Clarification of datasheet noise specification for MMA8452Q accelerometer

[dylanpfeifer]

Freescale:

Can you guys clarify the specification for noise in your MMA8452Q datasheet?  It says 99 micro-g per root Hz.  Where are you getting the bandwidth root Hz from?  The sheet says the bandwidth of the accelerometer is min ODR/3 and max ODR/2.  Assuming ODR is set to 100Hz, this would seem to indicate to me the bandwith is ~25 Hz?  Or is the bandwidth in the root Hz taken from something else?

We are trying to estimate the lowest measureable acceleration without noise in the 12 bit part...

Thanks

[reyes]

Hi Dylan Pfeifer,

Please take a look at the application note AN4075:  http://cache.freescale.com/files/sensors/doc/app_note/AN4075.pdf

Chapter 4 of the application note explains the noise specification for accelerometers.

Chapter 4.2 describes how the noise is usually measured. The 99ug/sqrt(Hz) has been measured with the ODR of 400Hz and LNOISE bit set.

The typical noise calculated in mg RMS is given in tables 3, 4, 5 and 6 of the AN4075 for all possible ODR/MODS/LNOISE combinations.

To calculate the RMS noise multiply the Power Spectral Density (PSD) value by the square root of the bandwidth (ODR/2):

Nrms = PSD * sqrt(ODR/2) = 126 * sqrt(800/2) = 2.52mg

Have a great day,
Jose Reyes

-----------------------------------------------------------------------------------------------------------------------
Note: If this post answers your question, please click the Correct Answer button. Thank you!
-----------------------------------------------------------------------------------------------------------------------