ADXL321EB datasheet(12/16 Pages) AD

12 / 16 page

ADXL321

Rev. 0 | Page 12 of 16

APPLICATIONS

POWER SUPPLY DECOUPLING

For most applications, a single 0.1 µF capacitor, CDC, adequately

decouples the accelerometer from noise on the power supply.

However, in some cases, particularly where noise is present at

the 140 kHz internal clock frequency (or any harmonic

thereof), noise on the supply may cause interference on the

ADXL321 output. If additional decoupling is needed, a 100 Ω

(or smaller) resistor or ferrite bead may be inserted in the

supply line. Additionally, a larger bulk bypass capacitor (in the

1 µF to 4.7 µF range) may be added in parallel to CDC.

SETTING THE BANDWIDTH USING CX AND CY

The ADXL321 has provisions for band-limiting the XOUT and

YOUT pins. Capacitors must be added at these pins to implement

low-pass filtering for antialiasing and noise reduction. The

equation for the 3 dB bandwidth is

F−3 dB = 1/(2π(32 kΩ) × C(X, Y))

or more simply,

F–3 dB = 5 µF/C(X,Y)

The tolerance of the internal resistor (RFILT) typically varies as

much as ±15% of its nominal value (32 kΩ), and the bandwidth

varies accordingly. A minimum capacitance of 2000 pF for CX

and CY is required in all cases.

Table 5. Filter Capacitor Selection, CX and CY

Bandwidth (Hz)

Capacitor (µF)

4.7

0.47

0.10

100

0.05

200

0.027

500

0.01

SELF-TEST

The ST pin controls the self-test feature. When this pin is set to

VS, an electrostatic force is exerted on the accelerometer beam.

The resulting movement of the beam allows the user to test if

the accelerometer is functional. The typical change in output is

315 mg (corresponding to 18 mV). This pin may be left open-

circuit or connected to common (COM) in normal use.

The ST pin should never be exposed to voltages greater than

VS + 0.3 V. If this cannot be guaranteed due to the system design

(for instance, if there are multiple supply voltages), then a low

VF clamping diode between ST and VS is recommended.

DESIGN TRADE-OFFS FOR SELECTING FILTER

CHARACTERISTICS: THE NOISE/BW TRADE-OFF

The accelerometer bandwidth selected ultimately determines

the measurement resolution (smallest detectable acceleration).

Filtering can be used to lower the noise floor, which improves

the resolution of the accelerometer. Resolution is dependent on

the analog filter bandwidth at XOUT and YOUT.

The output of the ADXL321 has a typical bandwidth of 2.5 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 may be further decreased to reduce noise and

improve resolution.

The ADXL321 noise has the characteristics of white Gaussian

noise, which contributes equally at all frequencies and is

described in terms of µg/√Hz (the noise is proportional to the

square root of the accelerometer’s bandwidth). The user should

limit bandwidth to the lowest frequency needed by the

application in order to maximize the resolution and dynamic

range of the accelerometer.

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

the ADXL321 is determined by

)

1.6

(

)

µg/

(320

rmsNoise

At 100 Hz bandwidth the noise will be

)

1.6

100

(

)

µg/

(320

rmsNoise

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

can only be estimated by statistical methods. A factor of 6 is

generally used to convert rms to peak-to-peak. Table 6 is useful

for estimating 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

4 × rms

4.6

6 × rms

0.27

8 × rms

0.006

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