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參數(shù)資料
| 型號： | ADXL50AH |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 模擬信號調(diào)理 |
| 英文描述： | Monolithic Accelerometer With Signal Conditioning |
| 中文描述： | SPECIALTY ANALOG CIRCUIT, MBCY10 |
| 封裝： | HERMETIC SEALED, METAL CAN, TO-100, 10 PIN |
| 文件頁數(shù)： | 12/16頁 |
| 文件大?。?/td> | 286K |
| 代理商： | ADXL50AH |
ADXL50
–12–
REV. B
DE MODULAT OR CAPACIT OR, C1
T he demodulator capacitor is connected across Pins 2 and 3 to
filter the demodulated signal from the sensor beam and to set
the bandwidth of the force balance control loop. T his capacitor
may be used to approximately set the bandwidth of the acceler-
ometer. A capacitor is always required for proper operation.
T he frequency response of the ADX L50 exhibits a single pole
roll-off response whose nominal 3 dB frequency is set by the
following equation:
f
3 dB
=
(28.60/C1
in
μ
F
)
±
40%
A nominal value of 0.022
μ
F is recommended for C1. In gen-
eral, the design bandwidth should be set 40% higher than the
minimum desired system bandwidth due to the
±
40% tolerance.
A minimum value of 0.015
μ
F is required, (over temperature
and system life), to prevent device instability or oscillation. T he
demodulation capacitor should be a low leakage, low drift ce-
ramic type with an NPO (best) or X 7R (good) dielectric.
In general, it’s best to use the recommended 0.022
μ
F capacitor
across the demodulator pins and perform any additional low-
pass filtering using the buffer amplifier. Using a large denomina-
tor capacitor for low-pass filtering has the disadvantage that the
capacitive sensor will be slow to respond to rapid changes in
acceleration and, therefore, the full shock survivability of the
device could be compromised. T he use of the buffer for low-
pass filtering generally results in smaller capacitance values and
better overall performance. It is also a convenient and more pre-
cise way to set the system bandwidth. Post filtering allows band-
width to be controlled accurately by component selection and
avoids the
±
40% demodulation tolerance. Note that signal noise
is proportional to the square root of the bandwidth of the
ADX L50 and may be a consideration in component selection—
see section on noise.
Care should be taken to reduce or eliminate any leakage paths
from the demodulator capacitor pins to common or to the +5 V
pin. Even a small imbalance in the leakage paths from these pins
will result in offset shifts in the zero-g bias level. As an example,
an unbalanced parasitic resistance of 30 M
from either
demodulator pin to ground will result in an offset shift at V
PR
of
approximately 50 mV. Conformal coating of PC boards with a
high impedance material is recommended to avoid leakage prob-
lems due to aging or moisture.
RE DUCING T HE AVE RAGE POWE R CONSUMPT ION OF
T HE ADX L50
T he ADX L50 is a versatile accelerometer that can be used in a
wide variety of applications. In some battery powered applica-
tions, such as shipping recorders, power consumption is a criti-
cal parameter. T he ADX L50 typically draws 10 mA current
from a 5 V power supply which may exceed the power budgeted
for the accelerometer.
For such applications, the ADX L50 can be successfully power
cycled, where the power is turned on only during the period
when data is sampled. Figure 21 illustrates the power-on settling
of the ADX L50 during cycling where the output amplifier has
a gain of one with no filtering. T he settling time-constant is
approximately 0.12 ms, waiting l ms before sampling ensures
maximally accurate readings.
For example, to reduce the average power to 5 mW from its
typical 50 mW, the power should be on 10% of the time. With
the power on for 1 ms and off for 9 ms, a maximum sample rate
of 100 Hz is achievable. Further reduction in average power can
be realized with lower sample rates.
T
0
1.0
0.8
0.6
0.4
0.2
POWER
SUPPLY
(V)
V
(V)
0V
+5V
TIME – ms
V
FINAL
Figure 21. Power-On Settling Time when Power Cycling
SY ST E M BANDWIDT H CONT ROL AND POST
FILT E RING
Unlike piezoresistive sensors, the resonant frequency of the
ADX L50’s capacitive sensor element is typically greater than
20 kHz and does not limit the useful bandwidth of the device.
Usually, the resonant frequency of the beam appears as a peak
in the bandwidth response at approximately 24 kHz with a Q of
3 to 4, as shown in Figure 22.
When using the recommended 0.022
μ
F demodulator capacitor,
be advised that the nominal 1300 Hz pole it establishes within
the device can vary
±
40%. T herefore, if additional low-pass
filtering is used—at frequencies much above 600 Hz—the two
poles may interact and result in a net circuit bandwidth that is
lower than expected.
190
19
100
1k
FREQUENCY – Hz
10k
1.9
0.19
AMPLITUDE
0.022
μ
F
0.015
μ
F
0.010
μ
F
0.005
μ
F
PHASE
S
0
–90
–180
P
Figure 22. Frequency Response of the ADXL50 for Various
Demodulator Capacitors
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