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ADXL001 noisy output

Dear community users,

at the moment I try to set up a vibration monitoring system using the MEMS accelerometer ADXL001. To "check" the ADXL001 I have just made a small pcb. On this pcb I soldered the ADXL001 and two capacitors. One capacitor at the power supply pin VDD to Ground (0.1uF) and the other one at the Output Pin Xout to Ground with 1000pF=1nF. The power for the ADXL001 is delivered by a linear regulator board with a 7805 fixed regulator. I  have connected 100nF capacitor from input to ground and output to ground of the 7805. My data acquisition board is a DAQ NI-9234 from National Instruments. The samplerate is 51.2k (maximum). My vibration source for the ADXL001 pcb board is a small electrodynamic shaker with sinus excitation.

In the time domain I see a lot of noise on the signal... I have just put the time domain and the FFT of the time signal in the attachment. The data acquisition and signal processing was performed with MATLAB.  Any ideas what to do against the noise ? I actually want to use the ADXL001 up to his 22kHz bandwiths but I get a lot of noise beginning at 1kHz... When I design a low pass / anti aliasing filter, then I have select the cut of frequency at 22kHz.. (lower than the half of the maximum samplerate).

The amplitude in the time domain plot is already calculated in acceleration g with the sensitivity setting of 24.2mV/g  at 5V power supply (refer to ADXL001 datasheet).

Thank you for your help and please apologize my bad english.

attachments.zip
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  • Hello NevadaMark,

    thank you for your quick reply.

    Sorry, that I forgot the important information ! => I am using the 70g Version.

    The electrodynamic shaker I have got in the lab only supports max. 1000 Hz sine motion. The first data I have attached in my initial posting was made with 70 Hz excitation sine of the electrodynamic shaker. I will check the the quality of the sine wave with a "calibrated" accelerometer like from Brüel&Kjaer or Endevco tomorrow.

    Can you give me more details about your noise calculations  ?! Where is the factor 1.57 (half of pi ?!) coming from.

    When I use 1000pF the cut-off frequency is 5kHz why ? How can I calculate the cut off frequency as a function of the capacitior value ?! In other ADXL sensor datasheeds there are some tables and equations to calculate this but in the ADXL001 datasheet I can't find any output resistance of the Xout Pin to calculate any cut-off frequency. Is it a simple RC Filter ?! With or without DC coupling ?!

    Thank you very much !

    best regards,

    opcode

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  • Hello NevadaMark,

    thank you for your quick reply.

    Sorry, that I forgot the important information ! => I am using the 70g Version.

    The electrodynamic shaker I have got in the lab only supports max. 1000 Hz sine motion. The first data I have attached in my initial posting was made with 70 Hz excitation sine of the electrodynamic shaker. I will check the the quality of the sine wave with a "calibrated" accelerometer like from Brüel&Kjaer or Endevco tomorrow.

    Can you give me more details about your noise calculations  ?! Where is the factor 1.57 (half of pi ?!) coming from.

    When I use 1000pF the cut-off frequency is 5kHz why ? How can I calculate the cut off frequency as a function of the capacitior value ?! In other ADXL sensor datasheeds there are some tables and equations to calculate this but in the ADXL001 datasheet I can't find any output resistance of the Xout Pin to calculate any cut-off frequency. Is it a simple RC Filter ?! With or without DC coupling ?!

    Thank you very much !

    best regards,

    opcode

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