ADXL78

Rev. B | Page 7 of 12

THEORY OF OPERATION

The ADXL78 provides a fully differential sensor structure and

circuit path, resulting in the industry’s highest resistance to

EMI/RFI effects. This latest generation uses electrical feedback

with zero-force feedback for improved accuracy and stability.

The sensor resonant frequency is significantly higher than the

signal bandwidth set by the on-chip filter, avoiding the signal

analysis problems caused by resonant peaks near the signal

bandwidth.

UNIT

SENSING

CELL

MOVABLE

FRAME

FIXED

PLATES

UNIT

FORCING

CELL

ANCHOR

MOVING

PLATE

PLATE

CAPACITORS

ANCHOR

Figure 5 is a simplified view of one of the differential sensor

elements. Each sensor includes several differential capacitor

unit cells. Each cell is composed of fixed plates attached

to the substrate and movable plates attached to the frame.

Displacement of the frame changes the differential capacitance,

which is measured by the on-chip circuitry.

Figure 5. Simplified View of Sensor Under Acceleration

Complementary 400 kHz square waves drive the fixed plates.

Electrical feedback adjusts the amplitudes of the square waves

such that the ac signal on the moving plates is 0. The feedback

signal is linearly proportional to the applied acceleration. This

unique feedback technique ensures that there is no net

electrostatic force applied to the sensor. The differential

feedback control signal is also applied to the input of the filter,

where it is filtered and converted to a single-ended signal.