, hello guys,  i have a problem and need help.
i an design a sigma-delta modulator for digital audio application, BW=25KHz, the goal is to achieve snr=90dB. i used mash2-1 , and osr=128,Fs=6.4MHz.
i built the modulator in the hspice with ideal blocks, when the switch on resistance was 1ohm, the result was 104dB(the matlab result was 116dB),the second stage(first orser) snr was less than that in tha matlab, the low freq noise floor was much higher than that in the matlab. (1).is that the problem of the time in the two stage interface?
(2). when change the switch on resistance to 50ohm and 100ohm.., the snr dramatically drop to 65dB, and the low freq noise floor became much high and not shapped, but the opamp ouput showed that the settling was complete, and 50ohm is not big value in the application spec, have you met this problem before? hope you guys can give me some help, thanks a lot!

the opamp i used in the hspice was eop op_ouyp op_outn opamp op_inp op_inn

Thanks for reply, and sorry for late answear.
The opamp i used was the ideal model in the HSPICE, and the circuit i built worked normally when the swotch on resistance was 1ohm, i think at least at some degree. if you think i used the wrong one then could u tell me where is the mistake and what is right one.
Another question, as we know the linear model we use to model the sigma delta modulator have nor delay in the quantizer, but when the modulator was implemented with the practical circuit it dohave one period delay in the DAC feedback signal, when i add a delay in the matlab  model, the performace will degrade dramatically, maybe i made mistake somewhere, have you guys ever notice this issue before?