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I have designed three continuuos time sigma delta modulators with the same structure, the same coefficients, same input signal and the same sampling frequency, the only difference is that the quantizers they use is 1 bit , 3bits and 5bits respectively. All the quantizers are described bye verilog-a and they have the same delay(0.1ns,the sample frequency is 48MHz), the same rising and falling time. I had think that the modulators with 3bits and 5bits quantizers would have better SNR performance than the single bit one. But simulation shows that they have the same SNR performance( the FFT points is also identical). I am really puzzled by this problem. Can anybody tell me the reason? thank you very much!