ADXL345's datasheet says "noise density" is 25 µg/√Hz. For the units, that looks to me like pink (1/√f) total noise power.
For noise density I would expect µg/Hz. Can anyone clarify that? I'd be working with negative SNR and really need to know what the noise is. More precisely, I'm interested in the noise spectrum between 0.1 Hz and some 20 Hz.