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4 Altera Corporation
Preliminary
Understanding CIC Compensation Filters
Equation 4 shows the magnitude response of an N-stage CIC filter at high
frequency (f
S
):
(4)
Figure 3 shows an example of a CIC filter magnitude response:
Figure 3. Magnitude Response of a CIC Filter with N = 9, R = 8, and M = 1
f For more information about CIC filters, refer to Matthew Donadio,
Cascaded Integrator-Comb (CIC) Filter Introduction, available at
www.dspguru.com/info/tutor/cic.htm.
CIC
Compensation
Filter Design
Figure 3 shows that when the number of stages is large, the CIC filter
frequency response does not have a wide, flat pass band. To overcome the
magnitude droop, a FIR filter that has a magnitude response that is the
inverse of the CIC filter can be applied to achieve frequency response
correction. Such filters are called “compensation filters.”
For data rate down conversion, the compensation filter follows the CIC
filter. For up sampling systems, the compensation FIR filter
pre-conditions the data and is followed by a CIC filter. In other words, the
compensation filter always operates at the lower rate in a rate conversion
Hf()
π
Mf()sin
πf
R
---- -
⎝⎠
⎛⎞
sin
-----------------------
N
=