ADXL335BCPZ datasheet(11/16 Pages) AD

tên linh kiện	ADXL335BCPZ

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ADXL335

Rev. B | Page 11 of 16

APPLICATIONS INFORMATION

POWER SUPPLY DECOUPLING

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

close to the ADXL335 supply pins adequately decouples the

accelerometer from noise on the power supply. However, in

applications where noise is present at the 50 kHz internal clock

frequency (or any harmonic thereof), additional care in power

supply bypassing is required because this noise can cause errors

in acceleration measurement.

If additional decoupling is needed, a 100 Ω (or smaller) resistor

or ferrite bead can be inserted in the supply line. Additionally, a

larger bulk bypass capacitor (1 μF or greater) can be added in

parallel to CDC. Ensure that the connection from the ADXL335

ground to the power supply ground is low impedance because

noise transmitted through ground has a similar effect to noise

transmitted through VS.

SETTING THE BANDWIDTH USING CX, CY, AND CZ

The ADXL335 has provisions for band limiting the XOUT, YOUT,

and ZOUT pins. Capacitors must be added at these pins to imple-

ment 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, Z))

or more simply

F–3 dB = 5 μF/C(X, Y, Z)

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 0.0047 μF for CX,

CY, and CZ is recommended in all cases.

Table 4. Filter Capacitor Selection, CX, CY, and CZ

Bandwidth (Hz)

Capacitor (μF)

1

4.7

10

0.47

50

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 −1.08 g (corresponding to −325 mV) in the X-axis, +1.08 g

(or +325 mV) on the Y-axis, and +1.83 g (or +550 mV) on the

Z-axis. This ST pin can be left open-circuit or connected to

common (COM) in normal use.

Never expose the ST pin 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 selected accelerometer bandwidth ultimately determines

the measurement resolution (smallest detectable acceleration).

Filtering can be used to lower the noise floor to improve the

resolution of the accelerometer. Resolution is dependent on

the analog filter bandwidth at XOUT, YOUT, and ZOUT.

The output of the ADXL335 has a typical bandwidth of greater

than 500 Hz. The user must filter the signal at this point to

limit aliasing errors. The analog bandwidth must be no more

than half the analog-to-digital sampling frequency to minimize

aliasing. The analog bandwidth can be further decreased to

reduce noise and improve resolution.

The ADXL335 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 bandwidth). The user should

limit bandwidth to the lowest frequency needed by the applica-

tion to maximize the resolution and dynamic range of the

accelerometer.

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

the ADXL335 is determined by

)

1.6

(

×

=

BW

Density

Noise

rms

It is often useful to know the peak value of the noise. Peak-to-

peak noise can only be estimated by statistical methods. Table 5

is useful for estimating the probabilities of exceeding various

peak values, given the rms value.

Table 5. 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

Số phần tương tự - ADXL335BCPZ

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