ADXL322_15 Datasheet, PDF (12/16 Pages) Analog Devices – Small and Thin 2 g Accelerometer

ADXL322
APPLICATIONS
POWER SUPPLY DECOUPLING
For most applications, a single 0.1 μF capacitor, CDC, 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
ADXL322 output. 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 (in the 1 μF to
4.7 μF range) can be added in parallel to CDC.
SETTING THE BANDWIDTH USING C_X AND C_Y
The ADXL322 has provisions for band-limiting the XOUT and
YOUT 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 (RFILT) typically varies as
much as ±15% of its nominal value (32 kΩ), and the bandwidth
varies accordingly. A minimum capacitance of 2000 pF for CX
and CY is required in all cases.
Table 5. Filter Capacitor Selection, CX and CY
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
300 mg (corresponding to 125 mV). This pin can be left open-
circuit or connected to common (COM) in normal use.
The ST pin should never be exposed 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 accelerometer bandwidth selected ultimately determines
the measurement resolution (smallest detectable acceleration).
Filtering can be used to lower the noise floor, which improves
the resolution of the accelerometer. Resolution is dependent on
the analog filter bandwidth at XOUT and YOUT.
The output of the ADXL322 has a typical bandwidth of 2.5 kHz.
To limit aliasing errors, the user must filter the signal at this
point. The analog bandwidth must be no more than half the
A/D sampling frequency to minimize aliasing. The analog
bandwidth can be further decreased to reduce noise and
improve resolution.
The ADXL322 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’s bandwidth). The user should
limit bandwidth to the lowest frequency needed by the applica-
tion in order to maximize the resolution and dynamic range of
the accelerometer.
With the single-pole, roll-off characteristic, the typical noise of
the ADXL322 is determined by
rmsNoise = (220 μg/ Hz )× ( BW ×1.6 )
At 100 Hz bandwidth the noise will be
rmsNoise = (220 μg/ Hz )×( 100 ×1.6 ) = 2.8 mg
Often, the peak value of the noise is desired. Peak-to-peak noise
can only be estimated by statistical methods. Table 6 is useful
for estimating the probabilities of exceeding various peak
values, given the rms value.
Table 6. 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. 0 | Page 12 of 16