I am inquiring about the mechanical resonant frequency of the mass spring associated with the measuring mechanism of the ADXL345? I have made a measurement with an ADXL345 on a vertical mass spring with the electronics of the ADXL345 representing the mass atop of a pole. The pole is 1/4" fully threaded steel rod fixed in a vice 33" below the top where the electronics are fixed. I pull the rod 5" off the center and let it go measuring the accelerations. I expected a largely under-damped 2nd order system mapping a single decaying resonant frequency decaying over a long time. Instead, the mapped accelerations indicate another frequency at the largest accelerations, letting go, and decaying to a single frequency, what I expected. I am wondering if I am measuring the beating frequencies of two mass springs, the system under test, and the ADXL345.
What is the mechanical resonant frequency of the ADXL345?