SC gain stage in pipeline ADC

For the SC gain stage(*2) in pipeline ADC,I used ideal switch and ideal opamp.But if input is 1v peak sin wave, the SNR of output is just 60dB.But if input sin wave is 0.5V, the ouput is above 90dB. Is that correct?I used this SC gain stage in pipeline stage,I can't get expected SNR.But I can't increase SNR for this circuit.Anyone can give me suggestion about where the problems are?

Thank you so much!