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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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  • UPDATE (2/14/13, NevadaMark): My assessment of the cut-off frequency (5kHz) in this post is not correct. The ADXL001 source impedance is not defined but it is much lower than 32kohms. When combined with the suggested external capacitor of 1000pF, it will not have an substantial impact on the frequency response expectations that are in the ADXL001 datasheet.

    Thank you for offering this information.  We may need or FormerMember to jump in on this, but this is what I would expect.  If we use a single-pole filter assumption, the noise bandwidth will be ~1.57 times greater than the cut-off frequency, which is ~5kHz when using a 1000pF capacitor. For the 70g version of the ADXL001, we can estimate the total noise of this configuration to be:

    Noise = 0.0033g/sqrt(Hz) x sqrt (1.57 x 5000) = 0.292g

    For the 250g version, this would be equal to ~0.321g and for the 500g version, this number would ~0.376g. Which version of the ADXL001 are you using? 

    Unfortunately, I was not able to grab a Matlab license so I was not able to compare the FFT from this most recent data. If this data indicates that the >1kHz noise increases when the device is in motion, the next step for consideration would be to determine if that response represents actual motion that the sensor is experiencing. The core sensor resonance is 22kHz. Based on your pictures, the PCB size could support resonance somewhere in the 1-5kHz region. It might help to determine the the quality of the sine-wave that your vibrating platform is capable of producing as well.

    Message was edited by: NevadaMark

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  • UPDATE (2/14/13, NevadaMark): My assessment of the cut-off frequency (5kHz) in this post is not correct. The ADXL001 source impedance is not defined but it is much lower than 32kohms. When combined with the suggested external capacitor of 1000pF, it will not have an substantial impact on the frequency response expectations that are in the ADXL001 datasheet.

    Thank you for offering this information.  We may need or FormerMember to jump in on this, but this is what I would expect.  If we use a single-pole filter assumption, the noise bandwidth will be ~1.57 times greater than the cut-off frequency, which is ~5kHz when using a 1000pF capacitor. For the 70g version of the ADXL001, we can estimate the total noise of this configuration to be:

    Noise = 0.0033g/sqrt(Hz) x sqrt (1.57 x 5000) = 0.292g

    For the 250g version, this would be equal to ~0.321g and for the 500g version, this number would ~0.376g. Which version of the ADXL001 are you using? 

    Unfortunately, I was not able to grab a Matlab license so I was not able to compare the FFT from this most recent data. If this data indicates that the >1kHz noise increases when the device is in motion, the next step for consideration would be to determine if that response represents actual motion that the sensor is experiencing. The core sensor resonance is 22kHz. Based on your pictures, the PCB size could support resonance somewhere in the 1-5kHz region. It might help to determine the the quality of the sine-wave that your vibrating platform is capable of producing as well.

    Message was edited by: NevadaMark

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