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Although a digital inverse sinc filter provides a convenient method to compensate for the sinc response, like everything else in the physical world there are tradeoffs involved. The first is that a digital filter requires hardware, which means additional power dissipation as well as some dedicated real estate on the integrated circuit. Second, because the digital filter samples at the DAC sample rate, it is limited to sinc compensation only within the first Nyquist zone (DC to one half Fs), which is demonstrated on the next slide. Another issue is that the gain of the digital filter must be constrained so its digital output does not exceed the numeric range of the DAC’s input bus. This would constitute a digital overflow (or underflow) and cause severe distortion in the DAC output spectrum. This constraint forces the compensation technique to be the administering of attenuation rather than amplification. Hence, the inverse sinc filter applies attenuation to low frequency signals and progressively less attenuation to higher frequency signals in such a way that it yields an inverse sinc response. This method of compensation means that the inverse sinc filter has an insertion loss of approximately 4dB, which corresponds to the amount of sinc roll off that must be compensated for at one half Fs (the Nyquist frequency).
