IIR filter design + please add me to the user list

[cedric.legendre at horiba.com]

 Good morning ,

I use IIR function to design digital filter.

My settings are :   Fsample = 100Hz , cut off freq = 30Hz, Low pass filter 
, 2nd Order ,  ELLIPTIC method , 

The transfer function given is : H(1/z) = 
                                                             2 
    0.600441568858968 + 1.19728045914772InvZ + 0.600441568858968InvZ 
 
--------------------------------------------------------------------------------------------------------------- 
 
                                     2 
           1 + 1.031059807280769InvZ + 0.391327664300703InvZ 



This filter work, but the step response, after N iterration (so the final 
value),  is 0.99 instead of 1.00.

To have the good final value, I have to insert an output gain (here: 
1.0101) to my filter.


I don't understand why I should add this gain ?

By using matlab function this output gain is not necessary


Merci d'avance for your answer


Best regards 

          Cedric  Legendre
________________________________________________
Electronic Engineer 

HORIBA - JOBIN-YVON
- Scientific Segment R&D -
 
5 Avenue Arago
ZA (Est)  de la Vigne aux Loups
F-91380 CHILLY MAZARIN - FRANCE
cedric.legendre at horiba.com
Phone : +33 (0) 1 64 54 13 00 then 89 08
http://www.horiba.com/us/en/scientific/
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