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Difference Between IIR and FIR Filters

iirfilter-pdIIR vs FIR Filters

IIR filters are difficult to control and have no particular phase, whereas FIR filters make a linear phase always possible. IIR can be unstable, whereas FIR is always stable. IIR, when compared to FIR, can have limited cycles, but FIR has no limited cycles. IIR is derived from analog, whereas FIR has no analog history. IIR filters make polyphase implementation possible, whereas FIR can always be made casual.

FIR filters are helpful to achieve fractional constant delays. #MAD stands for a number of multiplications and additions, and is used as a criterion for an IIR and FIR filter comparison. IIR filters require more #MAD when compared to FIR, because FIR is of a higher order in comparison to IIR, which is of lower order, and uses polyphase structures.

FIR filters are dependent upon linear-phase characteristics, whereas IIR filters are used for applications which are not linear. FIR’s delay characteristics is much better, but they require more memory. On the other hand, IIR filters are dependent on both i/p and o/p, but FIR is dependent upon i/p only. IIR filters consist of zeros and poles, and require less memory than FIR filters, whereas FIR only consists of zeros. IIR filters can become difficult to implement, and also delay and distort adjustments can alter the poles & zeroes, which make the filters unstable, whereas FIR filters remain stable. FIR filters are used for tapping of a higher-order, and IIR filters are better for tapping of lower-orders, since IIR filters may become unstable with tapping higher-orders.

FIR stands for Finite IR filters, whereas IIR stands for Infinite IR filters. IIR and FIR filters are utilized for filtration in digital systems. FIR filters are more widely in use, because they differ in response. FIR filters have only numerators when compared to IIR filters, which have both numerators and denominators.

Where the system response is infinite, we use IIR filters, and where the system response is zero, we use FIR filters. FIR filters are also preferred over IIR filters because they have a linear phase response and are non recursive, whereas IIR filters are recursive, and feedback is also involved. FIR cannot simulate analog filter responses, but IIR is designed to do that accurately. IIR’s impulse response when compared to FIR is infinite.

The high computational efficiency of IIR filters, with short delays, often make the IIR popular as an alternative. FIR filters have become too long in digital feedback systems, as well as in other applications, and cause problems.


1. IIR is infinite and used for applications where linear characteristics are not of concern.

2. FIR filters are Finite IR filters which are required for linear-phase characteristics.

3. IIR is better for lower-order tapping, whereas the FIR filter is used for higher-order tapping.

4. FIR filters are preferred over IIR because they are more stable, and feedback is not involved.

5. IIR filters are recursive and used as an alternate, whereas FIR filters have become too long and cause problems in various applications.

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  1. Its good. If i have any query. I will ask u. But plz let me knw that ur email id & send it on my mail id so i can ask u. Thanx

  2. I’m surprised how accurate this article is. Granted, it isn’t complete by any means, and doesn’t go into the mathematical aspects of it, but it deals with the application domain knowledge of the FIR and IIR filters pretty well.

  3. thanks…………………

  4. Just a note: the last word of the 1st paragraph should be *causal*, not casual :)

    • Hi.. I noticed that causal spelling error too.
      Jesse.. are you available to do a filter design for a start up.. rather complicated beast?


  5. FIR
    no feedback.
    easy to control.
    only zero so required much memory.

    non recursive.
    have feedback.
    difficult to control.
    have zero and poles so required less memory.

    • @Rohit

      I like your table….

      But, I believe you associated “recursive” with the wrong filter type.

      My understanding is that FIR filters are not recursive
      where as IIR filters are recursive.

  6. Thanks a ton! :)

  7. cool….thnx

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