HOW TO SET LOW-PASS FILTER OF MMA8451

594160903 · ‎06-20-2017

I am using mma8451 to detection earthquakes，But，there are too many noise in result . and the frequency of the earthquake is the infrasound.So,I want to use LOW-PASS FILTER to remove noise，but the data sheet have a little message about ,please tell me how to set it,there are data sheet about LOW-PASS.

david_diaz · ‎06-28-2017

Hello Kaner,

The low-pass filtered data is done internally to the device. The data is basically averaged.

Registers 0x01 through 0x06 are used to read the X, Y, Z data. The device can be configured to produce high-pass filtered data or low-pass filtered data by setting or clearing the HPF_Out bit in the XYZ_Data_Cfg Register 0x0E.

Please let me know if this information helps.

Regards,

David

594160903 · ‎06-28-2017

Hello Davidm,

Thank you very much for your reply，and I want to know how to set the cutoff frequency of the low pass filter.

Thanks,

Kaner

david_diaz · ‎07-24-2017

Hello Kaner,

My apologies for the delayed response.

In this case, there is no other control of the LPF than to switch it on or off. As a side note, this LPF is always enabled for all the other functions (landscape/portrait, transient, free fall/motion).

The low pass filter in question is a simple 1st order decimation filter, i.e. a moving average filter followed by a down-sampling. Its cut-off frequency depends on ODR and MODS settings.

If you look at the Table 67 of the datasheet, following the CTRL_REG2 description, "MODS Oversampling Modes Current Consumption and Averaging Values for each ODR", you'll find a column "OS Ratio" for all possible ODR/MODS combinations. This table allows you to calculate the ADC frequency, following the formula:

ADC freq = OS Ratio * ODR

The low pass filter in question does a decimation by either 2 or 4, according to the following rule, and the cut-off frequency depends on the decimation rate:

OS Ratio = 2: decimation by 2, cut-off freq = 0.25 * ADC freq

OS Ratio >= 4: decimation by 4, cut-off freq = 0.114 * ADC freq

I hope this information helps.

Regards,

David