FXOS8700CQ Response to High Rate of Change of Magnetic Field Direction

[georgebrand]

Hi. I have a question regarding the FXOS8700CQ 3-axis magentometer and accelerometer.  I am considering using this sensor to detect the Earth's magnetic field in a device that can rotate as fast as 7200 degrees/s.   Can the magnetometers respond fast enough to provide an accurate measurement when the magnetic field direction is changing this rapidly?  I do not see anything in the datasheet that addresses this.  Thanks.

[georgebrand]

Tomas,

Thanks for you response.  I have several follow-up questions on this topic.

1. I see on the FXOS8700CQ datasheet where the output noise is specified at 1.5 uT rms at an 800 Hz ODR but can you explain how you calculated a 3 degree rms uncertainty based on a 1.5 uT rms noise level?
2. Why does the noise increase with ODR?
3. What limits the ODR to 800 Hz?  Is it the power consumption, the noise level, or something different?  SPI and I2C can communicate much faster than this so it doesn't seem to be a limitation of the communications interface.  I would like to be able to sample at 4 kHz but that doesn't seem likely with any of the 3-axis magnetometer sensors with digital (i.e. SPI or I2C) output interfaces that I have seen.

Thanks!

George

[TomasVaverka]

Hi George,

The main limitation is the maximum ODR which is 800Hz in magnetometer mode only. Considering your device rotates as fast as 7200 degrees/s, 1 sample corresponds to 9 degrees (7200/800), so the output would be in steps of 9 degree rotation. Noise needs to be taken into account at this ODR, as it increases with ODR. At 800Hz it reaches a typical 1.5uT rms which translates into a 3° rms uncertainty. So the FXOS8700CQ could be a good solution if accuracy requirement is not too stringent. 

Regards,

Tomas

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