How to calculate SNR of dead-band quantizer?
Time align the quantized signal with your input signal and subtract to create an error signal (match the amplitudes such as to minimize the error signal). The SNR can then be determined as the ratio of the standard deviation of the input signal (after amplitude matching) to the standard deviation of the error signal (20Log10 of this ratio to be in dB).

The top diagram that I have shows this for a complex signal example, but it is also equally applicable to real signals.