Noise estimation at different data rates

Hi Killertechno! 

Thanks for your question. A few thoughts here. The datasheet for the ADXL355 specs the noise spectral density at 22.5ug/sqrt(Hz) or 25ug/sqrt(Hz) depending on the range of accelerometer. See page 6 here: https://www.analog.com/media/en/technical-documentation/data-sheets/adxl354_adxl355.pdf. 

When considering the RMS noise, you're looking at either bandwidth of the sensor or the output datarate. The formula is specified here https://www.analog.com/en/products/landing-pages/001/accelerometer-specifications-definitions.html under the Acelerometer Noise Density  section [sic]. What bandwidth are you running for your sensor? Remember that BW is ODR / 2 due to Nyquist theory. I think you were referencing this discussion here: https://ez.analog.com/mems/f/q-a/89125/adis16375-output-noise in which case the calculations assume datarate rather than BW. In that case the ODR is 2000 / (100 + 1) ~= 20. This is then divided by 2 to get the BW used in the formula. So use the coefficient of either 22.5 or 25ug/sqrt(Hz) depending on the accelerometer limits. 

In terms of comparing accelerometers, the RMS noise value is specified for a specific BW or ODR. So for a comparison, you need to keep the settings the same between accelerometers (in terms of BW and ODR). Additionally noise is related to temperature so making sure both are at the same temp is important.

Thank you  for the clear answer.

So, for instance, having an ADXL355 accelerometer ( noise = 25 ug/sqrt(Hz) ) set at ODR = 125 (address 0x28 = xxxx0101), the theoretical RMS noise will be 25*sqrt(1.57*125/2) = 0.248 mg, is that right ?

Yup! your math looks correct and the formula from the link here:  ADIS16375 output noise was used correctly!  

Yup! your math looks correct and the formula from the link here:  ADIS16375 output noise was used correctly!  

   thanks a million for the quick reply !

However, it seems that BW = ODR/4 in ADXL355, so the noise is

25*sqrt(1.57*125/4) = 0.176 mg