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ADXL05JH Arkusz danych(PDF) 9 Page - Analog Devices |
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ADXL05JH Arkusz danych(HTML) 9 Page - Analog Devices |
9 / 20 page ADXL05 –9– REV. B THEORY OF OPERATION The ADXL05 is a complete acceleration measurement system on a single monolithic IC. It contains a polysilicon surface- micro machined sensor and signal conditioning circuitry which implements a force-balance control loop. The ADXL05 is ca- pable of measuring both positive and negative acceleration to a maximum level of ±5 g. Figure 16 is a simplified view of the ADXL05’s acceleration sensor at rest. The actual structure of the sensor consists of 46 unit cells and a common beam. The differential capacitor sensor consists of independent fixed plates and central plates attached to the main beam that moves in response to an applied accelera- tion. The two capacitors are series connected, forming a capacitive divider with a common movable central plate. The sensor’s fixed capacitor plates are driven differentially by a 1 MHz square wave: the two square wave amplitudes are equal but are 180 ° out of phase from one another. When at rest, the values of the two capacitors are the same, and therefore, the voltage output at their electrical center (i.e., at the center plate) is zero. CS1 CS2 BEAM FIXED OUTER PLATES UNIT CELL CS1 = CS2 DENOTES ANCHOR CENTER PLATE Figure 16. A Simplified Diagram of the ADXL05 Sensor at Rest Figure 17 shows the sensor responding to an applied accelera- tion. When this occurs, the common central plate or “beam” moves closer to one of the fixed plates while moving further from the other. This creates a mismatch in the two capacitances, resulting in an output signal at the central plate. The output amplitude of the signal varies directly with the amount of accel- eration experienced by the sensor. BEAM FIXED OUTER PLATES UNIT CELL CS1 < CS2 TOP VIEW APPLIED ACCELERATION CS1 CS2 DENOTES ANCHOR CENTER PLATE Figure 17. The ADXL05 Sensor Momentarily Responding to an Externally Applied Acceleration Figure 18 shows a block diagram of the ADXL05. The voltage output from the central plate of the sensor is buffered and then applied to a synchronous demodulator which is clocked, in phase, with the same oscillator that drives the fixed plates of the sensor. If the applied voltage is in sync and in phase with the clock, a positive output will result. If the applied volt- age is in sync but 180 ° out of phase with the clock, then the demodulator’s output will be negative. All other signals will be rejected. An external capacitor, C1, sets the bandwidth of the demodulator. CS1 CS2 +3.4V +1.8V +0.2V COM +3.4V VREF +5V INTERNAL REFERENCE +1.8V VIN– BUFFER AMPLIFIER VOUT 1MHz OSCILLATOR +0.2V 0 ° 180 ° +1.8V 3M Ω EXTERNAL DEMODULATION CAPACITOR C1 C1 33k Ω 33k Ω +5V PREAMP +1.8V LOOP GAIN = 10 VPR INTERNAL FEEDBACK LOOP RST SYNCHRONOUS DEMODULATOR +3.4V 75 Ω +3.4V SYNC COMMON +5V +5V EXTERNAL OSCILLATOR DECOUPLING CAPACITOR C2 DENOTES EXTERNAL PIN CONNECTION +3.4V SELF–TEST (ST) 50k Ω BEAM Figure 18. Functional Block Diagram |
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