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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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  • We appreciate your feedback on the ADXL001 datasheet and are always looking for ways to improve their value. I didn't produce this datasheet but I suspect that this information was not provided, since the resonance at 22kHz introduces a more complex variable in the bandwidth estimation process.  This will be passed on to our team for future update consideration. Again, thanks!  Back to your questions:

    UPDATE (2/14/2013, NevadaMark): The accuracy of the following three lines (which have been changed to a lighter shade of gray)  has been questioned. I sincerely apologize for this, but it appears like the ADXL001's output impedance is not 32kohms.  Please see most recent post in this discussion for follow-up insights and suggestions. Thanks to for sharing this with me.

    ************************

    The output impedance of the ADXL001 is 32kohms, which is consistent with may other accelerometer products, including the ADXL203.

    The 5 kHz came from calculating the cut-off frequency (-3dB) of the RC combination (32k/1000pF).

    fc = 1/(2*pi*R*C) = 1/(2*3.1416*32000*10^-9) = ~5000Hz

    ************************

    If you are unfamiliar with this, the following reference may be helpful:

    http://en.wikipedia.org/wiki/RC_circuit#Frequency-domain_considerations

    For a basic description of the relationship between "noise bandwidth" and "cut-off frequency," check out the following link.

    http://www.k-state.edu/ksuedl/publications/Technote%201%20-%20Equivalent%20Noise%20Bandwidth.pdf

    For a more detailed, academic study of these principles, I learned them from the following textbook:

    http://www.wiley.com/WileyCDA/WileyTitle/productCd-0471577421.html

    Please keep in mind that the resonant behavior at 22kHz will extend the effective noise bandwidth and increase the total noise estimate.  Calculating a more precise expectation would require a more complex approach and may not yield any further value in this discussion.  My primary purpose was to provide a simple model for estimating the noise so we could ask, "are we seeing a reasonable amount of noise when the device is still?" I believe that the answer to that question is, "yes."  You are already working to answer the next question, which is, "what type of acceleration is my ADXL001-70 setup experiencing," through the use of your reference accelerometer.


    I hope that helps.

    Best,
    NevadaMark

    Message was edited by: NevadaMark

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  • We appreciate your feedback on the ADXL001 datasheet and are always looking for ways to improve their value. I didn't produce this datasheet but I suspect that this information was not provided, since the resonance at 22kHz introduces a more complex variable in the bandwidth estimation process.  This will be passed on to our team for future update consideration. Again, thanks!  Back to your questions:

    UPDATE (2/14/2013, NevadaMark): The accuracy of the following three lines (which have been changed to a lighter shade of gray)  has been questioned. I sincerely apologize for this, but it appears like the ADXL001's output impedance is not 32kohms.  Please see most recent post in this discussion for follow-up insights and suggestions. Thanks to for sharing this with me.

    ************************

    The output impedance of the ADXL001 is 32kohms, which is consistent with may other accelerometer products, including the ADXL203.

    The 5 kHz came from calculating the cut-off frequency (-3dB) of the RC combination (32k/1000pF).

    fc = 1/(2*pi*R*C) = 1/(2*3.1416*32000*10^-9) = ~5000Hz

    ************************

    If you are unfamiliar with this, the following reference may be helpful:

    http://en.wikipedia.org/wiki/RC_circuit#Frequency-domain_considerations

    For a basic description of the relationship between "noise bandwidth" and "cut-off frequency," check out the following link.

    http://www.k-state.edu/ksuedl/publications/Technote%201%20-%20Equivalent%20Noise%20Bandwidth.pdf

    For a more detailed, academic study of these principles, I learned them from the following textbook:

    http://www.wiley.com/WileyCDA/WileyTitle/productCd-0471577421.html

    Please keep in mind that the resonant behavior at 22kHz will extend the effective noise bandwidth and increase the total noise estimate.  Calculating a more precise expectation would require a more complex approach and may not yield any further value in this discussion.  My primary purpose was to provide a simple model for estimating the noise so we could ask, "are we seeing a reasonable amount of noise when the device is still?" I believe that the answer to that question is, "yes."  You are already working to answer the next question, which is, "what type of acceleration is my ADXL001-70 setup experiencing," through the use of your reference accelerometer.


    I hope that helps.

    Best,
    NevadaMark

    Message was edited by: NevadaMark

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