Hi, all
I was simulated two second-order feedforward sigma delta modulators recently. the model shown in sdm_ff.jpg, the spectrum shown in fft_sdm_ff.jpg, the result is correct. When i add the input signal feedforward path to the quantizer shown in sdm_ffxin.jpg, the result can not display the second order noise shaping effect. But this structure can usually be seen in papers or other situations. But why the result is so unreasonable?? All comments are welcome. thanks in advance.

sdm_ffxin.jpg

1: Check if overload occur for 1/(1-Z)  which map to SW-OP output and comparator/AD ?
2: scaling you input signal to be a small range .
3: calculate you NTF ,  stability? find good scaling factors for both  in-band SNR requirment and  stability.
4: use multilevel comparator/AD.

If no overload occur ,  STF and NTF have not stability problem and also have resonable target in-band SNR, but then you get bad simulink result, maybe you should check your fft window function and more fft caculation points.

Hi,

I think the coefficient of your direct-feed-forward path (according to your figure equals one)
is wrong. When multiplying the gains of the integrators I obtain 1/15. For a loop-gain of 1
your quantizer-gain should be 15. Well, the quantizer-gain of a single-bit quantizer isn't
defined. Nevertheless I think you have a coefficient of 1/15 for your direct feed-forward path.

And your are right, the additional path doesn't affect the NTF at all, and therefore also
the stability should be unchanged.

The quantizer gain has an influence on the transfer functions (NTF/STF).
Unfortunately, the gain of a 1bit quantizer isn't well defined. But usually
you derive you filter coefficients assuming that the quantizer gain is equal one.
After dynamic-range scaling you quantizer gain will probably change. In order
to have the proper STF (equals 1 in your case), the direct feed-forward path
times the quantizer gain should be one.
But I cannot predict from scratch how the loop stability is influenced in your case.
Of course, without the input signal feedforward you get a well designed feedforward
modulator, while with the input signal feedforward you STF with probably even amplify
the input signal.

I hope that helps, Regards.