EVAL-ADXL325Z Analog Devices Inc, EVAL-ADXL325Z Datasheet - Page 11

• Manufacturer Part Number: EVAL-ADXL325Z
• Description: Accelerometer Eval. Board
• Manufacturer: Analog Devices Inc
• Series: iMEMS®r

Specifications of EVAL-ADXL325Z

• Silicon Manufacturer: Analog Devices
• Application Sub Type: Accelerometer - Three-Axis
• Kit Application Type: Sensing - Motion / Vibration / Shock
• Silicon Core Number: ADXL325
• Sensor Type: Accelerometer, 3 Axis
• Sensing Range: ±6g
• Interface: Analog
• Sensitivity: 174mV/g
• Voltage - Supply: 1.8 V ~ 3.6 V
• Embedded: *
• Utilized Ic / Part: ADXL325
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

APPLICATIONS INFORMATION
POWER SUPPLY DECOUPLING
For most applications, a single 0.1 μF capacitor, C
close to the ADXL325 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
from the ADXL325 ground to the power supply ground is low
impedance because noise transmitted through ground has a
similar effect as noise transmitted through V
SETTING THE BANDWIDTH USING C
The ADXL325 has provisions for band limiting the X
Y
implement low-pass filtering for antialiasing and noise reduction.
The 3 dB bandwidth equation is
or more simply
The tolerance of the internal resistor (R
much as ±15% of its nominal value (32 kΩ), and the bandwidth
varies accordingly. A minimum capacitance of 0.0047 μF for C
C
Table 4. Filter Capacitor Selection, C
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
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 −190 mV) in the X axis,
+1.08 g (+190 mV) on the Y axis, and +1.83 g (+320 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
this cannot be guaranteed due to the system design (for instance,
there are multiple supply voltages), then a low V
diode between ST and V
OUT
Y
S
, an electrostatic force is exerted on the accelerometer beam.
, and C
, and Z
f
f
−3 dB
–3 dB
= 1/(2π(32 kΩ) × C
= 5 μF/C
Z
is recommended in all cases.
OUT
pins. Capacitors must be added at these pins to
(X, Y, Z)
S
is recommended.
(X, Y, Z)
)
DC
Capacitor (μF)
4.7
0.47
0.10
0.05
0.027
0.01
. Ensure that the connection
X
FILT
, C
) typically varies as
Y
, and C
S
.
X
, C
F
DC
Y
clamping
, AND C
S
Z
, placed
+ 0.3 V. If
OUT
,
Z
Rev. 0 | Page 11 of 16
X
,
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
The output of the ADXL325 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 ADXL325 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 ADXL325 is determined by
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
USE WITH OPERATING VOLTAGES OTHER THAN 3 V
The ADXL325 is tested and specified at V
powered with V
performance parameters change as the supply voltage is varied.
The ADXL325 output is ratiometric; therefore, the output
sensitivity (or scale factor) varies proportionally to the supply
voltage. At V
At V
The zero g bias output is also ratiometric; therefore, the zero g
output is nominally equal to V
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
is typically 200 μg/√Hz, while at V
noise density is typically 300 μg/√Hz.
rms Noise = Noise Density ×
S
= 2 V, the output sensitivity is typically 116 mV/g.
S
= 3.6 V, the output sensitivity is typically 209 mV/g.
S
as low as 1.8 V or as high as 3.6 V. Note that some
S
= 3.6 V, the X- and Y-axis noise density
% of Time That Noise Exceeds
Nominal Peak-to-Peak Value
32
4.6
0.27
0.006
OUT
S
, Y
/2 at all supply voltages.
(
OUT
S
= 2 V, the X- and Y-axis
BW
, and Z
S
×
= 3 V; however, it can be
1.6
OUT
)
.
ADXL325