ADXL50

–8–

REV. B

THEORY OF OPERATION

The ADXL50 is a complete acceleration measurement system

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

cro machined sensor and signal conditioning circuitry. The

ADXL50 is capable of measuring both positive and negative ac-

celeration to a maximum level of

±50 g.

Figure 16 is a simplified view of the ADXL50’s acceleration

sensor at rest. The actual structure of the sensor consists of 42

unit cells and a common beam. The differential capacitor sensor

consists of independent fixed plates and a movable “floating”

central plate which deflects in response to changes in relative

motion. The two capacitors are series connected, forming a ca-

pacitive divider with a common movable central plate. A force

balance technique counters any impeding deflection due to ac-

celeration and servos the sensor back to its 0 g position.

CS1

CS2

CENTER

PLATE

FIXED

OUTER

PLATES

UNIT CELL

CS1 = CS2

BEAM

TETHER

CS1

CS2

CENTER

PLATE

TOP VIEW

DENOTES ANCHOR

Figure 16. A Simplified Diagram of the ADXL50 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. The sensor’s fixed capacitor plates are driven

deferentially by a 1 MHz square wave: the two square wave am-

plitudes 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.

When the sensor begins to move, a mismatch in the value of

their capacitance is created producing an output signal at the

central plate. The output amplitude will increase with the

amount of acceleration experienced by the sensor. Information

concerning the direction of beam motion is contained in the

phase of the signal with synchronous demodulation being used

to extract this information. Note that the sensor needs to be po-

sitioned so that the measured acceleration is along its sensitive

axis.

Figure 18 shows a block diagram of the ADXL50. The voltage

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

applied to a synchronous demodulator. The demodulator is also

supplied with a (nominal) 1 MHz clock signal from the same

oscillator which drives the fixed plates of the sensor. The

demodulator will rectify any voltage which is in sync with its

clock signal. If the applied voltage is in sync and in phase with

the clock, a positive output will result. If the applied voltage is in

sync but 180

° out of phase with the clock, then the demodu-

lator’s output will be negative. All other signals will be rejected.

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

The output of the synchronous demodulator drives the preamp

—an instrumentation amplifier buffer which is referenced to

+1.8 volts. The output of the preamp is fed back to the sensor

through a 3 M

Ω isolation resistor. The correction voltage re-

quired to hold the sensor’s center plate in the 0 g position is a

direct measure of the applied acceleration and appears at the

CS1

CS2

UNIT CELL

CS1 < CS2

BEAM

TOP VIEW

DENOTES ANCHOR

APPLIED

ACCELERATION

Figure 17. The ADXL50 Sensor Momentarily Responding

to an Externally Applied Acceleration

When the ADXL50 is subjected to an acceleration, its capacitive

sensor begins to move creating a momentary output signal. This

is signal conditioned and amplified by the demodulator and

preamp circuits. The dc voltage appearing at the preamp output

is then fed back to the sensor and electrostatically forces the

center plate back to its original center position.

At 0 g the ADXL50 is calibrated to provide +1.8 volts at the

to the voltage required to hold the sensor stationary for the du-

ration of the acceleration and provides an output which varies

directly with applied acceleration.

The loop bandwidth corresponds to the time required to apply

feedback to the sensor and is set by external capacitor C1. The

loop response is fast enough to follow changes in g level up to

and exceeding 1 kHz. The ADXL50’s ability to maintain a flat

response over this bandwidth keeps the sensor virtually motion-

less. This essentially eliminates any nonlinearity or aging effects

due to the sensor beam’s mechanical spring constant, as com-

pared to an open-loop sensor.

An uncommitted buffer amplifier provides the capability to ad-

just the scale factor and 0 g offset level over a wide range. An in-

ternal reference supplies the necessary regulated voltages for

powering the chip and +3.4 volts for external use.

OBSOLETE