MXA2500U Просмотр технического описания (PDF) - Unspecified
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MXA2500U Datasheet PDF : 8 Pages
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AOUTX
AOUTY
C
R
C
R
AOUTX
Filtered
O u tp u t
AOUTY
Filtered
O u tp u t
Figure 3: High Pass Filter
Low Pass Filter: An external low pass filter is useful in
low frequency applications such as tilt or inclination. The
low pass filter limits the noise floor and improves the
resolution of the accelerometer. The low pass filter shown
in Figure 4 has a –3dB breakpoint given by the equation:
f
=
1
2πRC
.
For the 200 Hz ratiometric output device
filter, C=0.1µF and R=8kΩ, ±5%, 1/8W.
AOUTX
R
AOUTX
Filtered
C
O u tp u t
AOUTY
R
AOUTY
Filtered
C
O u tp u t
Figure 4: Low Pass Filter
USING THE ACCELEROMETER IN VERY LOW
POWER APPLICATIONS (BATTERY OPERATION)
In applications with power limitations, power cycling can
be used to extend the battery operating life. One important
consideration when power cycling is that the accelerometer
turn on time limits the frequency bandwidth of the
accelerations to be measured. For example, operating at
2.7V the turn on time is 40mS. To double the operating
time, a particular application may cycle power ON for
40mS, then OFF for 40mS, resulting in a measurement
period of 80mS, or a frequency of 12.5Hz. With a
frequency of measurements of 12.5Hz, accelerations
changes as high as 6.25Hz can be detected.
Power cycling can be used effectively in many inclinometry
applications, where inclination changes can be slow and
infrequent.
COMPENSATION FOR EXTENDING THE
FREQUENCY RESPONSE
The response of the thermal accelerometer is a function of
the internal gas physical properties, the natural convection
mechanism and the sensor electronics. Since the gas
properties of MEMSIC's mass produced accelerometer are
uniform, a simple circuit can be used to equally compensate
all sensors. For most applications, the compensating circuit
does not require adjustment for individual units.
A simple compensating network comprising two
operational amplifiers and a few resistors and capacitors
provides increasing gain with increasing frequency (see
Figure 5). The circuit shown is for an absolute output
accelerometer operating at 5 V supply. It provides a DC
gain of X2, so the offset at the output is 2.5V and the
sensitivity is doubled. The 14.3 KΩ and the 5.9KΩ
resistors along with the non-polarized 0.82µF capacitors
tune the gain of the network to compensate for the output
attenuation at the higher frequencies. The resistors and the
capacitors provide noise reduction and stability.
14.3K
5.9K
1.5uF
0.01uF
Aout X or Y 8.06K
0.047uF
UA
160K
-
+
0.0022uF
14.3K
1.5uF
5.9K
0.01uF
UB
-
+
8.06K
Freq. Comp. Output
0.047uF
Figure 5: Frequency Response Extension Circuit
The accelerometer response (bottom trace), the network
response (top trace) and the compensated response (middle
trace) are shown in Figure 6. The amplitude remains above
–3db beyond 100 Hz, and there is useable signal well
after this frequency.
MEMSIC MXA2500U Rev A
Page 6 of 8
05/02
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