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ADXL50 Datasheet(Arkusz danych) 8 Page - Analog Devices

Numer części ADXL50
Szczegółowy opis  Monolithic Accelerometer With Signal Conditioning
Pobierz  16 Pages
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Producent  AD [Analog Devices]
Strona internetowa  http://www.analog.com
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ADXL50
–8–
REV. B
THEORY OF OPERATION
The ADXL50 is a complete acceleration measurement system
on a single monolithic IC. It contains a polysilicon surface-mi-
cro machined sensor and signal conditioning circuitry. The
ADXL50 is capable of measuring both positive and negative ac-
celeration to a maximum level of
±50 g.
Figure 16 is a simplified view of the ADXL50’s acceleration
sensor at rest. The actual structure of the sensor consists of 42
unit cells and a common beam. The differential capacitor sensor
consists of independent fixed plates and a movable “floating”
central plate which deflects in response to changes in relative
motion. The two capacitors are series connected, forming a ca-
pacitive divider with a common movable central plate. A force
balance technique counters any impeding deflection due to ac-
celeration and servos the sensor back to its 0 g position.
CS1
CS2
CENTER
PLATE
FIXED
OUTER
PLATES
UNIT CELL
CS1 = CS2
BEAM
TETHER
CS1
CS2
CENTER
PLATE
TOP VIEW
DENOTES ANCHOR
Figure 16. A Simplified Diagram of the ADXL50 Sensor at
Rest
Figure 17 shows the sensor responding to an applied accelera-
tion. When this occurs, the common central plate or “beam”
moves closer to one of the fixed plates while moving further
from the other. The sensor’s fixed capacitor plates are driven
deferentially by a 1 MHz square wave: the two square wave am-
plitudes are equal but are 180
° out of phase from one another.
When at rest, the values of the two capacitors are the same and
therefore, the voltage output at their electrical center (i.e., at the
center plate) is zero.
When the sensor begins to move, a mismatch in the value of
their capacitance is created producing an output signal at the
central plate. The output amplitude will increase with the
amount of acceleration experienced by the sensor. Information
concerning the direction of beam motion is contained in the
phase of the signal with synchronous demodulation being used
to extract this information. Note that the sensor needs to be po-
sitioned so that the measured acceleration is along its sensitive
axis.
Figure 18 shows a block diagram of the ADXL50. The voltage
output from the central plate of the sensor is buffered and then
applied to a synchronous demodulator. The demodulator is also
supplied with a (nominal) 1 MHz clock signal from the same
oscillator which drives the fixed plates of the sensor. The
demodulator will rectify any voltage which is in sync with its
clock signal. If the applied voltage is in sync and in phase with
the clock, a positive output will result. If the applied voltage is in
sync but 180
° out of phase with the clock, then the demodu-
lator’s output will be negative. All other signals will be rejected.
An external capacitor, C1, sets the bandwidth of the demodulator.
The output of the synchronous demodulator drives the preamp
—an instrumentation amplifier buffer which is referenced to
+1.8 volts. The output of the preamp is fed back to the sensor
through a 3 M
Ω isolation resistor. The correction voltage re-
quired to hold the sensor’s center plate in the 0 g position is a
direct measure of the applied acceleration and appears at the
VPR pin.
CS1
CS2
UNIT CELL
CS1 < CS2
BEAM
TOP VIEW
DENOTES ANCHOR
APPLIED
ACCELERATION
Figure 17. The ADXL50 Sensor Momentarily Responding
to an Externally Applied Acceleration
When the ADXL50 is subjected to an acceleration, its capacitive
sensor begins to move creating a momentary output signal. This
is signal conditioned and amplified by the demodulator and
preamp circuits. The dc voltage appearing at the preamp output
is then fed back to the sensor and electrostatically forces the
center plate back to its original center position.
At 0 g the ADXL50 is calibrated to provide +1.8 volts at the
VPR pin. With an applied acceleration, the VPR voltage changes
to the voltage required to hold the sensor stationary for the du-
ration of the acceleration and provides an output which varies
directly with applied acceleration.
The loop bandwidth corresponds to the time required to apply
feedback to the sensor and is set by external capacitor C1. The
loop response is fast enough to follow changes in g level up to
and exceeding 1 kHz. The ADXL50’s ability to maintain a flat
response over this bandwidth keeps the sensor virtually motion-
less. This essentially eliminates any nonlinearity or aging effects
due to the sensor beam’s mechanical spring constant, as com-
pared to an open-loop sensor.
An uncommitted buffer amplifier provides the capability to ad-
just the scale factor and 0 g offset level over a wide range. An in-
ternal reference supplies the necessary regulated voltages for
powering the chip and +3.4 volts for external use.




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