Thanks for the fast reply.
It seems I'm missing something, what is the difference between an idealized sampler and the zero order hold?
From what you I are saying, if the integrate and dump circuit has a pulse response, then it should have the sinc(x) response in frequency, but this isn't the case.
For the integrating ADC, I believe the resulting spectrum is that of sinc(x) not sinc
2(x); Check Figure 3 in at this link
http://www.analog.com/static/imported-files/tutorials/MT-027.pdf.
Thanks a lot Ken.