ADXL206

Rev. 0 | Page 9 of 12

THEORY OF OPERATION

The ADXL206 is a complete acceleration measurement system

on a single, monolithic IC. The part contains a polysilicon, surface-

micromachined sensor and signal conditioning circuitry to imple-

ment an open-loop acceleration measurement architecture. The

output signals are analog voltages proportional to acceleration.

The ADXL206 is capable of measuring both positive and negative

accelerations to at least ±5 g. The accelerometer 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 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 accel-

eration. 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.

PERFORMANCE

Rather than using additional temperature compensation circuitry,

innovative design techniques have been used to ensure that high

performance is built in. As a result, there is essentially no quantiza-

tion error or nonmonotonic behavior, and temperature hysteresis

is very low (typically less than 2 mg over the −40°C to +175°C

temperature range).

Figure 12 shows the 0 g output performance of eight parts over

the −40°C to +175°C temperature range.

Figure 11 and Figure 14 show the typical sensitivity shift over