FIR_LP Problem

[Altera_Forum]

I have a problem of gain with a fir_lp (value < expected value )!!! 

config : filtre_type : low pass sample rate :10khz cutoff freq :500hz coeff : 15 input number system : unsigned binary  ast_sink_data :INPUT data width 16 bits ( data signal 50hz ampl : 2.5V --> 65535 ) ast_source_data :OUTPUT width 27 bits full resolution   when i calculate RMS (value < expected value) : { data in for RMS : lp_out(lp_out'high downto lp_out'high-15) }  thinks for help

[Altera_Forum]

Cutoff freq is 1/20 of sample frequency, it's impossible to design a good FIR filter with 15 or 30 taps. A first or second order IIR filter will perform much better in this case.

[Altera_Forum]

Thanks for replying, 

Can i generate irr filter from megawizard or i should design it ?

[Altera_Forum]

There is no Altera IP for IIR filters as far as I know. MatLab or special filter tools can be used to calculate the difference equation coefficients.

[Altera_Forum]

ok thanks, 

So it's impossible to use altera FIR compiler with Cutoff freq 1/20 of sample frequency even if i change coeff?

[Altera_Forum]

fir attenuation image (http://img3.imageshack.us/img3/4210/firlp.jpg)  

in this image udat : input signal 16hz (16 bits) fir_lp : width 37 bits coeff : 100  (i have always an attenuation even with others coeff)

[Altera_Forum]

we should start with specifications. 

 sampling rate:  pass band:  stop band:  in band ripple:  stop band attenuation:   then we can define how many taps you need, how many bits to use, etc.