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ADXL105 Datasheet(Arkusz danych) 6 Page - Analog Devices

Numer części ADXL105
Szczegółowy opis  High Accuracy 61 g to 65 g Single Axis iMEMS Accelerometer with Analog Input
Pobierz  8 Pages
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Producent  AD [Analog Devices]
Strona internetowa  http://www.analog.com
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REV. A
ADXL105
–6–
THEORY OF OPERATION
The ADXL105 is a complete acceleration measurement system
on a single monolithic IC. It contains a polysilicon surface-
micromachined sensor and BiMOS signal conditioning circuitry
to implement an open loop acceleration measurement architec-
ture. The ADXL105 is capable of measuring both positive and
negative accelerations to a maximum level of
±5 g. The acceler-
ometer also measures static acceleration such as gravity, allow-
ing it to be used as a tilt sensor.
The sensor is a surface micromachined polysilicon structure
built on top of the silicon wafer. Polysilicon springs suspend the
structure over the surface of the wafer and provide a resistance
against acceleration-induced forces. Deflection of the structure
is measured with a differential capacitor structure that consists
of two independent fixed plates and a central plate attached to
the moving mass. A 180
° out-of-phase square wave drives the
fixed plates. An acceleration causing the beam to deflect, will
unbalance the differential capacitor resulting in an output square
wave whose amplitude is proportional to acceleration. Phase sensi-
tive demodulation techniques are then used to rectify the signal
and determine the direction of the acceleration.
An uncommitted amplifier is supplied for setting the output
scale factor, filtering and other analog signal processing.
A ratiometric voltage output temperature sensor measures the
exact die temperature and can be used for optional calibration
of the accelerometer over temperature.
VDD
The ADXL105 has two power supply (VDD) pins, 13 and 14.
The two pins should be connected directly together. The output
of the ADXL105 is ratiometric to the power supply. Therefore a
0.22
µF decoupling capacitor between V
DD and COM is re-
quired to reduce power supply noise. To further reduce noise,
insert a resistor (and/or a ferrite bead) in series with the VDD
pin. See the EMC and Electrical Noise section for more details.
COM
The ADXL105 has two common (COM) pins, 4 and 7. These
two pins should be connected directly together and Pin 7
grounded.
ST
The ST pin (Pin 6) controls the self-test feature. When this pin
is set to VDD, an electrostatic force is exerted on the beam of the
accelerometer causing the beam to move. The change in output
resulting from movement of the beam allows the user to test for
mechanical and electrical functionality. This pin may be left
open-circuit or connected to common in normal use. The self-
test input is CMOS and TTL compatible.
AOUT
The accelerometer output (Pin 8) is set to a nominal scale fac-
tor of 250 mV/g (for VDD = 5 V). Note that AOUT is guaranteed
to source/sink a minimum of 50
µA (approximately 50 kΩ out-
put impedance). So a buffer may be required between AOUT and
some A-to-D converter inputs.
VMID
VMID is nominally VDD/2. It is primarily intended for use as a
reference output for the on board uncommitted amplifier (UCA)
as shown in Figures 14a and 14b. Its output impedance is ap-
proximately 10 k
Ω.
VDD
TOUT
ST
COM
AOUT
VMID
VIN
VNIN
UCAOUT
UNCOMMITTED
AMPLIFIER
ADXL105
X SENSOR
TEMP
SENSOR
OUTPUT
VDD
R1
R2
+V
COM
0.22 F
GAIN
1
2
3
4
250
500
750
1000
50k
50k
50k
50k
50k
100k
150k
200k
SCALE – mV/
g
R1
R2
a. Using the UCA to Change the Scale Factor
VDD
TOUT
ST
COM
AOUT
VMID
VIN
VNIN
UCAOUT
UNCOMMITTED
AMPLIFIER
ADXL105
X SENSOR
TEMP
SENSOR
OUTPUT
VDD
R1
R2
+V
COM
0.22 F
+V
10k
R3
R3 = 5R1
R1 > 20k
SCALE =
(250) R2
R1
mV/
g
b. Using the UCA to Change the Scale Factor
and Zero g Bias
Figure 14. Application Circuit for Increasing Scale Factor
TOUT
The temperature sensor output is nominally 2.5 V at +25
°C and
typically changes 8 mV/
°C, and is optimized for repeatability
rather than accuracy. The output is ratiometric with supply
voltage.
Uncommitted Amplifier (UCA)
The uncommitted amplifier has a low noise, low drift bipolar
front end design. The UCA can be used to change the scale
factor of the ADXL105 as shown in Figure 14. The UCA may
also be used to add a 1- or 2-pole active filter as shown in Fig-
ures 15a through 15d.




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