Hi,
When you have no input, (called idle-channel) the reported SNR will not be 0 dBFS due to the definition of dBFS. You may want to choose to disply the SNR in dBc instead of dBFS and see if that is more like what you expect to see. There is a menu item for dBFS under the Test Options menu that is checked by default. When SNR is calculated it is first calculated in dBc which is the ratio of the power of the fundamental to the sum of the power of the rest of the bins of the FFT neglecting harmonics. If the SNR is to be displayed in terms of dBFS, then the dBFS value is the dBc value plus the amount that the fundamental is below full scale. in other words, the dBFS value is an optimistic report of what the SNR would be if the fundamental were exactly up to the full scale level. Since with an idle-channel input the power of the fundamental is down in the noise floor, the amount that is added to the fundamental to get the dBFS value will be a large number. I think the dBc value for SNR is what you are lookign for.
Regards,
RIchard P.