how to design a low pass digital filter with smallest delay

[Altera_Forum]

Hello, All 

 I want to design a digital filter with the cutoff frequency 20Hz. The sampling frequency of my acquiring system is very high, up to 8192 Hz.   The important requirement is the delay introduced by the filter should be as small as possible.   The filter will be used online, and therefore I don't want to use the inverse model of the filter to cancel the delay.  I tried the elliptic filter, and the delay is about 20 to 30 ms. Too large for me.  What type of filters can I use?  Thank you for your help

[Altera_Forum]

In my last post, I wrote the transition band is 8Hz, from 20Hz to 28Hz

[Altera_Forum]

i understand, this is why i said designing a filter with a transition band of 1/1000 of the sample rate is not a simple task

[Altera_Forum]

It simply involves a considerable filter order. To reduce the resource consumption, one would prefer a multi-rate design usually

[Altera_Forum]

in this case you can do a down sample by 256 (try a whole series of half band filters too). i designed a filter with a transition band of 1/1000 of the sample rate (1 dB in band ripple, 100 dB stop band attenuation) and couldn't decimate (the passband was just over the quarter sample rate). it was a challenge

[Altera_Forum]

If you indeed wanted to go for decimation then a decimation polyphase structure will reduce multipliers by decimation factor. Using cascaded halfband is an overkill. 

 However, I feel an IIR filter is best option provided phase linearity is not a problem and provided the maximum delay in the band of interest is kept minimum.

[Altera_Forum]

Many thanks to everyone! 

 Now I am using two filters in parallel. It is not so bad and I have to accept the delay.  Thank you again.