Sigma delta modulator - SNR or SNDR??

[ljy4468]

sndr sigma.delta

Dear all members.

I've a question about SNR or SNDR.

In Nyquist converter, there are difference between SNR and SNDR, of course.

But In SDM, Almost all do not use the term 'SNDR'.
and ENOB can be obtained through SNR.

I think it is because all distortion is out of band.
Is it right??

Thanks in advance

 

[panditabupesh]

sigma delta sndr

In SDM anything out of signal bandwidth is supposed to be filtered out and hence is not used in SNR calculation- however, any tone in signal bandwidth will still affect SNR and SNDR.
In Nyquist sampling the whole bandwidth is signal bandwidth and hence has to be counted.

Bupesh

 

[elbadry]

In normal ADC's, the quantization noise is not shaped so you obtain the famous resolution equation

snr = 6n dB

However, in sigma-delta shaping is used reducing the in-band quantization noise. Besides, Oversampling is also used which help reduce the quantiztion noise density.

In both cases sndr is important but sigma-delta modulators are less-prone to non-linearity than nyquist rate ADCs ( as far as i know, not quite sure ). Perhaps that's way sndr in not often used