Filtering out 20-200Hz vibration from MMA8652FC

[timtanner]

Hi,

We are planning to use the MMA8652FC in a system where there is large amplitude vibration in the range 20-200Hz. We are interested in measuring accelerations from DC to 1Hz and rejecting the vibration.

The data sheet and application notes mention low pass filtering as part of the tap detection function but it is not very clear if this will do what we want or whether the filtering will be adequate.

With frequency components up to 200Hz we will presumably need to sample at 400Hz or greater to avoid aliasing? We don't want 400Hz ODR so some sort of oversampling will be required.

There is a table of HP_FILTER_CUTOFF setting options but nothing about setting the low pass filter.

Q1. Can I assume that the cut-off frequencies mentioned in Table 10 are also the low pass cut-off frequencies?

Q2. What is the roll off of the LP filter? We probably need 40dB of attenuation at 20Hz. Will this limit the cut-off frequency to less than 1Hz?

Kind regards,

Tim.

[anthonyduhamel]

Tim,

Low-Pass filter can not be configured, it's is linked with the Output Data Rate (ODR). For your application you don't care about Pulse_LPF_EN and Pulse_HPF_BY bits because they are dedicated to Pulse Detection only.

Q1. Can I assume that the cut-off frequencies mentioned in Table 10 are also the low pass cut-off frequencies?

No.

Q2. What is the roll off of the LP filter? We probably need 40dB of attenuation at 20Hz. Will this limit the cut-off frequency to less than 1Hz?

You can't use the LPF by grabbing data from OUT_X_XXX registers.

I drew a short basic diagram which would be equivalent to the register configuration effects.

You can implement a digital Low-pass filter in the MCU firmware.

Anthony

[anthonyduhamel]

Hi Tim,

Yes, the datasheet is not clear about that point. Initially, the HPF and the LPF were used for tap (pulse) detection. But (it sounds) you can redirect High-pass or Low-pass filtered data to the classical register (0x01 to 0x06) by setting the HPF_OUT bit in XYZ_DATA_CFG  (check out app note AN4083) .

About LPF configuration, I'm looking forward to get more information: not sure it fits with ths HLP cut-off frequencies table because HPF & LPP seem to be actionable altogether.

I'm coming back to you asap.

Anthony