FXOS8700CQ Accelerometer calibration

[hans-henrikfuxe]

After a reflow process with a rebuild simple owen with temperature regulation I got some issues with off set values with accelerometer values X,Y and Z. Ramp down is very coarse - just opening the owen door slightly

Raw values of Accelerometer in 4G-range  is X=-470, Y=-642 and Z=-252

These values are so off, that it is not possible to adjust the with OFF_X, OFF_Y and OFF_Z

The best I get is:

    SPI_WriteFXOS(OFF_X, 0b01110111); =>     ~0 mg
    SPI_WriteFXOS(OFF_Y, 0b01111111); => ~-130 mg
   SPI_WriteFXOS(OFF_Z, 0b00111111); =>       ~0 mg

Q1) How much does the PCB footprint affect calibration values in production with proper reflow?? ( for simplicity of soldering, pads are much longer now)

Q2) For testing purposes, just adding +130 to Y-value, what other affects can it have beside shrinking the dynamic range in Y-led??

[hans-henrikfuxe]

Thanx David for your answer!

I have looked for the correct reflow temperature curve for production and have only found general directions. Do you happens to have a reflow temperature graph for FXOS8700?

Best Regards,

Hans-Henrik

[david_diaz]

Hello Hans,

I hope it is not too late for the response.

Printed circuit board (PCB) layout is a critical portion of the total design. The footprint for the surface mount packages must be the correct size to ensure proper solder connection interface between the PCB and the package. With the correct footprint, the packages will self-align when subjected to a solder reflow process.

Please review the application note below as a guideline for soldering and mounting the device. I recommend reviewing and apply the procedures mentioned on the document.

Design Checklist and Board Mounting Guidelines

Please note that the Sensitivity Accuracy for the acceleration data can be up to ±2.5% and the Zero-g Level Offset Accuracy Post Board Mount rises to ±30 mg.

After the accelerometer has been mounted onto a PCB there may be a small offset shift. If additional calibration is required there are three registers to zero out the offset. Further I would recommend the accelerometer offset calibration procedure mentioned in the example project below:  

FXOS8700CQ - Bare metal example project

Please let me know if the information provided was useful for you or if you have any further question.

Regards,

David