Page 205 - MEMS Mechanical Sensors

194 Inertial Sensors

Figure 8.18 The ADXL202 dual-axis accelerometer. The proof mass is compliant to move in both
in-plane directions and has interdigitated fingers on all four sides. (Courtesy Analog Devices, Inc.
From: http://www/analog.com.)

Figure 8.19 The suspension system of the ADXL202. (Courtesy Analog Devices, Inc. From:
http://www.analog.com.)

The bandwidth of the ADXL202 may be set from 0.01 Hz to 6 kHz via external
capacitors. The typical noise floor is 500 µg√Hz, allowing signals below 5 mg to be
resolved for bandwidths below 60 Hz.
The latest model, introduced in January 2003, is the ADXL311, which is priced
at only $2.50 in quantities greater than 10,000 units. It is also a dual-axis sensor and
the working principle is very similar to the previous models. Improved fabrication
tolerance controls have allowed improved performance. The main differences are
that the noise floor has dropped to 300 µg√Hz and the sensor can now be operated
from a single 3V power supply.
Two other companies offer commercial surface-micromachined accelerometers:
Motorola and Bosch. The latter have only recently started selling their sensors sepa-
rately. Previously they were only available embedded in complete automotive safety
systems (e.g., for airbag release). Little more information is available other than that
given on the datasheets.
Motorola’s MMA1201P is a single-axis, surface-micromachined MEMS accel-
erometer rated for ±40G and is packed in a plastic 16-lead DIP package. The oper-
ating temperature range is –40°C to +85°C with a storage temperature range of
–40°C to +105°C. The sensing element can sustain accelerations up to 2,000G from
any axis and unpowered and powered accelerations up to 500G. The main compo-
nents of the MMA1201P consist of a surface-micromachined capacitive sensing cell
(g-cell) and a CMOS signal conditioning ASIC. The g-cell’s mechanical structure is
composed of three consecutive semiconductor plates, defining sensitivity along the