naderi
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Posts: 8
Canada
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Hello all,
In design of a delta sigma modulator, if we look for a SNDR>85 dB for a 10 kHz bandwidth (BW), then the output integrated in-band noise (IBN) must be at least less than -85dB. This means 10 log (BW x IBN) < -85. If BW=10kHz, then IBN<-125dB/Hz. IBN includes both quantization noise and circuit noise.
As the input referred noise (IRN) of the first integrator reaches to the output of the modulator, the IRN must be less than -125dB/Hz.
I have a continuous time (CT) third order modulator that produces SNDR=98dB when the circuit noise is not considered in the a transient simulation. When I use transient analysis with transient noise in Cadence-spectre, the SNR drops to 60dB. Replacing the circuits with VerilogA modules showed the noise is due to the Active-RC integrator. However, small signal noise analysis of the integrator shows IRN < -140dB/Hz.
I wonder if the transient noise analysis is producing correct results. Please let me know your ideas. Thank you, Ali
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