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

damaging the ADXL313. All possible power-on modes are

summarized in Table 5. The interface voltage level is set with

ensure that the ADXL313 does not create a conflict on the

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. Current Consumption vs. Data Rate

Output Data

Rate (Hz)

Bandwidth (Hz)

Rate Code

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

Output Data

Rate (Hz)

Bandwidth (Hz)

Rate Code

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

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.