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In my filter design, FIR gives order 14 and IIR gives order 8 or 4. so what are the disadvantages of IIR design?

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  • $\begingroup$ I plotted the phase responses of the FIR and IIR filter. IIR filter does not provide linear phase in passband, while FIR does. why is this? $\endgroup$ – Jayesh Parmar Oct 16 '14 at 16:03
  • $\begingroup$ i would find it amazing if a 14-tap FIR could do the job of an order-8 IIR regarding sharpness of cutoff. not even close. $\endgroup$ – robert bristow-johnson Oct 16 '14 at 16:39
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Typically the advantages of an FIR filter are that it is easy to obtain a linear phase response, and numerical stability is not normally a problem.

An IIR filter typically requires fewer taps (as you have observed), so is more efficient computationally, but the phase response tends to be somewhat erratic, and numerical stability is more likely to be an issue.

So it really depends on whether phase response or compute bandwidth are more important to you, and whether you are using limited precision arithmetic (e.g. on a microcontroller) that might cause problems with an IIR filter.

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Disadvantage of IIR:

  1. Phase delay dependent on frequency
  2. Ripple in passband

Advantages IIR:

  1. Requires less RAM to execute
  2. Requires fewer multiply and add steps

Advantages of FIR:

  1. Can specify passband and stop band ripple
  2. Can specify phase delay
  3. Constant delay possible (absolute time for signal to emerge from filter)

Disadvantages of FIR:

  1. Requires more taps to reach the same performance as an IIR - means more computations per sample (more work for the processor)
  2. Requires more taps to reach the same performance as an IIR - means you need more memory to do the job. Not so much a problem on a PC, more of a problem in smaller systems where memory is tight.
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  • 3
    $\begingroup$ not all IIR filters have ripples in the passband. the two issues are not related. (consider Tchebyshev Type 2 or a Butterworth.) $\endgroup$ – robert bristow-johnson Oct 16 '14 at 16:38

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