ADXL327 データシートの表示（PDF） - Analog Devices

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ADXL327

Small, Low Power, 3-Axis ±2 g Accelerometer

Analog Devices 

ADXL327

APPLICATIONS INFORMATION

POWER SUPPLY DECOUPLING

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

close to the ADXL327 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 ADXL327 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 ADXL327 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 3 dB bandwidth equation is

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 ADXL327 has a typical bandwidth 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 ADXL327 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.

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

whether the accelerometer is functional. The typical change in

output is −1.08 g (corresponding to −450 mV) in the X axis,

+1.08 g (+450 mV) on the Y axis, and +1.83 g (+770 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,

there are multiple supply voltages), then a low VF clamping

diode between ST and VS is recommended.

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

the ADXL327 is determined by

rms Noise = Noise Density × ( BW ×1.6 )

Often, the peak value of the noise is desired. 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

USE WITH OPERATING VOLTAGES OTHER THAN 3 V

The ADXL327 is tested and specified at VS = 3 V; however, it can be

powered with VS as low as 1.8 V or as high as 3.6 V. Note that some

performance parameters change as the supply voltage is varied.

The ADXL327 output is ratiometric; therefore, the output

sensitivity (or scale factor) varies proportionally to the supply

voltage. At VS = 3.6 V, the output sensitivity is typically 500 mV/g.

At VS = 2 V, the output sensitivity is typically 289 mV/g.

The zero g bias output is also ratiometric; therefore, 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 = 3.6 V, the X- and Y-

axis noise density is typically 200 μg/√Hz, while at VS = 2 V, the

X- and Y-axis noise density is typically 300 μg/√Hz.

Rev. 0 | Page 11 of 16

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