john_ana
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Hi Berti,
I agree that for high or moderate OSR, the difference is not big. For low OSRs, how do you achieve the desired NTF?
1) use CRFF or CRFB structure, which has a z/(z-1)^2 in their transfer function. It is easy to realize z^-1/(1-z^-1), how do you realize 1/(1-z^-1). I know some switch-capacitor implementation can realize z^1/2, is there any other way?
2) use CIFB structure, but you have need extra DACs need to build and Feed forward structure can achieve lower distortion since it only process mostly quantization noise only.
3) how do you achieve the NTF in continuous-time Sigma-Delta? I know you can use d2c with zoh to converter the NTF from Z domain to S domain and the 'zoh' will take care NRZ dac pulse, however, usually you get a transfer function H(s) that has poles like Num(H(s))=s(a*s^2+b*s +c). However, if you use two 1/s in cascade and feedback from the output of the second 1/s to the input of the first 1/s, you will get something like K(s)=(x*s^2+y), unless you feedback from the output of the second 1/s to the input of 1/s as well. However, if you look at most published continuous-time sigma-delta, nobody actually feedback from the output of the second 1/s to the input of the second 1/s. All of them only feedback from the output of the second 1/s to the input of first 1/s.
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