MMA7260 dynamic vs. static acceleration

[johnpaul013]

I am using the MMA7260 accelerometer located on the DEMOJM evaluation board.

The volatge limits go from .85 to 2.45 volts with 1.65 volts at the 0g point.

If I shake the accelerometer about the x-acis then I see the voltage swings from .85 to 2.45 volts.

So far so good.

Now I tilt the board 90 degress to each side of horizontal and the voltage does change in the correct direction BUT

it never goes to the limits of .85 and 2.45. It stops short by about .3 volts.

Does anyone know why?

Am I doing something wrong?

THANK YOU...

[bigmac]

Hello,

If you have selected the 1.5g range, and you are applying a 1g offset due to gravity, overload in one direction will occur with additional acceleration of 0.5g.  Your "shaking" may well exeed this level, resulting in saturation of tha analog output signal.

Perhaps you should try one of the less sensitive settings of the acelerometer to ascertain if this is the cause.

Regards,

Mac

[PeterHouse]

JohnPaul,

When you tilt it +/- 90 degrees you are applying +/- 1G of acceleration.  By my calculations you are getting .5v per G out of the sensor.  When you are shaking it, it seems to be maxing out at 8/5ths of a G or 1.6G.

What is the volts per G supposed to be?

Regards,

Peter House

[johnpaul013]

For the sensitivity that I am using, it should be .8 volts per G.

The datasheet is as follows:

   x-axis parallel to gravity = 1.65V (mid-supply) of the 0 to 3.3 reference voltage.

   tilted 90 left on x-axis should be +1G or 2.45 volts.

   tilted 90 right on x-axis should be -1G or .85 volts.

   So at the current sensitivity of .8 volts per G then these numbers make sense.

   However, in real life when I tilt the accelerometer 90 degrees I see 2.15 volts to 1.15 volts.

   But shaking it very hard along the x-axis produces the 2.45 volts to .85 volts.

  Any help would be appreciated.

  Thanks,

              --JP