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192 Inertial Sensors

a small volume of air, which responds to acceleration. The temperature distribution
under acceleration of the heated air bubble becomes asymmetric with respect to the
heater and can be measured by temperature sensors placed symmetrically around the
heater. Simple piezoresistors can be used both for heating and temperature sensing
[49]. The sensor has a relatively low bandwidth from dc to 20 Hz. The authors claim,
however, that with design modifications this can be extended to several hundred
hertz and sensitivities in the microG range are possible.
Finally, there are sensors that sense the motion of the proof mass by electro-
magnetic means. Abbaspour-Sani et al. [50] designed an accelerometer with two
12-turn coils, one located on the proof mass, the other one on the substrate. Accel-
eration causes changes in the distance between the two coils, which results in a
change of the mutual inductance. They achieved a sensitivity 0.175 V/G with a
dynamic range of 0G to 50G. An advantage of this approach is the simple read-out
electronics.

8.2.3 Commercial Micromachined Accelerometer
In this section, a selective overview of commercially available micromachined accel-
erometers is given. Often, detailed information about the design and fabrication
process is not readily available, as this is often considered proprietary.
One of the most successful ranges of micromachined accelerometer was intro-
duced by Analog Devices and is termed the ADXL range. These devices are primarily
aimed at the automotive market; the first commercial device was the ADXL50,
released in 1991. It is based on a surface micromachined technology with the sensing
electronics integrated on the same chip. It is operated in an analog force-balancing
closed loop control system and has a ±50G dynamic range with a 6.6-mG/√Hz noise
floor, a bandwidth of 6 kHz, and a shock survivability of more than 2,000G, mak-
ing it suitable for airbag deployment. The nominal sense capacitance is 100 fF and
the sensitivity is 19 mV/G. A simplified control system block diagram is shown in
Figure 8.17.
The sensor’s fixed electrodes are excited differentially with a 1-MHz square
wave, which are equal in amplitude but 180° out of phase. If the proof is not
deflected, the two capacitors are matched and the resulting output voltage of the
buffer is zero. If the proof is displaced from the center, the amplitude of the buffer
voltage is proportional to the mismatch in capacitance. The buffer voltage is
demodulated and amplified by an instrumentation amplifier referenced to 1.8V; this
signal is fed back to the proof mass througha3MΩ isolation resistor. This results in
an electrostatic force that maintains the proof mass virtually motionless over the
dynamic range. The output signal for 0G is +1.8V with an output swing of ±0.95V
for ±50G acceleration; with an internal buffer and level shifter this can be amplified
to an output range from 0.25V to 4.75V. The sensor additionally has a self-test
capability where a transistor-transistor logic (TTL) “high” signal is applied to one of
the pins, which results in an electrostatic force approximately equal to a –50G iner-
tial force. If the sensor operates correctly, a –1-V output signal is produced. The sen-
sor is available in a standard 10-pin TO100 metal package.
Subsequently, Analog Devices has introduced a range of other micromachined
accelerometers. The ADXL05 works in the same way as the ADXL50 but has a