ADXL325

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THEORY OF OPERATION

The ADXL325 is a complete 3-axis acceleration measurement

system. The ADXL325 has a measurement range of ±5 g

minimum. It contains a polysilicon surface micromachined

sensor and signal conditioning circuitry to implement an open-

loop acceleration measurement architecture. The output signals

are analog voltages that are proportional to acceleration. The

accelerometer can measure the static acceleration of gravity in

tilt-sensing applications, as well as dynamic acceleration, resulting

from motion, shock, or vibration.

The sensor is a polysilicon surface micromachined structure

built on top of a 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 measured

using a differential capacitor that consists of independent fixed

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

are driven by 180° out-of-phase square waves. Acceleration deflects

the moving mass and unbalances the differential capacitor resulting

in a sensor output whose amplitude is proportional to acceleration.

Phase-sensitive demodulation techniques are then used to determine

the magnitude and direction of the acceleration.

The demodulator output is amplified and brought off-chip through

a 32 kΩ resistor. The user then sets the signal bandwidth of the

device by adding a capacitor. This filtering improves measurement

resolution and helps prevent aliasing.

MECHANICAL SENSOR

The ADXL325 uses a single structure for sensing the X, Y, and Z axes.

As a result, the three axes sense directions are highly orthogonal

with little cross-axis sensitivity. Mechanical misalignment of the

sensor die to the package is the chief source of cross-axis sensitivity.

Mechanical misalignment can, of course, be calibrated out at

the system level.

PERFORMANCE

Rather than using additional temperature compensation circuitry,

innovative design techniques ensure that high performance is

built-in to the ADXL325. As a result, there is neither quantization

error nor nonmonotonic behavior, and temperature hysteresis is

very low (typically <3 mg over the −25°C to +70°C temperature

range).