First order digital filter

[omid2006]

Hello friends
I found a unknown digital low pass filter in a sample code but I can't adapt it's formula with common filters like IIR because it's like a IIR filter with negative coefficient. it's like feedback filter but feedback filter is a highpass in ordinary and this filter is low pass. I'll write it's formula and I appreciate if anyone can help.

out[n]+=(in[n]-out[n-1])*500*Ts

and Ts is the sampling frequency thus execution rate of filter.

 

[danadakk]

3 different results from the following :

out[n]+=(in[n]-out[n-1])*500*Ts

OR

out[n]=out[n] +(in[n]-out[n-1])*500*Ts

 

[FvM]

Second equation doesn't correspond to feasible time discrete system. Only useful interpretation in post #7, #17, #19.

 

[danadakk]

The second equation is the first equation, post #1, just the += expanded.

So yes, not feasible, except by, as MATLAB complained, adding in all paths a unit delay.
Which I did and got it to solve. But then, and here is where I am over my head, I think it
just causes group delay, but magnitude response still accurate......not sure.

The above is why I recommended to OP to do a recursive solution in Excel to look at samples
generated, and hence plots.

Makes me wonder if there is a typo here from OP or his source.

 

[FvM]

The second equation is the first equation, post #1, just the += expanded.

No, it makes no sense to write u[n] on RHS of the assignment, must be u[n-1]. There's one unit delay created by performing the assignment periodically.

Step response is monotical.

 

[omid2006]

Hi Friends
Thanks for your attention all.
I've found the answer, that was just a 1/(1+tS) which descretized with tustin method and some simplifications. The attached link shows how it works.

https://www.pomad.fr/tutorials/stm32/digital_filtering

regards