I was wondering whether there is a better way of obtaining the noise transfer function (NTF) for a given sigma delta modulator structure. Meaning the transfer function from the input of the comparator to the output of the modulator.

At the moment I replace the comparator with a summer adding a PAC input signal (quantization error) and the modulator signal, ground the overall input and have spectre simulate the transfer function from the input of the removed comparator to the output. This works fine, however, I always need to re-do the circuit (take out the comp., put in a SC adder to include the quantization error......).

Is there a way where I could use the exact same circuit as in a transient simulation, leave everything as it is, and just tell spectre to give me the transfer function from the comparator node to the output node?

Thanks.