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regardin the SNR of ADC

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manissri

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hi one query
for 10bit Nyquist Rate ADC .
i am giving 1k signal input and sampling frequency is 4MHz.
then what snr i expect therotically.
snr = 6.02N+1.76 ==> 61.96dB
but i have seen the snr equation is SNR=6.02N+1.76+10log(OSR)
here i am applying the input fin=1k and fsamp= 4M
so the OSR = fsamp/2*fin = 4M/2K = 2000
hence total SNR = 6.02N+1.76+33= 94.96dB
but in real i am getting the only 58dB.
so where is the problem in understaning in snr calculation
Please clarify the same
 

manissri said:
... i have seen the snr equation is SNR=6.02N+1.76+10log(OSR)
Click to expand...
Where did you find this equation? A multiplication factor of 10 in front of a log ratio within an SNR or SINAD calculation seems rather weird, s. e.g. Eq. 2 on p.6 of AD's tutorial on this stuff.
 

The oversampling gain only is referred to the bandwidth of the signal. If you are still integrating your noise over the Nyquist band, you do not get any gain from oversampling but if you integrate your noise up to, say, 2kHz you should get 30dB more.
Moreover, an ADC specified for 10 bits will not provide 10 effective bits. The equation you quote is for quantization noise only and it does not include thermal noise, mismatch, non-linearity and so forth...
 

thanks JoaneePaul for your quick answer
as i am calculatiing the snr from dc to fs/2 ( i.e upto 2 MHZ)
as i am doing ovesampling 2000 times i.e. i am taking samples 2000 times for one input signal cycle. so due to over sampling the quantization noise floor will be reduced upto fs/2 frequency range hence snr would be increased upto 33dB.
but in real this is not happening i.e. i am getting only 58dB which sounds (6.03N+1.76 --> comming form this formula) not from 6.02N+1.76+10logOSR
can you please elaborate the things more so that the understanding would be proper
Thanks in Advance
 

You are integrating the noise over 2MHz therefore there is no oversampling gain: fs/(2*BW)=1, e.g. no oversampling.
If you want to take advantage of the oversampling gain you need to consider a smaller bandwidth, for instance between 0 and 2KHz (digitally filtering the output of your ADC with a 2KHz filter) and calculate the SNR between 0 and 2KHz.
 

Thanks Joan
for your comment
 

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