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ADXL345 Output Noisy at 3200Hz Data Rate

Question asked by RIH7777 on Jan 16, 2013

With ADXL345 stationary observe much more variation in output at 3200Hz data rate than 800 or 1600 Hz rate.

I'm reading data via SPI with SPI Clk = 4 MHz. (I am pretty confident in my SPI code:  I can write ADXL345 registers and then read back the expected values, and I ran the self-test very carefully, (at 800Hz data rate) and measured the results to be in the expected ranges.)

For both experiments shown below we have:  FIFO_CTL = 0x90, DATA_FORMAT = 0x0b, POWER_CTL = 0x08.    On scope measure > 5.2 microseconds between FIFO reads.   I have a SW test for FIFO overflow and I am sure this is not occurring.  Notice almost +- .5g variation at 3200 Hz.  Everything in system is identical for both experiments except setting of BW_RATE.  Note I see a similar pattern for X and Y data: much greater variation from baseline values at 3200Hz vs 800 Hz.

Questions:

1.)Does output look 'reasonable/correct' for 800 Hz plot?

2.)Why so much  more variation at 3200 Hz? 

 

Experiment 1:  BW_RATE = 0x0f , (3200Hz). 

Z_3200_16g_FS.jpg

Experiment 2: BW_RATE = 0x0d, (800 Hz)

Z_800_16g_fs.jpg

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