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NTamb.2
Visitor II
March 1, 2022
Solved

H3LIS331DLTR produces abnormal readings

  • March 1, 2022
  • 2 replies
  • 954 views

Hi!

We have been evaluating a few accelerometers from ST.

So far we have seen reasonable results from H3LIS100 and H3LIS200 devices.

However, H3LIS331DLTR is behaving contrary to our expectations.

We have the H3LIS331DLTR device on a PCB, interfaced via SPI with a microcontroller.

The H3LIS331DLTR device is mounted on the PCB such that the Z-axis of the H3LIS331DLTR device is perpendicular to the ground. The X and Y axes are in the plane of the PCB.

We have configured the H3LIS331 device with the following settings:

  • High-pass filter disabled (this is intentional)
  • Output data rate: 400Hz
  • Reference mode: normal
  • Block Data Update bit is set, so that both high and low byte of measurement can be read
  • Full-Scale bits are set to 00 (hence +/-100G full scale measurements)
  • Acquisition mode: polling by checking XYZ data ready flag in status register

However, we are reading +30G on the Z-axis measurement.

When we flip the PCB over, we are reading +15G on the Z-axis measurement.

The delta between the two measurements is much larger than 1G.

We are seeing these measurements on 2 PCBs, so it's not a one-off.

I've decided to post here, in the hope that someone will be able to guide us and point out something we may be missing.

Thanks in advance!

    This topic has been closed for replies.
    Best answer by Eleon BORLINI

    Hi @NTamb.2​ ,

    are you correctly converting the LSB units into physical units?

    Please note that the data output for the H3LIS100 and the H3LIS200 device is on 8 bits, so that the sensitivity is respectively 780mg/digit and 1560mg/digit, while the H3LIS331DLTR data is on 12 bits, so the sensitivity is reduced by a factor 12. If you don't consider this difference and uses the same libraries, you will have a 16x value on this last sensor. You can refer to the C drivers on Github for the implementation of the conversion of LSB into physical units --> h3lis331dl_reg.c.

    float_t h3lis331dl_from_fs100_to_mg(int16_t lsb)
{
 return ((float_t)lsb * 3.0625f);
}

    Can be this one the issue?

    -Eleon

    2 replies

    E
    Eleon BORLINIAnswer
    ST Employee
    March 4, 2022

    Hi @NTamb.2​ ,

    are you correctly converting the LSB units into physical units?

    Please note that the data output for the H3LIS100 and the H3LIS200 device is on 8 bits, so that the sensitivity is respectively 780mg/digit and 1560mg/digit, while the H3LIS331DLTR data is on 12 bits, so the sensitivity is reduced by a factor 12. If you don't consider this difference and uses the same libraries, you will have a 16x value on this last sensor. You can refer to the C drivers on Github for the implementation of the conversion of LSB into physical units --> h3lis331dl_reg.c.

    float_t h3lis331dl_from_fs100_to_mg(int16_t lsb)
{
 return ((float_t)lsb * 3.0625f);
}

    Can be this one the issue?

    -Eleon

    N
    NTamb.2Author
    Visitor II
    March 4, 2022

    Hi,

    Yes, I found out a couple of days ago that the conversion was incorrect.

    I was using the correct scaling factor (49mg/bit).

    However the data I was receiving was left justified by 4 bits. Once I shifted the data by 4 bits, all the measurements looked correct.

    I noticed in the datasheet mentioned that the resolution was 12-bits; but hadn't realized that the data would be left-justified when it arrives via the SPI interface (might be helpful if this detail is added).

    Thanks for your response!

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