Facing issue of very high filter result with LowPass2.0 Filter

[harkhka]

Hi,

I saw your video on YouTube (https://www.youtube.com/watch?v=eM4VHtettGg&ab_channel=CurioRes) demonstrating the LPF2.0.

I just tried your filter on my STM32 board to filter the ADC measurement of a current sensor (ACS758).

Just like in your video, I set up the 2nd-order LPF with a 4 Hz cutoff, as shown below:

LowPass <2> current_sens_adc_filter (4, 4e3, true);

However, I noticed that occasionally, the filter result "jumps" to a very high result despite the input being normal, or despite the input being zero (ADC pin pulled down to GND).

STM32 has a 12-bit ADC, so 4095 is the maximum number that can be input to the filter as a raw value, but sometimes I see that the output reaches even 36000.

I figured out that this happens only when I use the 2nd-order filter object of your class.
If I use a first-order filter object with the same cutoff frequency, the output is normal.
If I cascade two first-order filters, then also the output is normal.

I can conclude that the result jumps only when I use the 2nd-order filter object.

Can you please check?

Thanks,
Divyanshu
www.dtpc.in

[fikrielektro21]

Hi brother, can you explain what is the difference between 1st and 2nd order LPF? I also want to make this tool in my final project, but seeing your comment I am afraid of losing the STM board.
Thank you.

[harkhka]

Hi @fikrielektro21,

A 2nd-order LPF is a cascaded filter in which 2 filters are connected together, such that the output of the first filter goes to the input of the second filter, and the production of the second filter is used.

Thanks,
Divyanshu