FXAS21002 gyro group delay versus ODR and lowpass filter setting

[jonbrunetti]

Hi,

I would like to know the group delay of the FXAS21002C for a given ODR (output data rate) and lowpass filter shown in Table 33 on page 39 of the data sheet. 

For example, at 400Hz ODR, Table 33 has the following entries for the lowpass filter bandwidth:

Table 33. FXAS21002C LPF Cutoff Frequency

BW	ODR = 400Hz
0b00	128
0b01	64
0b1x	32

The group delay likely increases by 2x for each 2x decrease in the LPF (lowpass filter) cutoff frequency. Also the group delay likely increases by 2x for each 2x decrease in the ODR setting.  Therefore, the group delay for any value in Table 33 would be highly useful.

If the z-transform of the lowpass filter and the sample rate before decimation is known, that would also answer the question.  For example, if a moving average filter is used as the lowpass filter (sometimes called a SINC filter), knowing the sample rate and filter length, prior to decimation to the ODR, would allow the group delay to be calculated.

Thanks in advance for your replies.

Jon

[TomasVaverka]

Hi Jon,

I just want to inform you that I have requested assistance from our Apps/Design team to clarify your question. I will update this thread once I have their answer.

Regards,

Tomas

[TomasVaverka · Accepted solution]

Hi Jon,

Here is the response from the Design team, sorry it has taken so long.

The LPF of the FXAS21002C is a CIC (cascaded integrator-comb) decimator of 2^nd order. It has the advantage of down-sampling and filtering lower than the Nyquist rate. This is not a simple moving average filter or sinc.

The group delay of this filter is “N*M-1” samples with N the decimation factor and M the differential delay.

- Knowing that the ADC run at 6400Hz, the decimation factor goes from 8x to 512x (ODR from 6400/800 to 6400/12.5).

- The differential delay is either 1, 2 or 4 for the BW = 00, 01 or 0x respectively.

Thus, group delay = (6400/ODR*M-1)*156.25us.

I hope it helps.

Regards,

Tomas

[jonbrunetti]

Thank you for the information.  That is very helpful.

Jon