ADXL311

Rev. B | Page 9 of 12

The output of the ADXL311 has a typical bandwidth of 3 kHz.

The user must filter the signal at this point to limit aliasing

errors. The analog bandwidth must be no more than half the

A/D sampling frequency to minimize aliasing. The analog

bandwidth can be further decreased to reduce noise and

improve resolution.

The ADXL311 noise has the characteristics of white Gaussian

noise that contribute equally at all frequencies and are described

in terms of µg/√Hz, i.e., the noise is proportional to the square

root of the bandwidth of the accelerometer. It is recommended

that the user limits the bandwidth to the lowest frequency

needed by the application to maximize the resolution and

dynamic range of the accelerometer.

With the single-pole roll-off characteristic, the typical noise of

the ADXL311 is determined by

(

) (

.

1

Hz

/

300

×

×

µ

=

BW

g

Noise

RMS

At 100 Hz the noise will be

(

)

g

g

Noise

RMS

m

8

.

3

6

.

1

100

Hz

/

300

=

×

×

µ

=

Often the peak value of the noise is desired. Peak-to-peak noise

can only be estimated by statistical methods. Table 6 shows the

probabilities of exceeding various peak values, given the rms value.

Table 6. Estimation of Peak-to-Peak Noise

Peak-to-Peak

Value

% of Time That Noise Exceeds Nominal

Peak-to-Peak Value

2 × rms

32

4 × rms

4.6

6 × rms

0.27

8 × rms

0.006

The peak-to-peak noise value gives the best estimate of the

uncertainty in a single measurement. Table 7 gives the typical

Bandwidth

(Hz)

(µF)

RMS Noise

(mg)

Peak-to-Peak Noise

Estimate (mg)

10

0.47

1.2

7.2

50

0.1

2.7

16.2

100

0.047

3.8

22.8

500

0.01

8.5

51

USING THE ADXL311 WITH OPERATING

VOLTAGES OTHER THAN 3 V

performance parameters change as the supply voltage varies.

The ADXL311 output is ratiometric, so the output sensitivity

(or scale factor) varies proportionally to the supply voltage. At

The 0 g bias output is also ratiometric, so the 0 g output is

The output noise is not ratiometric, but absolute in volts;

therefore, the noise density decreases as the supply voltage

increases. This is because the scale factor (mV/g) increases

while the noise voltage remains constant.

The self-test response is roughly proportional to the square of

approximately equivalent to 750 mg (typical).

The supply current increases as the supply voltage increases.

USING THE ADXL311 AS A DUAL-AXIS

TILT SENSOR

One of the most popular applications of the ADXL311 is tilt

measurement. An accelerometer uses the force of gravity as an

input vector to determine the orientation of an object in space.

An accelerometer is most sensitive to tilt when its sensitive axis

is perpendicular to the force of gravity, i.e., parallel to the earth’s

surface. When the accelerometer is oriented parallel to the gravity

vector, i.e., near its +1 g or –1 g reading, the change in output

acceleration per degree of tilt is negligible. When the acceler-

ometer is perpendicular to gravity, its output changes nearly

17.5 mg per degree of tilt, but at 45° degrees, it changes only

12.2 mg per degree, and resolution declines.

Dual-Axis Tilt Sensor: Converting

Acceleration to Tilt

When the accelerometer is oriented so both its X-axis and Y-axis

are parallel to the earth’s surface, it can be used as a two-axis tilt

sensor with a roll axis and a pitch axis. Once the output signal

from the accelerometer has been converted to an acceleration

that varies between –1 g and +1 g, the output tilt in degrees is

calculated as follows:

(

A

SIN

Pitch

X

1

/

A

=

(

A

SIN

Roll

Y

1

/

A

=

Be sure to account for overranges. It is possible for the

accelerometers to output a signal greater than ±1 g due to

vibration, shock, or other accelerations.