ADXL204

Rev. A | Page 9 of 12

THEORY OF OPERATION

EARTH'S SURFACE

TOP VIEW

(Not to Scale)

PIN 8

PIN 8

PIN 8

PIN 8

Figure 22. Output Response vs. Orientation

The ADXL204 is a complete acceleration measurement system on

a single monolithic IC. The ADXL204 is a dual-axis accelerometer.

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 proportional to acceleration. The ADXL204 is

capable of measuring both positive and negative accelerations to

at least ±1.7 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

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 10 shows the zero g output performance of eight parts

(X-axis and Y-axis) over a –40°C to +125°C temperature range.

Figure 13 demonstrates the typical sensitivity shift over tem-

than ±1% over temperature.