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ADXL323KCPZ

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

Analog Devices 

ADXL323

APPLICATIONS

POWER SUPPLY DECOUPLING

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

close to the ADXL323 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 ADXL323 ground to the power supply

ground is low impedance because noise transmitted through

ground has an effect similar to that of noise transmitted

through VS.

SETTING THE BANDWIDTH USING CX, CY, AND CZ

The ADXL323 has provisions for band limiting the XOUT pin

and the YOUT pin. 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 5. 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

−500 mg (corresponding to −150 mV) in the X-axis, and 500 mg

(or 150 mV) on the Y-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

example, if there are multiple supply voltages), 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 and YOUT.

The output of the ADXL323 has a typical bandwidth of greater

than 1600 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 ADXL323 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 ADXL323 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 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

32

4 × rms

4.6

6 × rms

0.27

8 × rms

0.006

USE WITH OPERATING VOLTAGES OTHER THAN 3 V

The ADXL323 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 5.25 V.

Note that some performance parameters change as the supply

voltage is varied.

Rev. 0 | Page 12 of 16

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