ADXL326
Rev. 0 | Page 11 of 16
APPLICATIONS INFORMATION
POWER SUPPLY DECOUPLING
For most applications, a single 0.1 µF capacitor, C
DC
, placed
close to the ADXL326 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 C
DC
. Ensure that the connection
from the ADXL326 ground to the power supply ground is low
impedance because noise transmitted through ground has a
similar effect as noise transmitted through V
S
.
SETTING THE BANDWIDTH USING C
X
, C
Y
, AND C
Z
The ADXL326 has provisions for band limiting the X
OUT
, Y
OUT
, and
Z
OUT
pins. Capacitors must be added at these pins to implement
low-pass filtering for antialiasing and noise reduction. The 3 dB
bandwidth equation is
f
−3 dB
= 1/(2π(32 kΩ) × C
(X, Y, Z)
)
or more simply
f
–3 dB
= 5 F/C
(X, Y, Z)
The tolerance of the internal resistor (R
FILT
) 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 C
X
,
C
Y
, and C
Z
is recommended in all cases.
Table 4. Filter Capacitor Selection, C
X
, C
Y
, and C
Z
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 accelerometer beam.
The resulting movement of the beam allows the user to test
whether the accelerometer is functional. The typical change in
output is −1.08 g (corresponding to −62 mV) in the X axis, +1.08 g
(+62 mV) on the Y axis, and +1.83 g (+105 mV) on the Z axis.
This ST pin can be left open circuit or connected to common
(COM) in normal use.
Never expose the ST pin to voltages greater than V
S
+ 0.3 V. If
this cannot be guaranteed due to the system design (for instance,
there are multiple supply voltages), then 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 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 X
OUT
, Y
OUT
, and Z
OUT
.
The output of the ADXL326 has a typical bandwidth 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 ADXL326 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 application to maximize
the resolution and dynamic range of the accelerometer.
With the single-pole roll-off characteristic, the typical noise of
the ADXL326 is determined by
rms Noise = Noise Density ×
)1.6( ×BW
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
% 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
USE WITH OPERATING VOLTAGES OTHER THAN 3 V
The ADXL326 is tested and specified at V
S
= 3 V; however, it can be
powered with V
S
as low as 1.8 V or as high as 3.6 V. Note that some
performance parameters change as the supply voltage is varied.
The ADXL326 output is ratiometric; therefore, the output
sensitivity (or scale factor) varies proportionally to the supply
voltage. At V
S
= 3.6 V, the output sensitivity is typically 68 mV/g.
At V
S
= 2 V, the output sensitivity is typically 38 mV/g.
The zero g bias output is also ratiometric; therefore, the zero g
output is nominally equal to V
S
/2 at all supply voltages.
The output noise is not ratiometric but is absolute in volts;
therefore, the noise density decreases as the supply voltage
increases. This is because the scale factor (mV/g) increases while
the noise voltage remains constant. At V
S
= 3.6 V, the X- and Y-
axis noise density is typically 120 µg/√Hz, while at V
S
= 2 V, the
X- and Y-axis noise density is typically 270 µg/√Hz.
