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ADXL103/ADXL203
APPLICATIONS
POWER SUPPLY DECOUPLING
For most applications, a single 0.1 μF capacitor, C
DC
, will
adequately decouple the accelerometer from noise on the power
supply. However in some cases, particularly where noise is pre-
sent at the 140 kHz internal clock frequency (or any harmonic
thereof), noise on the supply may cause interference on the
ADXL103/ADXL203 output. If additional decoupling is needed,
a 100 (or smaller) resistor or ferrite beads may be inserted in
the supply line of the ADXL103/ADXL203. Additionally, a
larger bulk bypass capacitor (in the 1 μF to 22 μF range) may be
added in parallel to C
DC
.
Rev. 0 | Page 9 of 12
SETTING THE BANDWIDTH USING C
X
AND C
Y
The ADXL103/ADXL203 has provisions for bandlimiting the
X
OUT
and Y
OUT
pins. Capacitors must be added at these pins to
implement low-pass filtering for antialiasing and noise
reduction. The equation for the 3 dB bandwidth is
F
–3
dB
= 1/(2π(32 k) ×
C
(
X
,
Y
)
)
or more simply,
F
–3
dB
= 5 μF/
C
(
X
,
Y
)
The tolerance of the internal resistor (R
FILT
) can vary typically as
much as ±25% of its nominal value (32 k); thus, the band-
width will vary accordingly. A minimum capacitance of 2000 pF
for C
X
and C
Y
is required in all cases.
Table 4. Filter Capacitor Selection, C
X
and C
Y
Bandwidth (Hz)
1
10
50
100
200
500
SELF TEST
The ST pin controls the self-test feature. When this pin is set to
V
S
, an electrostatic force is exerted on the beam of the accelero-
meter. The resulting movement of the beam allows the user to
test if the accelerometer is functional. The typical change in
output will be 750 m
g
(corresponding to 750 mV). This pin may
be left open-circuit or connected to common in normal use.
Capacitor (μF)
4.7
0.47
0.10
0.05
0.027
0.01
The ST pin should never be exposed to voltage greater than
V
S
+ 0.3 V. If the system design is such that this condition
cannot be guaranteed (i.e., multiple supply voltages present), a
low V
F
clamping diode between ST and V
S
is recommended.
DESIGN TRADE-OFFS FOR SELECTING FILTER
CHARACTERISTICS: THE NOISE/BW TRADE-OFF
The accelerometer bandwidth selected will ultimately
determine the measurement resolution (smallest detectable
acceleration). Filtering can be used to lower the noise floor,
which improves the resolution of the accelerometer. Resolution
is dependent on the analog filter bandwidth at X
OUT
and Y
OUT
.
The output of the ADXL103/ADXL203 has a typical bandwidth
of 2.5 kHz. The user must filter the signal at this point to limit
aliasing errors. The analog bandwidth must be no more than
half the A/D sampling frequency to minimize aliasing. The
analog bandwidth may be further decreased to reduce noise and
improve resolution.
The ADXL103/ADXL203 noise has the characteristics of white
Gaussian noise, which contributes equally at all frequencies and
is described in terms of μ
g
/√Hz (i.e., the noise is proportional to
the square root of the accelerometer’s bandwidth). The user
should limit bandwidth to the lowest frequency needed by the
application in order to maximize the resolution and dynamic
range of the accelerometer.
With the single pole roll-off characteristic, the typical noise of
the ADXL103/ADXL203 is determined by
)
6
×
(
)
/
μg
110
(
×
=
BW
Hz
rmsNoise
At 100 Hz, the noise is
g
4
=
)
6
×
100
(
)
/
μg
110
(
m
Hz
rmsNoise
×
=
Often, the peak value of the noise is desired. Peak-to-peak noise
can only be estimated by statistical methods. Table 5 is useful
for estimating the probabilities of exceeding various peak
values, given the rms value.
Table 5. Estimation of Peak-to-Peak Noise
Peak-to-Peak Value
2 × RMS
4 × RMS
6 × RMS
8 × RMS
% of Time That Noise Will Exceed
Nominal Peak-to-Peak Value
32
4.6
0.27
0.006