ADXL001

Rev. A | Page 11 of 16

THEORY OF OPERATION

DESIGN PRINCIPLES

The ADXL001 accelerometer provides a fully differential sensor

structure and circuit path for excellent resistance to EMI/RFI

interference.

This latest generation SOI MEMS device takes advantage

of mechanically coupled but electrically isolated differential

sensing cells. This improves sensor performance and size

because a single proof mass generates the fully differential

signal. The sensor signal conditioning also uses electrical

feedback with zero-force feedback for improved accuracy

and stability. This force feedback cancels out the electrostatic

forces contributed by the sensor circuitry.

Figure 19 is a simplified view of one of the differential sensor

cell blocks. Each sensor block includes several differential

capacitor unit cells. Each cell is composed of fixed plates attached

to the device layer and movable plates attached to the sensor

frame. Displacement of the sensor frame changes the differential

capacitance. On-chip circuitry measures the capacitive change.

MECHANICAL SENSOR

The ADXL001 is built using the Analog Devices SOI MEMS

sensor process. The sensor device is micromachined in-plane

in the SOI device layer. Trench isolation is used to electrically

isolate, but mechanically couple, the differential sensing elements.

Single-crystal silicon springs suspend the structure over the

handle wafer and provide resistance against acceleration forces.

UNIT

SENSING

CELL

MOVABLE

FRAME

FIXED

PLATES

UNIT

FORCING

CELL

ANCHOR

MOVING

PLATE

PLATE

CAPACITORS

ANCHOR

Figure 19. Simplified View of Sensor Under Acceleration