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    bma456 - filtered v pre-filtered data

    TMunb
    New Poster

    bma456 - filtered v pre-filtered data

     

    hello Bosch,

    in headerless watermark mode I can program an interrupt on int1 and read the resulting data when the bma456 is ready. The bma456 datasheet states "It is possible to select filtered or pre-filtered data as an input to the FIFO." - what does this mean in particular what is filtered versus pre-filtered data ?

    Choosing between the two data types can be done through bit acc_fifo_filt_data which if 0, meaning unfiltered data, results in a sample rate of 1600Hz. This bit is by default 1 meaning data is filtered, so I believe this is the setting used in your example code fifo_watermark_headerless_mode.c ?

    I already choose an output data rate through acc_adr in register ACC_CONF and this can be, for example, 100Hz. Another option for acc_adr is 1600Hz, but I believe this is a different thing compared to the 1600Hz "sample rate" referred to with respect to unfiltered data - is that correct, and if so what is the difference ? I also set the bandwidth parameter, acc_bwp, which depends on acc_perf_mode. The accelerometer digital filter can be configured through Register ACC_CONF - so what is the difference between filtered or pre-filtered data, and how does it relate to these other configuration parameters set through ACC_CONF ? If fifo_watermark_headerless_mode.c is by default with filtered data (please confirm) then what sample rate is used seeing as it is not 1600Hz ?

    4 REPLIES 4
    BSTRobin
    Community Moderator

    Re: bma456 - filtered v pre-filtered data

    Hello TMunb,

    Normally, acc_fifo_filt_data was enable. If you used pre-filtered data, sample rate was fixed with 1600 Hz. If you used un-filtered data, sample rate was fixed with 1600 Hz.

    When you used filtered data, sample rate and band width could be selected according the following descriptin.

    BMA456 accelerometer data processing for low power mode.pngBMA456 accelerometer data processing for performance mode.png

    TMunb
    New Poster

    Re: bma456 - filtered v pre-filtered data


    hi BSTRobin,

    Ok, acc_fifo_filt_data is by default = 1 meaning Filtered Data. The datasheet mentions two different terms but I'm assuming 'pre-filtered' and 'unfiltered' are the same thing - is that correct ? If unfiltered data is chosen then the datasheet says the sample rate is 1600Hz, ok. Are you saying it is also fixed to 1600Hz with the filtered data option ? But ... that with the filtered data setting I would also have the option of changing the sample rate and bandwidth as per those register settings - is that correct ? So is the 'sample rate' as referred to when the datasheet discusses filtered v pre-filtered data just another name for the 'output data rate' or odr ?

     

    TMunb
    New Poster

    Re: bma456 - filtered v pre-filtered data


    hi BSTRobin,

    when you have some time can you please get back to me in relation to the questions in my previous post ?

    Thank-you.

    BSTRobin
    Community Moderator

    Re: bma456 - filtered v pre-filtered data

    Hello TMunb,

    Ok, acc_fifo_filt_data is by default = 1 meaning Filtered Data. The datasheet mentions two different terms but I'm assuming 'pre-filtered' and 'unfiltered' are the same thing - is that correct ?
    Reply: 'pre-filtered' and 'unfiltered' indicates that the data is not filtered.

    If unfiltered data is chosen then the datasheet says the sample rate is 1600Hz, ok. Are you saying it is also fixed to 1600Hz with the filtered data option ? But ... that with the filtered data setting I would also have the option of changing the sample rate and bandwidth as per those register settings - is that correct ? So is the 'sample rate' as referred to when the datasheet discusses filtered v pre-filtered data just another name for the 'output data rate' or odr ?
    Reply: sampling rate refers to the sampling frequency of ADC, which is at the front end; ODR refers to the frequency of sensor data output after passing through the filter, which is at the back end.

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