Data Sheet ADXL377
Rev. 0 | Page 9 of 12
APPLICATIONS INFORMATION
POWER SUPPLY DECOUPLING
For most applications, a single 0.1 µF capacitor, C
DC
, placed
close to the ADXL377 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. In addition, a
larger bulk bypass capacitor (1 µF or greater) can be added in
parallel to C
DC
. Ensure that the connection from the ADXL377
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 ADXL377 has provisions for band-limiting the X
OUT
, Y
OUT
,
and Z
OUT
pins. A capacitor must be added at each of 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
)
or more simply,
f
−3 dB
= 5 µF/C
x
The tolerance of the internal resistor (R
FILT
) typically varies by as
much as ±15% of its nominal value (32 kΩ), and the bandwidth
varies accordingly. A minimum capacitance of 1000 pF for C
X
,
C
Y
, and C
Z
is recommended in all cases.
Table 4. Filter Capacitor Selection for C
X
, C
Y
, and C
Z
Bandwidth (Hz) Capacitor (µF)
50 0.10
100 0.05
200 0.025
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 −6.5 mV) for the x-axis,
+1.08 g (or +6.5 mV) for the y-axis, and +1.83 g (or +11.5 mV)
for the z-axis. The ST pin can be left open circuit or connected
to ground (GND) 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 guaran-
teed (for example, if multiple supply voltages are present), it is
recommended that a clamping diode with low forward voltage
be connected between ST and V
S
.
SELECTING FILTER CHARACTERISTICS:
NOISE/BANDWIDTH TRADE-OFF
The selected accelerometer bandwidth ultimately determines
the measurement resolution (smallest detectable acceleration).
Filtering can be used to lower the noise floor, thereby improving
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 ADXL377 has a typical bandwidth of 1000 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 decreased further to reduce noise and
improve resolution.
The ADXL377 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 the bandwidth to the
lowest frequency required 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 ADXL377 is determined by
rms Noise = Noise Density ×
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
can be used to estimate the probability of exceeding various peak
values, given the rms value.
Table 5. Estimation of Peak-to-Peak Noise
Peak-to-Peak Value
Percentage 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
