XMMA1000P Datasheet, PDF (5/8 Pages) Motorola, Inc – MICROMACHINED ACCELEROMETER

PRINCIPLE OF OPERATION
The Motorola accelerometer is a surface–micromachined
integrated–circuit accelerometer.
The device consists of a surface micromachined capaci-
tive sensing cell (G–cell) and a CMOS signal conditioning
ASIC contained in a single integrated circuit package. The
sensing element is sealed hermetically at the wafer level
using a bulk micromachined “cap’’ wafer.
The G–Cell is a mechanical structure formed from semi-
conductor materials (polysilicon) using semiconductor pro-
cesses (masking and etching). It consists of two stationary
plates with a moveable plate in–between. The center plate
can be deflected from its rest position by subjecting the sys-
tem to an acceleration (Figure 1).
When the center plate deflects, the distance from it to one
fixed plate will increase by the same amount that the dis-
tance to the other plate decreases. The change in distance is
a measure of acceleration.
The G–Cell plates form two back–to–back capacitors
(Figure 2). As the center plate moves with acceleration, the
distance between the plates changes and each capacitor’s
value will change, (C = Aε/D). Where A is the area of the
plate, ε is the dielectric constant, and D is the distance
between the plates.
The CMOS ASIC uses switched capacitor techniques to
measure the G–Cell capacitors and extract the acceleration
data from the difference between the two capacitors. The
ASIC also signal conditions and filters (switched capacitor)
the signal, providing a high level output voltage that is ratio-
metric and proportional to acceleration.
Acceleration
Figure 1.
Figure 2.
SPECIAL FEATURES
Filtering
The Motorola accelerometers contain an onboard 4–pole
switched capacitor filter. A Bessel implementation is used
because it provides a maximally flat delay response (linear
phase) thus preserving pulse shape integrity. Because the fil-
ter is realized using switched capacitor techniques, there is
no requirement for external passive components (resistors
and capacitors) to set the cut–off frequency.
Noise Calculation
The noise for the Motorola accelerometer is specified as
an rms value which is a statistical value of a gaussian noise
source. To convert the rms values to a peak to peak value at
a particular confidence level refer to Table 1. A sample cal-
culation at a 99.9% confidence level is shown.
XMMA1000P XMMA2000W
Table 1.
Nominal Peak to Peak Value
2.0 rms
3.0 rms
4.0 rms
5.0 rms
6.0 rms
6.6 rms
% Confidence Level
68%
87%
95.40%
98.80%
99.73%
99.90%
Noise rms = 3.5mVrms
Noise peak to peak at a 99.9% confidence level:
3.5mVrms* 6.6 = 23.1mVpp
Self–Test
XMMA sensors provide a self–test feature that allows the
verification of the mechanical and electrical integrity of the
accelerometer at any time before or after installation. This
feature is critical in applications such as automotive airbag
systems where system integrity must be ensured over the life
of the vehicle. A fourth “plate’’ is used in the g–cell as a self–
test plate. This plate is fixed and is located under an ex-
tended portion of the center (moveable) plate. When the user
applies a logic high input to the self–test pin, a calibrated po-
tential is applied across the self–test plate and the moveable
plate. The resulting electrostatic force (Fe = ^1/2 AV2/d2)
causes the center plate to deflect. The resultant deflection, is
measured by the accelerometer’s control ASIC and a propor-
tional output voltage results. This procedure assures that
both the mechanical (g–cell) and electronic sections of the
sensor are functioning.
Ratiometricity
The XMMA1000P and XMMA2000W are designed to be
“ratiometric’’. That is, their transfer function will be propor-
tional to the applied supply voltage. This feature allows easy
interfacing to common microcontrollers that use ratiometric
A/D converters for system cost benefits.
In operation, a ratiometric sensor’s gain or “sensitivity’’ will
change 1:1 with applied supply voltage and the zero signal
output will be at midsupply. (2.5 V for a 5 V VDD and 2.625 V
for a 5.25 VDD).
Minimum G Range Calculation
To calculate the minimum g range values of an accelerom-
eter several factors have to be taken into consideration.
These considerations include, the supply voltage, the
device’s sensitivity, offset voltage and output rail. A sample
calculation for the minimum g range is shown below.
To complete the calculation the rail and offset voltages
must be subtracted from the supply voltage, then divided by
the supply voltage multiplied by the device’s worst case
(highest) sensitivity.
* * * V_DD 0.56V_DD 0.3V
ń ń ^{+ + ǒ * Ǔ} V_DD(8.64mV V g)
0.44V_DD 0.3V
V_DD(0.00864)
50.93
34.72
V_DD
g
Using the standard five volt power supply, the minimum g
range is calculated to be:
* + [ 50.926
34.722
5.00
43.98
44g
Motorola Sensor Device Data
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