ADXL204EB Analog Devices Inc, ADXL204EB Datasheet - Page 10

• Manufacturer Part Number: ADXL204EB
• Description: EVAL BOARD
• Manufacturer: Analog Devices Inc
• Series: iMEMS®r

Specifications of ADXL204EB

• Silicon Manufacturer: Analog Devices
• Application Sub Type: Accelerometer - Dual-Axis
• Kit Application Type: Sensing - Motion / Vibration / Shock
• Silicon Core Number: ADXL204
• Silicon Family Name: IMEMS
• Sensor Type: Accelerometer, 2 Axis
• Sensing Range: ±1.7g
• Interface: Analog
• Sensitivity: 620mV/g
• Voltage - Supply: 3 V ~ 6 V
• Embedded: No
• Utilized Ic / Part: ADXL204
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

ADXL204
APPLICATIONS
POWER SUPPLY DECOUPLING
For most applications, a single 0.1 μF capacitor, C
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 ADXL204
output. If additional decoupling is needed, a 100 Ω, or smaller,
resistor or ferrite bead can be inserted in the supply line of the
ADXL204. Additionally, a larger bulk bypass capacitor, in the
1 μF to 22 μF range, can be added in parallel to C
SETTING THE BANDWIDTH USING C
The ADXL204 has provisions for bandlimiting the X
Y
low-pass filtering for antialiasing and noise reduction. The
equation for the 3 dB bandwidth is
or more simply,
The tolerance of the internal resistor (R
much as ±25% of its nominal value (32 kΩ); thus, the band-
width varies accordingly. A minimum capacitance of 2000 pF
for C
Table 6. 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
meter. The resulting movement of the beam allows the user to
test if the accelerometer is functional. The typical change in
output is 325 mg (corresponding to 200 mV). This pin can be
left open-circuit or connected to common in normal use.
The ST pin should never be exposed to voltage greater than
V
cannot be guaranteed (that is, multiple supply voltages present),
a low V
OUT
S
S
, an electrostatic force is exerted on the beam of the accelero-
+ 0.3 V. If the system design is such that this condition
F
F
pins. Capacitors must be added at these pins to implement
X
–3 dB
–3 dB
and C
F
clamping diode between ST and V
= 1/(2π(32 kΩ) × C
= 5 μF/C
Y
is required in all cases.
(X, Y)
(X, Y)
)
Capacitor (μF)
4.7
0.47
0.10
0.05
0.027
0.01
X
FILT
and C
) can vary typically as
S
is recommended.
X
Y
AND C
DC
DC
.
, adequately
OUT
Y
and
Rev. A | Page 10 of 12
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 X
The output of the ADXL204 has a typical bandwidth of 2.5 kHz.
The user must filter the signal at this point to limit aliasing
errors. 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 ADXL204 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’s 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 ADXL204 is determined by
At 100 Hz the noise is
Often, the peak value of the noise is desired. Peak-to-peak noise
can only be estimated by statistical methods. Table 7 is useful
for estimating the probabilities of exceeding various peak
values, given the rms value.
Table 7. Estimation of Peak-to-Peak Noise
Peak-to-Peak Value
2 × rms
4 × rms
6 × rms
8 × rms
rmsNoise = (170 μg/√Hz) × (√BW×1.6)
rmsNoise = (170 μg/√Hz) × (√BW×1.6) = 2.15 mg
% of Time Noise Exceeds
Nominal Peak-to-Peak Value
32
4.6
0.27
0.006
OUT
and Y
OUT
.