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ADXL313 Datasheet(Arkusz danych) 8 Page - Analog Devices

Numer części ADXL313
Szczegółowy opis  3-Axis, ±0.5 g/±1 g/±2 g/±4 g Digital Accelerometer
Pobierz  28 Pages
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Producent  AD [Analog Devices]
Strona internetowa  http://www.analog.com
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ADXL313
Data Sheet
Rev. 0 | Page 8 of 28
THEORY OF OPERATION
The ADXL313 is a complete 3-axis acceleration measurement
system with a selectable measurement range of ±0.5 g, ±1 g,
±2 g, or ±4 g. It measures both dynamic acceleration resulting
from motion or shock and static acceleration, such as gravity,
which allows it to be used as a tilt sensor.
The sensor is a polysilicon surface-micromachined 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 differential
capacitors that consist of independent fixed plates and plates
attached to the moving mass. Acceleration deflects the beam
and unbalances the differential capacitor, resulting in a sensor
output whose amplitude is proportional to acceleration. Phase-
sensitive demodulation is used to determine the magnitude and
polarity of the acceleration.
POWER SEQUENCING
Power can be applied to VS or VDD I/O in any sequence without
damaging the ADXL313. All possible power-on modes are
summarized in Table 5. The interface voltage level is set with
the interface supply voltage, VDD I/O, which must be present to
ensure that the ADXL313 does not create a conflict on the
communication bus. For single-supply operation, VDD I/O can be
the same as the main supply, VS. In a dual-supply application,
however, VDD I/O can differ from VS to accommodate the desired
interface voltage, as long as VS is greater than or equal to VDDI/O.
After VS is applied, the device enters standby mode, where power
consumption is minimized and the device waits for VDD I/O to be
applied and for the command to enter measurement mode to be
received. (This command can be initiated by setting the measure
bit in the POWER_CTL register (Address 0x2D).) In addition, any
register can be written to or read from to configure the part while
the device is in standby mode. It is recommended that the device
be configured in standby mode before measurement mode is
enabled. Clearing the measure bit returns the device to the standby
mode.
POWER SAVINGS
Power Modes
The ADXL313 automatically modulates its power consumption
in proportion to its output data rate, as outlined in Table 5. If
additional power savings are desired, a lower power mode is
available. In this mode, the internal sampling rate is reduced,
allowing for power savings in the 12.5 Hz to 400 Hz data rate
range at the expense of slightly greater noise. To enter low
power mode, set the LOW_POWER bit (Bit 4) in the BW_RATE
register (Address 0x2C). The current consumption in low power
mode is shown in Table 6 for cases where there is an advantage
to using low power mode. Use of low power mode for a data
rate not shown in Table 6 does not provide any advantage over
the same data rate in normal power mode. Therefore, it is
recommended that only data rates shown in Table 6 be used in
low power mode. The current consumption values shown in
Table 5 and Table 6 are for a VS of 3.3 V.
Table 5. Current Consumption vs. Data Rate
(TA = 25°C, VS = VDDI/O = 3.3 V)
Output Data
Rate (Hz)
Bandwidth (Hz)
Rate Code
IDD (µA)
3200
1600
1111
170
1600
800
1110
115
800
400
1101
170
400
200
1100
170
200
100
1011
170
100
50
1010
170
50
25
1001
115
25
12.5
1000
82
12.5
6.25
0111
65
6.25
3.125
0110
57
Table 6. Current Consumption vs. Data Rate, Low Power Mode
(TA = 25°C, VS = VDDI/O = 3.3 V)
Output Data
Rate (Hz)
Bandwidth (Hz)
Rate Code
IDD (µA)
400
200
1100
115
200
100
1011
82
100
50
1010
65
50
25
1001
57
25
12.5
1000
50
12.5
6.25
0111
43
Table 7. Power Sequencing
Condition
VS
VDDI/O
Description
Power Off
Off
Off
The device is completely off, but there is a potential for a communication bus conflict.
Bus Disabled
On
Off
The device is on in standby mode, but communication is unavailable, and the device creates a conflict
on the communication bus. Minimize the duration of this state during power-up to prevent a conflict.
Bus Enabled
Off
On
No functions are available, but the device does not create a conflict on the communication bus.
Standby or
Measurement
On
On
The device is in standby mode, awaiting a command to enter measurement mode, and all sensor
functions are off. After the device is instructed to enter measurement mode, all sensor functions are
available.




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