 16 and Table 17.
