LSM9DS1 - Understanding gyro configuration

PRebo.1 · ‎2020-09-29

I have an application using an LSM9DS1 sensor, where I'm only interested in ready gyro values in order to determine angular velocity around the Z-axis.

Looking at the datasheet in page 45, table 46, what does it mean the cutoff frequency in that table ?

That readings with an update frequency above the cutoff are not supported or reliable ?

Eleon BORLINI · ‎2020-09-29

Hi @PRebo.1 ,

The cut off frequency is, technically, the frequency at which you start to loss -3dB in the signal amplitude (a signal coming from an object vibrating/rotating at that frequency), i.e. after which the signal is attenuated more and more.

Typically, if you were able to not apply any low pass filter (LPF1 and LPF2, as for sensors other than LSM9DS1), you will be able to acquire signals of a vibration (mg) or a rotation (dps) up to ODR/2.

But from the Block diagram you can find in the datasheet p.22, the data necessarily passes through the LPF1 filter, so that your in the ODR max configuration is limited to 100Hz (a variation of 100Hz in the data, for example if you perform the FFT of the acquired signal).

-Eleon

View solution in original post

Eleon BORLINI · ‎2020-09-29

Hi @PRebo.1 ,

The cut off frequency is, technically, the frequency at which you start to loss -3dB in the signal amplitude (a signal coming from an object vibrating/rotating at that frequency), i.e. after which the signal is attenuated more and more.

Typically, if you were able to not apply any low pass filter (LPF1 and LPF2, as for sensors other than LSM9DS1), you will be able to acquire signals of a vibration (mg) or a rotation (dps) up to ODR/2.

But from the Block diagram you can find in the datasheet p.22, the data necessarily passes through the LPF1 filter, so that your in the ODR max configuration is limited to 100Hz (a variation of 100Hz in the data, for example if you perform the FFT of the acquired signal).

-Eleon

PRebo.1 · ‎2020-09-29

I get it. Still related to this, where can I get the values table for INT_SEL and OUT_SEL in CTRL_REG2_G (11h) ?

Thanks

Eleon BORLINI · ‎2020-09-29

Hi @PRebo.1 ,

the INT_SEL and OUT_SEL meanings are explained in the diagram of Figure 28.

OUT_SEL value determines the filters applied on the output data stream
- 00 dataout with LPF1 only applied
- 01 dataout with HPF (and LPF1) applied
- 10 or 11 dataout with LPF2 (and LPF1) applied
INT_SEL value determines the filters applied on the data that generate the interrupts
- 00 interrupt with LPF1 only applied
- 01 interrupt with HPF (and LPF1) applied
- 10 or 11 interrupt with LPF2 (and LPF1) applied

-Eleon

PRebo.1 · ‎2020-09-30

Sorry to comeback again on this...

By a variation of 100hz in the data, you mean that in a second I can only get a maximum of 100 distinct readings ?

Eleon BORLINI · ‎2020-10-01

Hi @PRebo.1 ,

Not exactly: the maximum distinct readings in a second is determined by the ODR, or output data rate, that can be configured in the CTRL_REG1_G (10h) register and that can go up to 952Hz when ODR_G [2:0] bits are "110". The "100Hz" (BW_G [1:0] = 11) limitation refers to the maximum "useful" signal you can detect. This means that if you have a vibration or rotation that is varying with a frequency of 100Hz, and you are running at 952Hz, you will have more or less 10 points for every cycle of the signal. On the other side, you will not be able to detect a 200Hz-signal even if you would be able to acquire it without alias, due to the low pass filter with cutoff frequency at 100Hz.

-Eleon