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8.3 Micromachined Gyroscopes                                                  205

                                 Voltage
                                 multiplier
                                                       Velocity amp
                                                                    Trim
                                                                    and
                                                                    test


                                                    Rate
                                         Drivers    sensor
                                 Regulators

                                                              Coriolis
                                                              amp

                                   Output
                                               Demodulator      Bias
                                                                References
                                                                Temperature
                                                      (a)

                                                                Inner frame

                                                                Resonating mass


                                                                Mass drive direction

                                                                Springs

                                                                Coriolis sense fingers
                                                      (b)
                  Figure 8.29  (a) Die photo of the surface-micromachined gyroscope from Analog Devices with
                  the interface and control electronics integrated on the same chip. It contains two identical
                  mechanical structures to achieve differential sensing. (b) Schematic drawing of one of the two
                  identical gyroscope elements. (Courtesy Analog Devices, Inc. Picture taken from ADI Web site,
                  http://www.analog.com.)




                  to +85°C and is available in a space-saving 32-pin Ball Grid Array surface-mount
                  package measuring 7 × 7 × 3 mm. Both are priced at approximately $30 per unit in
                  thousand-piece quantities. Because the internal resonators require 14V to 16V for
                  proper operation, ADI includes on-chip charge pumps to boost an applied TTL-
                  level voltage. Both the ADXRS150 and ADXRS300 are essentially z-axis gyro-
                  scopes based on the principle of resonant-tuning-fork gyroscopes. In these systems,
                  two polysilicon sensing structures each contain a so-called dither frame that is
                  driven electrostatically to resonance. Interestingly, the gyroscope includes two iden-
                  tical structures to enable differential sensing in order to reject environmental shock
                  and vibration. Figure 8.29(b) shows one structure schematically.
                      A rotation about the z-axis, normal to the plane of the chip, produces a Coriolis
                  force that displaces the inner frame perpendicular to the vibratory motion. This
                  Coriolis motion is detected by a series of capacitive pick-off structures on the edges
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