There are several reasons IMO:
1.) Nothing ever works as good in practice as it should work in theory. If it really would, then the Sharp D-S amps would have a much better SNR than their 100dB (Theoretically 270 dB @ 20 kHz).
2.) The chosen dither is certainly not optimal.
3.) The chosen NTF may not be optimal either since I didn't spend too much time on detemining it. I just chose 200 kHz cutoff and fourth-order Butterworth for the NTF and then derived the coefficients for achieving it. This choice is quite critical: the lower the cutoff frequency - the worse the SNR, the higher it is (i.e. aggressive noise-shaping) - the more the loop is prone to instability.
Regards
Charles
1.) Nothing ever works as good in practice as it should work in theory. If it really would, then the Sharp D-S amps would have a much better SNR than their 100dB (Theoretically 270 dB @ 20 kHz).
2.) The chosen dither is certainly not optimal.
3.) The chosen NTF may not be optimal either since I didn't spend too much time on detemining it. I just chose 200 kHz cutoff and fourth-order Butterworth for the NTF and then derived the coefficients for achieving it. This choice is quite critical: the lower the cutoff frequency - the worse the SNR, the higher it is (i.e. aggressive noise-shaping) - the more the loop is prone to instability.
Regards
Charles
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