Data Sheet

ADXL103/ADXL203

Rev. D | Page 13 of 16

THEORY OF OPERATION

The ADXL103/ADXL203 are complete acceleration measurement

systems on a single, monolithic IC. The ADXL103 is a single-

axis accelerometer, and the ADXL203 is a dual-axis accelerometer.

Both parts contain a polysilicon surface-micro-machined 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

ADXL103/ADXL203 are capable of measuring both positive

and negative accelerations from ±1.7 g to at least ±18 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 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 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

quantization error or nonmonotonic behavior, and temperature

hysteresis is very low (typically less than 10 mg over the −40°C

to +125°C temperature range).

Figure 11 shows the 0 g output performance of eight parts

(x and y axes) over a −40°C to +125°C temperature range.

Figure 13 demonstrates the typical sensitivity shift over

EARTH’S SURFACE

TOP VIEW

(Not to Scale)

PIN 8

PIN 8

PIN 8

PIN 8

Figure 39. Output Response vs. Orientation