MPL3115A2 OFF_H register

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MPL3115A2 OFF_H register

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marconapoleone
Contributor I

Hi all, I developed a simple project with the MPL3115A2 sensor and I only need to detect little altitude differences.

I read in the application note that the manipulation of OFF_H register cause the sensor to be more stable reading small altitutes. I don't understand the reason for wich subtracting or adding an offset shuold give this result. My sensor continues to derive even if I set -127 to that register.

In my application I want only to detect altitudes between 0 and 40 meters, but the sensor still derive from the first sltitude mesurement.

Thus, I can't understand how can I correct offsets > 128 or <-127, using a register taht is only 1 byte in size. I could need greater offset.

Thanks

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TomasVaverka
NXP TechSupport
NXP TechSupport

Hi Marco,

In your case, the offset shifts are shared in the two absolute measurements and negate each other during the calculation, so there is indeed no need to make use of the OFF_H register.

It is good for absolute altitude measurements, allowing to remove the long term offset shift in the range of -128 to 127 meters.

To detect small altitude differences, I would just recommend using a maximum oversampling (OS[2:0] = 0b111).

Regards,

Tomas

PS: If my answer helps to solve your question, please mark it as "Correct". Thank you.

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marconapoleone
Contributor I

Hi Tomas. I already use the suggested sample, and I realized that the sensor accuracy decrease if I increase the sample rate. Anyway I can't get the sensor to work correctly. After a few minutes, the measured value is changed by 4-5 meters wiyhout moving the PCB from the table. How can I discriminate small altitudes into a building?

Thanks

Marco

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TomasVaverka
NXP TechSupport
NXP TechSupport

Hi Marco,

Such drift is probably related to the ambient pressure changes.

However, when using relative readings (subtracting the ending altitude from the starting altitude), that kind of slow drift should not affect the application.

There may be also another source of inaccuracy related to a little-known fact about light sensitivity. The sensor die is light sensitive and direct light exposure through the port hole can lead to varied accuracy of pressure measurement. My recommendation is to avoid such exposure to the port during normal operation. For instance you can put a small piece of foam over the port to block out light, but still allow the air to reach the sensor.

Regards,

Tomas

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