ADXL337 Просмотр технического описания (PDF) - Analog Devices

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ADXL337

Small, Low Power, 3-Axis ±3 g Accelerometer

Analog Devices 

ADXL337

APPLICATIONS INFORMATION

POWER SUPPLY DECOUPLING

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

close to the ADXL337 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 ADXL337 ground to the power supply ground is low

impedance because noise transmitted through ground has a

similar effect as noise transmitted through VS.

SETTING THE BANDWIDTH USING CX, CY, AND CZ

The ADXL337 has provisions for band limiting the XOUT, YOUT,

and ZOUT 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, 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 mg (or +550 mV) on the

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

common (GND) 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 ADXL337 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 decreased further to reduce noise

and improve resolution.

The ADXL337 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 application to maximize

the resolution and dynamic range of the accelerometer.

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

the ADXL337 is determined by

rms Noise = Noise Density × ( BW ×1.6 )

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

Percent 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

Rev. 0 | Page 11 of 16

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