Why notches in sinc filter response Below response shows sinc1,sinc2 and sinc3 filter response.
I understand that notches in the frequency response happens at the output data rate of the sigma-delta ADC.

But I am not able to figure out why there is a notch ? All articles I read mentions that notch happens at integral multiples of output data rate but doesn't give explanation.
Also why notch width increases as order of the sinc filter increases ?

Please let me know if I am missing basic concepts that I should read before finding answer to this question.
Look at the equation for the frequency magnitude response of a sinc filter. It will be of the form:
$$\frac{\sin(\alpha)}{ \alpha}$$
Look carefully at the $\alpha$ term and determine for what frequencies the $\sin(\alpha)$ numerator will be equal to zero. The magnitude response notches occur at those frequencies.