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ADXL212AEZ Datasheet(Arkusz danych) 9 Page - Analog Devices

Numer części ADXL212AEZ
Szczegółowy opis  Precision ±2 g Dual Axis, PWM Output Accelerometer
Pobierz  12 Pages
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Producent  AD [Analog Devices]
Strona internetowa  http://www.analog.com
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ADXL212
Rev. 0 | Page 9 of 12
THEORY OF OPERATION
EARTH'S SURFACE
TOP VIEW
(Not to Scale)
PIN 8
XOUT = 50%
YOUT = 62.5%
XOUT = 50%
YOUT = 50%
PIN 8
XOUT = 50%
YOUT = 37.5%
PIN 8
XOUT = 62.5%
YOUT = 50%
PIN 8
XOUT = 37.5%
YOUT = 50%
Figure 20. Output Response vs. Orientation
The ADXL212 is a complete dual axis acceleration measure-
ment system on a single monolithic IC. It contains a polysilicon
surface-micromachined sensor and signal conditioning circuitry
to implement an open-loop acceleration measurement archi-
tecture. The output signals are duty cycle modulated digital
signals proportional to the acceleration. The ADXL212 is capable
of measuring both positive and negative accelerations to ±2 g.
The accelerometer can measure static acceleration forces such
as gravity, allowing the ADXL212 to be used as a tilt sensor.
The sensor is a surface-micromachined polysilicon structure
built on top of a silicon wafer. Polysilicon springs suspend the
structure over the surface of the wafer and provide a resistance
against acceleration forces. Deflection of the structure is measured
using a differential capacitor that consists of independent fixed
plates and plates attached to the moving mass. The fixed plates
are driven by 180° out-of-phase square waves. Acceleration
deflects the beam and unbalances the differential capacitor,
resulting in an output square wave with an amplitude that is
proportional to acceleration. Phase sensitive demodulation tech-
niques are used to rectify the signal and determine the direction
of the acceleration.
The output of the demodulator is amplified and brought off
chip through a 32 kΩ resistor, at which point the user can set
the signal bandwidth of the device by adding a capacitor. This
filtering improves measurement resolution and helps prevent
aliasing.
After being low-pass filtered, the analog signals are converted to
duty cycle modulated outputs that can be read by a counter.
A single resistor (RSET) sets the period for a complete cycle (t2)
according to the following equation:
t2 (nominal) = RSET/125 MΩ
A 0 g acceleration produces a 50% nominal duty cycle. The
acceleration can be determined by measuring the length of the
positive pulse width (t1) and the period (t2). The nominal
transfer function of the ADXL212 is
Acceleration = ((t1/t2) − Zero g Bias)/Sensitivity
where:
Zero g Bias = 50% nominal.
Sensitivity = 12.5%/g nominal.
PERFORMANCE
High performance is built into the device through innovative
design techniques rather than by using additional temperature
compensation circuitry. As a result, there is essentially no quantiza-
tion error or nonmonotonic behavior, and temperature hysteresis
is very low (typically less than 10 mg over the −40°C to +85°C
temperature range).
Figure 10 shows the zero g output performance of eight parts
(x-axis and y-axis) over a –40°C to +85°C temperature range.
Figure 13 demonstrates the typical sensitivity shift over temper-
ature for VS = 5 V. Sensitivity stability is optimized for VS = 5 V
but remains very good over the specified range; it is typically
better than ±2% over temperature at VS = 3 V.




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