ADXL321EB datasheet(13/16 Pages) AD

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ADXL321

Rev. 0 | Page 13 of 16

Peak-to-peak noise values give the best estimate of the

uncertainty in a single measurement. Table 7 gives the typical

noise output of the ADXL321 for various CX and CY values.

Table 7. Filter Capacitor Selection (CX, CY)

Bandwidth

(Hz)

CX, CY

(µF)

RMS Noise

(mg)

Peak-to-Peak

Noise Estimate

(mg)

0.47

1.3

7.8

0.1

2.9

17.4

100

0.047

500

0.01

9.1

54.6

USE WITH OPERATING VOLTAGES OTHER THAN 3 V

The ADXL321 is tested and specified at VS = 3 V; however, it can

be powered with VS as low as 2.4 V or as high as 6 V. Note that

some performance parameters change as the supply voltage is

varied.

The ADXL321 output is ratiometric, so the sensitivity (or scale

factor) varies proportionally to supply voltage. At VS = 5 V, the

sensitivity is typically 100 mV/g. At VS = 2.4 V, the sensitivity is

typically 45 mV/g.

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

nominally equal to VS/2 at all supply voltages.

The output noise is not ratiometric but is 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. At VS = 5 V, the noise

density is typically 190 µg/√Hz, while at VS = 2.4 V, the noise

density is typically 400 µg/√Hz,

Self-test response in g is roughly proportional to the square of

the supply voltage. However, when ratiometricity of sensitivity is

factored in with supply voltage, the self-test response in volts is

roughly proportional to the cube of the supply voltage. For

example, at VS = 5 V, the self-test response for the ADXL321 is

approximately 80 mV. At VS = 2.4 V, the self-test response is

approximately 8 mV.

The supply current decreases as the supply voltage decreases.

Typical current consumption at VS = 5 V is 750 µA, and typical

current consumption at VS = 2.4 V is 350 µA.

USE AS A DUAL-AXIS TILT SENSOR

An accelerometer is most sensitive to tilt when its sensitive axis

is perpendicular to the force of gravity (that is, when it is

parallel to the earth’s surface). At this orientation, its sensitivity

to changes in tilt is highest. When the accelerometer is oriented

on axis to gravity (near its +1 g or −1 g reading), the change in

output acceleration per degree of tilt is negligible. When the

accelerometer is perpendicular to gravity, its output changes

nearly 17.5 mg per degree of tilt. At 45°, its output changes at

only 12.2 mg per degree of tilt, and resolution declines.

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 2-axis

tilt sensor with both a roll axis and 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

PITCH = arcsine(AX/1 g)

ROLL = arcsine(AY/1 g)

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.

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