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ADXL357 additional peaks in FFT


My name is Maciej.

I tested ADXL356 and ADXL357 accelerometers. The test consisted in placing the sensors on a professional vibration exciter from TIRA GmbH. The sensors were soldered to the EVAL-ADXL356 / EVAL-ADXL357 evaluation board, and the board was screwed to the metal base (as shown in the attached picture).

The metal base was screwed to the Tira vibration exciter.

In the attached figure I present the FFT of signals when the vibration excitation was set to 500 Hz.

As can be seen in the case of ADXL357, there are additional excitations that look like amplitude modulation. These extra peaks appear for all set frequencies - the magnitude of the peaks increases as the frequency increases. I did tests for three ADXL357 accelerometers, two ADXL356 accelerometers and three metal bases. All results are similar: ADXL356 - no additional peaks, ADXL357 - additional peaks for the three metal bases. The same metal base don’t give peaks for the ADXL356 and produces peaks for the ADXL357. What could be the reason for this behaviour? Incorrect installation of the ADXL357 or incorrect configuration? If the assembly was incorrect, the same metal base should give additional peaks for the ADXL356 and ADXL 357, which is not the case.

ADXL357 initialization commands are as follows:

1) Reset command: address  0x2F, value 0

2) Set Range command: address  0x2C,  value 1  (ten G)

3) Set Power Control: address 0x2D,  value 0 (measurement mode)

According to datasheet after reset: no high-pass filter enabled, ODR and low-pass filter corner is 4000 Hz and 1000 Hz.

I read ADXL357 via SPI with freqency 2kHz.

Can you help me understand this strange behavior?

Regards Maciek