ADXL05

–9–

REV. B

THEORY OF OPERATION

The ADXL05 is a complete acceleration measurement system

on a single monolithic IC. It contains a polysilicon surface-

micro machined sensor and signal conditioning circuitry which

implements a force-balance control loop. The ADXL05 is ca-

pable of measuring both positive and negative acceleration to a

maximum level of

±5 g.

Figure 16 is a simplified view of the ADXL05’s acceleration

sensor at rest. The actual structure of the sensor consists of 46

unit cells and a common beam. The differential capacitor sensor

consists of independent fixed plates and central plates attached

to the main beam that moves in response to an applied accelera-

tion. The two capacitors are series connected, forming a

capacitive divider with a common movable central plate. The

sensor’s fixed capacitor plates are driven differentially by a

1 MHz square wave: the two square wave amplitudes are equal

but are 180

° out of phase from one another. When at rest, the

values of the two capacitors are the same, and therefore, the

voltage output at their electrical center (i.e., at the center plate)

is zero.

CS1

CS2

BEAM

FIXED

OUTER

PLATES

UNIT CELL

CS1 = CS2

DENOTES ANCHOR

CENTER PLATE

Figure 16. A Simplified Diagram of the ADXL05

Sensor at Rest

Figure 17 shows the sensor responding to an applied accelera-

tion. When this occurs, the common central plate or “beam”

moves closer to one of the fixed plates while moving further

from the other. This creates a mismatch in the two capacitances,

resulting in an output signal at the central plate. The output

amplitude of the signal varies directly with the amount of accel-

eration experienced by the sensor.

BEAM

FIXED

OUTER

PLATES

UNIT CELL

CS1

TOP VIEW

APPLIED

ACCELERATION

CS1

CS2

DENOTES ANCHOR

CENTER PLATE

Figure 17. The ADXL05 Sensor Momentarily Responding

to an Externally Applied Acceleration

Figure 18 shows a block diagram of the ADXL05. The voltage

output from the central plate of the sensor is buffered and then

applied to a synchronous demodulator which is clocked, in

phase, with the same oscillator that drives the fixed plates

of the sensor. If the applied voltage is in sync and in phase

with the clock, a positive output will result. If the applied volt-

age is in sync but 180

° out of phase with the clock, then the

demodulator’s output will be negative. All other signals will be

rejected. An external capacitor, C1, sets the bandwidth of the

demodulator.

CS1

CS2

+3.4V

+1.8V +0.2V

COM

+3.4V

+5V

INTERNAL

REFERENCE

+1.8V

BUFFER

AMPLIFIER

1MHz

OSCILLATOR

+0.2V

0

°

180

°

+1.8V

3M

Ω

EXTERNAL

DEMODULATION

CAPACITOR

C1

C1

33k

Ω

33k

Ω

+5V

PREAMP

+1.8V

LOOP GAIN = 10

INTERNAL

FEEDBACK

LOOP

RST

SYNCHRONOUS

DEMODULATOR

+3.4V

75

Ω

+3.4V

SYNC

COMMON

+5V

+5V

EXTERNAL

OSCILLATOR

DECOUPLING

CAPACITOR

C2

DENOTES EXTERNAL

PIN CONNECTION

+3.4V

SELF–TEST

(ST)

50k

Ω

BEAM

Figure 18. Functional Block Diagram