ADXL321

Rev. 0 | Page 11 of 16

THEORY OF OPERATION

The ADXL321 is a complete acceleration measurement system

on a single monolithic IC. The ADXL321 has a measurement

range of ±18 g. 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 measures static acceleration forces, such as

gravity, which allows it to be used as a tilt sensor.

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 beam and unbalances 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 demodulator’s 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.

PERFORMANCE

Rather than using additional temperature compensation

circuitry, innovative design techniques have been used to ensure

high performance is built-in. As a result, there is neither

quantization error nor nonmonotonic behavior, and

temperature hysteresis is very low (typically less than 10 mg

over the −20°C to +70°C temperature range).

Figure 10 shows the zero g output performance of eight parts

(X- and Y-axis) over a −20°C to +70°C temperature range.

Figure 13 demonstrates the typical sensitivity shift over

temperature for supply voltages of 3 V. This is typically better

than ±1% over the −20°C to +70°C temperature range.