Theory Of Operation; Overview; Acceleration Sensor - Analog Devices ADXL180 iMEMS Manual

ADXL180

THEORY OF OPERATION

OVERVIEW

The ADXL180 is a complete satellite system, including
acceleration sensor, data filtering, digital protocol functionality,
and a 2-wire, high-voltage, current-modulated bus interface
communications port.

ACCELERATION SENSOR

The ADXL180 provides a fully differential sensor structure and
circuit path. This device uses electrical feedback with zero force
feedback. Figure 4 is a simplified view of one of the differential
sensor elements. Each sensor includes several differential capa-
citor 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 the on-
chip circuitry measures.
Complementary signals drive the fixed capacitor plates. The
relative phasing between the two halves of the differential
ACCELERATION SENSING AXIS
+X-AXIS SEN
SPRING
ISOLATED
MECHANICAL
COUPLINGS
–X-AXIS SENSOR
A NCHOR
PLATE
CAPACITORS
UNIT SENSING
CELL
MOVING
PLATE
Figure 4. Simplified View of ADXL180 Sensor Under Acceleration
+
SOR
0
0
–
Figure 5. Differential Acceleration Sensing
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sensor is such that the displacement signal is differential
between the two measurement channels. Using the fully
differential sensor and an antiphase clocking scheme helps
reject electrical environmental noise (see Figure 5).
The ADXL180 acceleration sensor uses two electrically isolated,
mechanically coupled sensors to measure acceleration as shown
in Figure 5. The clock phasing of the readout is such that the
electrical signal due to acceleration is differential between the
channels and environmental disturbances couple in as a common-
mode signal. The following differential amplifier can then extract
the acceleration signal while suppressing the environmental noise.
Electrical feedback adjusts the amplitudes of the fixed capacitor
plates' drive signals such that the ac signal on the moving plates
is zero. The feedback signal is linearly proportional to the applied
acceleration. This feedback technique ensures that there is no
net electrostatic force applied to the sensor.
MOVABLE
FRAME
FIXED
PLATES
UNIT SELF-TEST
FORCING CELL
ANCHOR
EMI DISTURBANCE RESPONSE
COMMON TO BOTH CHANNELS
+
AMP
–
ACCELERATION RESPONSE
DIFFERENTIAL BETWEEN CHANNELS
V
OUT