Datasheet

ADXL103/ADXL203 Data Sheet
Rev. D | Page 14 of 16
APPLICATIONS INFORMATION
POWER SUPPLY DECOUPLING
For most applications, a single 0.1 μF capacitor, C
DC
, adequately
decouples the accelerometer from noise on the power supply.
However, in some cases, particularly where noise is present at
the 140 kHz internal clock frequency (or any harmonic thereof),
noise on the supply can cause interference on the ADXL103/
ADXL203 output. If additional decoupling is needed, a 100 Ω
(or smaller) resistor or ferrite beads can be inserted in the supply
line of the ADXL103/ADXL203. Additionally, a larger bulk
bypass capacitor (in the 1 μF to 22 μF range) can be added in
parallel to C
DC
.
SETTING THE BANDWIDTH USING C
X
AND C
Y
The ADXL103/ADXL203 has provisions for band limiting 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 kΩ) × 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 kΩ); thus, the bandwidth
varies accordingly. A minimum capacitance of 2000 pF for C
X
and
C
Y
is required in all cases.
Table 7. Filter Capacitor Selection, C
X
and C
Y
Bandwidth (Hz) Capacitor (μF)
1 4.7
10 0.47
50 0.10
100 0.05
200 0.027
500 0.01
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 accelerometer.
The resulting movement of the beam allows the user to test if
the accelerometer is functional. The typical change in output is
750 mg (corresponding to 750 mV). This pin can be left open-
circuit or connected to common in normal use.
Never expose the ST pin to voltages greater than V
S
+ 0.3 V. If
the system design is such that this condition cannot be guaranteed
(that is, multiple supply voltages are present), a low V
F
clamping
diode between ST and V
S
is recommended.
DESIGN TRADE-OFFS FOR SELECTING FILTER
CHARACTERISTICS: THE NOISE/BANDWIDTH
TRADE-OFF
The accelerometer bandwidth selected ultimately determines
the measurement resolution (smallest detectable acceleration).
Filtering can be used to lower the noise floor, improving 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 analog-to-digital sampling frequency to minimize
aliasing. The analog bandwidth can 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 μg/√Hz (that is, the noise is proportional to
the square root of the accelerometer bandwidth). Limit bandwidth
to the lowest frequency needed by the application 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
rmsNoise = (110 μg/√Hz) × (
6.1BW ×
)
At 100 Hz, the noise is
rmsNoise = (110 μg/√Hz) × (
6.1100×
) = 1.4 mg
Often, the peak value of the noise is desired. Peak-to-peak noise
can only be estimated by statistical methods. Table 8 is useful
for estimating the probabilities of exceeding various peak values,
given the rms value.
Table 8. Estimation of Peak-to-Peak Noise
Peak-to-Peak Value
% of Time That Noise Exceeds
Nominal Peak-to-Peak Value
2 × rms 32
4 × rms 4.6
6 × rms 0.27
8 × rms 0.006
Peak-to-peak noise values give the best estimate of the uncertainty
in a single measurement; peak-to-peak noise is estimated by
6 × rms. Table 9 gives the typical noise output of the ADXL103/
ADXL203 for various C
X
and C
Y
values.
Table 9. Filter Capacitor Selection (C
X
, C
Y
)
Bandwidth (Hz)
C
X
, C
Y
(μF)
RMS Noise
(mg)
Peak-to-Peak Noise
Estimate (mg)
10 0.47 0.4 2.6
50 0.1 1.0 6
100 0.047 1.4 8.4
500 0.01 3.1 18.7