What is 12dB SINAD?
SINAD is a measurement of signal, noise and distortion. It is defined as:
SINAD value = Signal + Noise + Distortion / Noise + Distortion
This is a power ratio so when there is no input signal to the receiver it would correspond to a SINAD value of 0 dB since there is only noise and distortion (Noise + Distortion / Noise + Distortion = 1). As we increase the input signal level to the receiver, the SINAD will increase from 0dB to some positive value. By this definition the SINAD can only be 0 or positive when expressed in dB.
The expression "12dB SINAD" means what input signal level (usually in uV) would produce a SINAD of 12 dB.
eg. 0.25uV at 12dB SINAD, meaning that a signal input level at the antenna would need to be 0.25uV to produce a SINAD of 12dB.