Hello,

I have a question about ADCs and SNRs. Normaly, when you see the specification of an ADC, they gives you an specific SNR in dBFS or dBFc. My first question is, ¿Is possible to detect a signal smaller than -SNR of the ADC when the SNR minimal needed for an specific standard in the receptor is, for example, 0dB?

I mean, imaging that the SNR in the ADC is 65dBFS and cames a signal with -70dBFS (so, it's smaller than the noise) and the minimum signal to noise ratio needed by standard is 0dB.

In the ADC the noise level doesn't depend of the bandwidth becouse when you sample, all the bands put in top of the others, but, if you make a digital filtering after this to take a channel much smaller than the total bandwidth (for example, sampling wiht 250Msps and taking channels with 5MHz). IN this case, ¿Could I be able to distinguish a signal smaller than minus the minimum specified in the ADC (-65dB in the example)?

Thanks very much!

Hi,

The SNR is the ratio of the rms signal amplitude to the rms value of the sum of all spectral components in the ADC output spectrum, which has an Fs/2 (sampling rate /2) bandwidth, except the first six harmonics and dc. Therefore the noise density at any particular frequency will be much lower, and you will be able to resolve signal levels lower than the SNR. I would recommend application note 835 on our website as a reference on how we define the specified SNR (page 9). And also this tutorial which explains how to calculate the expected noise spectral density for bandwidths less than Fs/2 (see page 42).:

http://www.analog.com/library/analogDialogue/archives/39-06/Chapter%202%20Sampled%20Data%20Systems%20F.pdf

If you have any further questions, let me know.

Regards,

David