Effect of oversampling on SFDR

Hi Rick

>   Just to add my two cents here, we should keep in 
>mind that traditional 'oversampling to improve 
>signal-to-quatization-noise ratio (SQNR) is a three 
>step process:
>
>1. Sample the analog signal at a higher sample rate 
>   that is required by the Nyquist criterion.
>
>2. Lowpass filter the sampled data.  Make sure that 
>   the number of bits in the filter's coefficients and 
>   the number of bits in the arithmetic results are 
>   large enough to maintain your desired high SQNR.
>
>3. Decimate the filtered sequence.

Thank you very much for clarifying that. That is actually what I'm doing
but I should have been more explicit about it in my original question.

What prompted me to ask the question originally was trying to decide what
stopband attenuation my lowpass filter should have. My initial thought was
that it should be at least:

SNR_ADC + 10log(R)

where SNR_ADC is the quoted SNR for the ADC and R is the decimation factor.
However, I realised that, for large decimation factors, this would give me
a stopband attenuation much greater than the ADC SFDR (and require a lot of
filter taps). My thinking was that, if SFDR is not improved by
oversampling, there is no point improving SNR to the extent that signals
smaller than the spurs can be detected and therefore that I could
potentially have a less stringent stopband attenuation (and save on filter
taps). This discussion has made me think that that thinking was probably
oversimplified!

Sharon

>Sharon
>
>In the case of overall INL errors that cause low-order distortion terms,
you can model this as a continuous-time non-linearity followed by the
sampling operation. Therefore distortion terms that exceed the Nyquist rate
will alias.
>
>
>Bob
>
>

Hi Bob

Thank you for the clarification. I think I see what you are getting at.

Sharon