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Test & measurement


implemented in the ADXL355, the flat bandwidth is limited by the user programmed ODR (bandwidth = ODR/4). In the plots shown here, an ODR selection of 4kHz results in a –3dB corner of approximately 1kHz. Vibrations at frequencies around the device resonance will encounter a resonant enhancement of sensitivity.


CALibrAtion


The dc sensitivity of the ADXL354 and ADXL355 is guaranteed in a range of ±8 per cent from nominal. Should the dc sensitivity not be calibrated, the maximum error in an acceleration measurement at dc is eight per cent. If higher accuracy is required, dc sensitivity calibration can be implemented by measuring at least two values per axis, by applying known input acceleration to the device. The simplest method to perform such a calibration is by orienting both directions (positive and negative), along each axis to the 1g gravity field. AC sensitivity variation in ADXL354


Figure 2. ADXL355 sensitivity vs. vibration frequency


Figure 3. ADXL354 part-to-part sensitivity variation vs. vibration frequency


vibration, using the setup described in the previous section. The frequency response for the sensitivity (normalised to the dc sensitivity) of the ADXL354 is shown in Figure 1. As can be inferred from the plot, the combination of the resonance enhancement and the attenuation, due to the analog antialiasing filter, limits the flat band (±5 per cent variation from dc) to roughly 1.3kHz. For example, the 3dB frequency corner (the frequency at which the sensitivity is twice the dc sensitivity) is roughly 2.1kHz for the x- and y-axes. The quality factor of the z-axis sensor is lower than the x- and y-axes sensors, and, thus,


the ac sensitivity does not equal twice the dc sensitivity at any frequency. The maximum sensitivity for the z-axis sensor is at its resonance frequency.


ADXL355


The parts were operated in ±8 g range with 5g peak excitation on the sinusoidal vibration, using the setup described above. The frequency response for the sensitivity of the ADXL355, shown in the Figure 2, is for an ODR selection of 4kHz. The plots show the sensitivity at all frequencies, normalised to the sensitivity at dc. Due to the additional digital filtering


essentially depends on the variation of the resonance frequency and the quality factor. The variation in these parameters is typically very small, governed by process variations. The frequency variation is typically less than two per cent across multiple devices, and the Q variation is typically less than 10 per cent across parts. Figure 3 shows a comparison of two ADXL354 parts (x- axis) with vastly different Q and resonance frequencies. The combination of the antialiasing filter, along with the resonance results in the normalised ac sensitivity at 2kHz, equal to 1.63 and 1.74 for both devices, a difference of approximately six per cent. Thus, if 100 mg of vibration is sensed by the accelerometer with higher Q at 2kHz, the other accelerometer will report the same signal as 94mg. In applications where absolute accuracy of the vibration content at a particular frequency is important, additional ac calibration with a precision shaker table is recommended. In conclusion, the decision to calibrate


or not to calibrate the accelerometer is dependent on the signal of interest. For vibration monitoring and structural health monitoring applications that require monitoring absolute magnitude of vibration harmonic frequencies, additional calibration is required. In applications that are intended to track relative shifts in natural oscillation, amplitude, and frequencies, the ADXL354 and ADXL355 accelerometers can be used with a baseline measurement, without additional calibration.


Analog Devices 28 analog.com April 2019 Instrumentation Monthly


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