ADXL335BCPZ-RL Datasheet, PDF (11/16 Pages) Analog Devices – Small, Low Power, Accelerometer

APPLICATIONS INFORMATION
POWER SUPPLY DECOUPLING
For most applications, a single 0.1 μF capacitor, CDC, placed
close to the ADXL335 supply pins adequately decouples the
accelerometer from noise on the power supply. However, in
applications where noise is present at the 50 kHz internal clock
frequency (or any harmonic thereof), additional care in power
supply bypassing is required because this noise can cause errors
in acceleration measurement.
If additional decoupling is needed, a 100 Ω (or smaller) resistor
or ferrite bead can be inserted in the supply line. Additionally, a
larger bulk bypass capacitor (1 μF or greater) can be added in
parallel to CDC. Ensure that the connection from the ADXL335
ground to the power supply ground is low impedance because
noise transmitted through ground has a similar effect to noise
transmitted through VS.
SETTING THE BANDWIDTH USING C_X, C_Y, AND C_Z
The ADXL335 has provisions for band limiting the XOUT, YOUT,
and ZOUT pins. Capacitors must be added at these pins to imple-
ment 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, Z))
or more simply
F–3 dB = 5 μF/C(X, Y, Z)
The tolerance of the internal resistor (RFILT) typically varies as
much as ±15% of its nominal value (32 kΩ), and the bandwidth
varies accordingly. A minimum capacitance of 0.0047 μF for CX,
CY, and CZ is recommended in all cases.
Table 4. Filter Capacitor Selection, CX, CY, and CZ
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
VS, an electrostatic force is exerted on the accelerometer beam.
The resulting movement of the beam allows the user to test if
the accelerometer is functional. The typical change in output
is −1.08 g (corresponding to −325 mV) in the X-axis, +1.08 g
(or +325 mV) on the Y-axis, and +1.83 g (or +550 mV) on the
Z-axis. This ST pin can be left open-circuit or connected to
common (COM) in normal use.
ADXL335
Never expose the ST pin to voltages greater than VS + 0.3 V.
If this cannot be guaranteed due to the system design (for
instance, if there are multiple supply voltages), then a low
VF clamping diode between ST and VS is recommended.
DESIGN TRADE-OFFS FOR SELECTING FILTER
CHARACTERISTICS: THE NOISE/BW TRADE-OFF
The selected accelerometer bandwidth ultimately determines
the measurement resolution (smallest detectable acceleration).
Filtering can be used to lower the noise floor to improve the
resolution of the accelerometer. Resolution is dependent on
the analog filter bandwidth at XOUT, YOUT, and ZOUT.
The output of the ADXL335 has a typical bandwidth of greater
than 500 Hz. 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 ADXL335 noise has the characteristics of white Gaussian
noise, which contributes equally at all frequencies and is
described in terms of μg/√Hz (the noise is proportional to the
square root of the accelerometer bandwidth). The user should
limit bandwidth to the lowest frequency needed by the applica-
tion to maximize the resolution and dynamic range of the
accelerometer.
With the single-pole, roll-off characteristic, the typical noise of
the ADXL335 is determined by
rms Noise = Noise Density × ( BW × 1.6 )
It is often useful to know the peak value of the noise. 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
% 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
Rev. B | Page 11 of 16