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# How ADXL354 Calculates Bandwidth and Sample Rate Correctly to Obtain Low Noise

I have an EVAL-ADXL354 and am using the ADS1256 for signal sampling.
Figure (1).

I don't understand how the so called noise density is calculated.

I found this formula " Noise = Noise Density * sqrt(C * 1.6)"

What I want to ask is, does the BW in the formula refer to the frequency width between DCHz and (Low Pass) Hz? Or does it depend on the Sample Rate of the ADC?

If it refers to frequency width between DCHz and (Low Pass) Hz, then EVAL-ADXL354 itself has a 32K internal resistance and an external 0.1uf capacitor, so the bandwidth (BW) is DCHz-49.8Hz.So "Noise = 20ug * sqrt(49.8Hz * 1.6)" = 178.5ug=0.000178g= 1.75×10^-3 m/s²=0.175cm/s2

So why does the noise floor of the data I observe fall about 0.6cm/s2s(ADS125 Sample Rate 100) 0.9 cm/s2s (ADS125  Sample Rate 500)

What am I thinking wrong? Or do you think the reference voltage is unstable or the ADC itself has too much noise?

sorry i really don't understand~