why is SFDR larger than SNR in a properly designed ADC ?

[henrywent]

hi there,
I found that the SFDR of ADC is larger , if not always, than SNR in the datasheet of commercially available ADCs or from my simulations. Is there a definite relation between these two parameters of ADCs ? why is SFDR larger?
Any comment is welcome!
regards,
henry

 

[JoannesPaulus]

The SFDR is the ratio between the power of the fundamental and the power of the highest tone (harmonic or not) generated by the ADC. The SNR is the ratio between the power of the signal and the power of all the noise (excluding harmonics). So, unless the ADC has an extremely strong tone the SFDR is going to be larger than the SNR.

 

[henrywent]

So they are totally unrelated to each other ,right ?

 

[haykding]

SFDR and SNR really aren't related much. SNR has to do with the signal
energy versus quantization noise. Some ADCs are noisier than others so they
have an effective number of bits that is less than the actual number of bits
per sample to represent that the noise is higher than what one would expect
for just quantization, e.g. an 8 bit ADC may only have 7.2 ENOB. The SNR
then should be given by ENOB*6 or 43.2 in this case.

 

[JoannesPaulus]

SNR has to do with the signal energy versus quantization noise.

Not only the quantization noise but all the noise sources in the ADC, e.g. mismatch, clock jitter, and so on...