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Digital
Accelerometers
Fabricated in a
2-micron CMOS
process, the digital ASIC functions as a sigma-delta type
capacitance-to-frequency converter. The ASIC modulates the capacitive
sense element and monitors the effect of accelerations on the sense
element via the sense amplifier. The comparator then demodulates the
output of the sense amplifier to form the (sigma-delta type) pulse
density digital output.
The digital section of the ASIC is driven by an external clock source
and controls the sequence in which the reference voltages are switched
onto the sense elements' capacitor plates. Non-volatile PROM and D/A
converters are used to generate these reference voltages and provide a
method of calibrating the accelerometer, thus compensating for the bias
and scale factor fabrication tolerances of the sense element.
| The
digital output from the electronics allows direct connection to an
inexpensive microprocessor or digital counter without requiring a
separate analog-to-digital converter. This type of output also provides
substantial insensitivity to electromagnetic interference (EMI)
compared with low level analog signals. |
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The
pulse density output signal consists of a series of logic pulses and
the pulse rate of this signal is linearly proportional to the applied
acceleration. The output pulse rate varies from no pulses per second
for negative full scale acceleration to the maximum pulse rate (equal
to the input clock frequency) for positive full scale acceleration. At
zero acceleration, the pulse rate is nominally equal to one-half the
input clock rate. |
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