Precision of MMA8451

[michelmeunier]

Hello,

I am very surprised by the very bad precision of the MMA8451 on the FRDM-KL25Z.

When the board is fixed, the (x,y,z) values of the 1g is changing quite a lot from one read to an other. For example the z value change between 3931 to  3968. It's about 1%. Why have 14bits precision, when the true precision is not good? I am in 2g mode to have the best precision.

Michel

[TomasVaverka]

Hi Michel,

For the best noise performance, it is recommended to select the lowest possible ODR and use both Low Noise (LNOISE bit = 1) and High Resolution (MODS [1:0] = 0b10) modes. For more information please refer to our AN4075.

Hope it helps.

Regards,

Tomas

[michelmeunier]

Hello

I have tried the Tomas' explainations:

  error=MMA1_Disable();

  error=MMA1_ReadReg8(XYZ_DATA_CFG,&valeur);

  valeur&=0xFC; //2g mode

  error=MMA1_WriteReg8(XYZ_DATA_CFG,valeur);

  error=MMA1_ReadReg8(CTRL_REG1,&valeur);

  valeur|=0x3C; //1.56Hz rate plus low noise

  error=MMA1_WriteReg8(CTRL_REG1,valeur);

  error=MMA1_ReadReg8(CTRL_REG2,&valeur);

  valeur&=0xF7;

  valeur|=0x10; //High resolution mode

  error=MMA1_WriteReg8(CTRL_REG1,valeur);

  error=MMA1_ReadReg8(F_SETUP,&valeur);

  valeur&=0xFC; //no FIFO

  error=MMA1_WriteReg8(F_SETUP,valeur);

  error=MMA1_Enable();

And it's only a little bit better:

my board is on a table and the z value is between 4026 and 4080! And, no I am not a the top of a buiding during an earthquake ;-)

I thing I am going to take an other system because the MMA8451 is really to bad and it hasn't enough precision for my goal.

Michel

[bobpaddock]

USB is not always the cleanest of power supplies. Take a look at the VTI/Now Murta SC3100 .  It can sense me typing from several feet away (at first I thought the part was junk and noisy tell I figured out the typing connection). http://www.murata.com/en-sg/products/sensor/accel/sca3100 Analog Devices makes parts in the aircraft grade with lots of zeros in the price tag if noise is that big of issue. Consider using a Median Filter:  http://embeddedgurus.com/stack-overflow/tag/median-filter/

[bobpaddock]

How clean is the power supply?

[michelmeunier]

It is powered by the FRDM-KL25 board. And The board is powered by USB of my computer.