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noise of sigma delta modulator

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chungming

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Hi~~
i have a question :
often say the first OTA of sigma delta modulator will dominate the noise ,it thermal & flicker noise
if i have a three stage modulator ,the ota1 has fllicker noise at -100dB and ota2&3 have -80dB
Is the modulator's low frequency would be -100dB only dominate by ota1 ?
thanks~~~
 

I think this depends on the gain of the 1st stage.

If you get the input referred noise of the circuit, then the input referred noise of each stage will be divided by the gain of the previous stages. Thus, if the 1st OTA has high enough gain then it will dominate the noise performance...

This is the same reason that an LNA is used as the 1st stage of an RF receiver since the 1st stage usually dominates the noise performance...
 

yes, the second stage noise will be divided by the first stage, which is an integrator. Only the first stage matters.
 

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