sigma-delta modulator snr test

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zml1985

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recently i design a third order sigma-delta modulator . when taping out,I test the modulator and get SNDR=70dB, but the simulated result is 94db,but the noise floor is very high. can u tell me the reason. thanks in advance.
 

power domain OK? can you find some tone same as power?
cap size OK?
thermal noise and flicker noise of OP OK?
 

here is SDM output spectrum,hope you can give me some advice.
thanks
 

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