ADXL311JE Ver la hoja de datos (PDF) - Analog Devices

Número de pieza

componentes Descripción

Fabricante

ADXL311JE

Low Cost, Ultracompact ±2 gDual-Axis Accelerometer

Analog Devices 

ADXL311

THEORY OF OPERATION

The ADXL311 is a complete, dual-axis acceleration measure-

ment system on a single monolithic IC. It contains a polysilicon

surface-micromachined sensor and signal conditioning cir-

cuitry to implement an open-loop acceleration measurement

architecture. The output signals are analog voltage proportional

to acceleration. The ADXL311 is capable of measuring both

positive and negative accelerations to at least ±2 g. The acceler-

ometer can measure static acceleration forces, such as gravity,

allowing it to be used as a tilt sensor.

The sensor is a surface-micromachined polysilicon structure

built on top of the silicon wafer. Polysilicon springs suspend the

structure over the surface of the wafer and provide a resistance

against acceleration forces. Deflection of the structure is meas-

ured using a differential capacitor that consists of independent

fixed plates and central plates attached to the moving mass. The

fixed plates are driven by 180° out of phase square waves. Accel-

eration will deflect the beam and unbalance the differential

capacitor, resulting in an output square wave whose amplitude is

proportional to acceleration. Phase sensitive demodulation

techniques are then used to rectify the signal and determine the

direction of the acceleration.

The output of the demodulator is amplified and brought off-

chip through a 32 kΩ resistor. At this point, the user can set the

signal bandwidth of the device by adding a capacitor. This

filtering improves measurement resolution and helps prevent

aliasing.

Applications

POWER SUPPLY DECOUPLING

For most applications, a single 0.1 µF capacitor, CDC, will ade-

quately decouple the accelerometer from noise on the power

supply. However, in some cases, particularly where noise is pre-

sent at the 100 kHz internal clock frequency (or any harmonic

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

ADXL311 output. If additional decoupling is needed, a 100 Ω

(or smaller) resistor or ferrite beads may be inserted in the sup-

ply line of the ADXL311. 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 ADXL311 has provisions for bandlimiting 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–3dB = 1/ 2π 32 kΩ ×C(X,Y )

or, more simply

F–3dB = 5 µF/C(X,Y )

The tolerance of the internal resistor (RFILT) can vary typically as

much as ±15% of its nominal value of 32 kΩ; thus, the band-

width will vary accordingly. A minimum capacitance of 1000 pF

for CX and CY is required in all cases.

Table 4. Filter Capacitor Selection, CX and CY

Bandwidth

Capacitor (µF)

10 Hz

0.47

50 Hz

0.10

100 Hz

0.05

200 Hz

0.027

500 Hz

0.01

5 kHz

0.001

SELF TEST

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

VDD, an electrostatic force is exerted on the beam of the acceler-

ometer. The resulting movement of the beam allows the user to

test if the accelerometer is functional. The typical change in

output will be 270 mg (corresponding to 45 mV). This pin may

be left open circuit or connected to common in normal use.

RBIAS SELECTION

A bias resistor (RBIAS) must always be used. If no resistor is pre-

sent, the ADXL311 may appear to work but will suffer degraded

noise performance. The value of the resistor used is not critical.

Any value from 50 kΩ to 2 MΩ can be used. Using a 2 MΩ

resistor rather than a 50 kΩ will save roughly 25 µA of supply

current.

Design Trade-Offs for Selecting Filter

Characteristics: The Noise/BW Trade-Off

The accelerometer bandwidth selected will ultimately determine

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 ADXL311 has a typical bandwidth of 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 ADXL311 noise has the characteristics of white Gaussian

noise that contributes equally at all frequencies and is described

in terms of µg/√Hz, i.e., the noise is proportional to the square

Rev. A | Page 7 of 12

Share Link: