MMA8451Q - Effective resolution calculation

[bytemanaccel]

Hi all,

I kundly ask about the effective resolution for this accelerometer model.

Suppose to work ad ODR=400 Hz hence BW=200 Hz.

From the device datasheet we have a PSD=99 ug/sqrt(Hz) @ 4g scale ODR=400 Hz

From the AN4075 the SNR(db) will be equal to:

  SNR(db)=20*log*(4g/(2sqrt(2)) / NRMS)

where:

  NRMS = PSD*SQRT(BW)=99u*SQRT(200)=1.4 mg

Hence:

  SNR(db)=60.087 dB

and finally we have for the bits number:

  n=(SNR(dB)-1.76)/6.02=9.68 bit

This means that instead of the theoretical 14 bit we have really usable 9 bit of resolution then 4g/2^9.68=4.87 mg of resolution.

Someone have carried out similar calculation?

Thanks in advance!

Best regards

Fabio

[TomasVaverka · Accepted solution]

Hi Fabio,

The number of effective bits and typical noise calculated in mg RMS is actually given in tables 3, 4, 5 and 6 of the AN4075 for all possible ODR/MODS/LNOISE combinations. As you can see, it is practically 14-bit true at 1.56Hz in Hi Resolution Mode with the Low Noise bit set. As the sample rate decreases the resolution improves. The best case scenario is 14-bit true at the lowest sample rate. 

Best regards,

Tomas

[bytemanaccel]

Hi Tomas,

thank you for your answer, hence the best mode is on the first row of table Table 4 for a setting of 4g -> 14.7 bit with ODR=1.56.

Best regards

Fabio