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AD22280 Datasheet(PDF) 8 Page - Analog Devices |
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AD22280 Datasheet(HTML) 8 Page - Analog Devices |
8 / 12 page ADXL78 Rev. B | Page 8 of 12 APPLICATIONS POWER SUPPLY DECOUPLING For most applications, a single 0.1 μF capacitor, CDC, adequately decouples the accelerometer from noise on the power supply. However, in some cases, particularly where noise is present at the 400 kHz internal clock frequency (or any harmonic thereof), noise on the supply can cause interference on the ADXL78’s output. If additional decoupling is needed, a 50 Ω (or smaller) resistor or ferrite bead can be inserted in the supply line. Additionally, a larger bulk bypass capacitor (in the 1 μF to 4.7 μF range) can be added in parallel to CDC. SELF-TEST The fixed fingers in the forcing cells are normally kept at the same potential as that of the movable frame. When the self-test digital input is activated, the voltage on the fixed fingers on one side of the moving plate in the forcing cells is changed. This creates an attractive electrostatic force, which causes the frame to move toward those fixed fingers. The entire signal channel is active; therefore, the sensor displacement causes a change in VOUT. The ADXL78 self-test function is a comprehensive method of verifying the operation of the accelerometer. Because electrostatic force is independent of the polarity of the voltage across capacitor plates, a positive voltage is applied in half of the forcing cells, and its complement in the other half of the forcing cells. Activating self-test causes a step function force to be applied to the sensor, while the capacitive coupling term is canceled. The ADXL78 has improved self-test functionality, including excellent transient response and high speed switching capabilities. Arbitrary force waveforms can be applied to the sensor by modulating the self-test input, such as test signals to measure the system frequency response or even crash signals to verify algorithms within the limits of the self-test swing. The ST pin should never be exposed to voltages greater than VS + 0.3 V. If this cannot be guaranteed due to the system design (for instance, if there are multiple supply voltages), then a low VF clamping diode between ST and VS is recommended. CLOCK FREQUENCY SUPPLY RESPONSE In any clocked system, power supply noise near the clock frequency may have consequences at other frequencies. An internal clock typically controls the sensor excitation and the signal demodulator for micromachined accelerometers. If the power supply contains high frequency spikes, they may be demodulated and interpreted as an acceleration signal. A signal appears as the difference between the noise frequency and the demodulator frequency. If the power supply spikes are 100 Hz away from the demodulator clock, there is an output term at 100 Hz. If the power supply clock is at exactly the same frequency as the accelerometer clock, the term appears as an offset. If the difference frequency is outside of the signal bandwidth, the filter attenuates it. However, both the power supply clock and the accelerometer clock may vary with time or temperature, which can cause the interference signal to appear in the output filter bandwidth. The ADXL78 addresses this issue in two ways. First, the high clock frequency eases the task of choosing a power supply clock frequency such that the difference between it and the accelero- meter clock remains well outside of the filter bandwidth. Second, the ADXL78 is the only micromachined accelerometer to have a fully differential signal path, including differential sensors. The differential sensors eliminate most of the power supply noise before it reaches the demodulator. Good high frequency supply bypassing, such as a ceramic capacitor close to the supply pins, also minimizes the amount of interference. The clock frequency supply response (CFSR) is the ratio of the response at VOUT to the noise on the power supply near the accelerometer clock frequency. A CFSR of 3 means that the signal at VOUT is 3× the amplitude of an excitation signal at VDD near the accelerometer internal clock frequency. This is analogous to the power supply response, except that the stimulus and the response are at different frequencies. The ADXL78’s CFSR is 10× better than a typical single-ended accelerometer system. SIGNAL DISTORTION Signals from crashes and other events may contain high amplitude, high frequency components. These components contain very little useful information and are reduced by the 2-pole Bessel filter at the output of the accelerometer. However, if the signal saturates at any point, the accelerometer output does not look like a filtered version of the acceleration signal. The signal may saturate anywhere before the filter. For example, if the resonant frequency of the sensor is low, the displacement per unit acceleration is high. The sensor may reach the mechanical limit of travel if the applied acceleration is high enough. This can be remedied by locating the accelerometer where it does not see high values of acceleration, and by using a higher resonant frequency sensor such as the ADXL78. Also, the electronics may saturate in an overload condition between the sensor output and the filter input. Ensuring that the internal circuit nodes operate linearly to at least several times the full-scale acceleration value can minimize electrical saturation. The ADXL78’s circuits are linear to approximately 8× full scale. |
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