Because the ADC outputs only 
levels there is inherently noise in the quantized
output signal. The ratio of the signal to this quantization noise is called SQNR. The
SQNR in dB is approximately equal to 6 times the number of bits of the ADC:
So for a 16 bit ADC this means that the SQNR is approximately equal to 96dB. There are, of course, other sources of noise that corrupts the output of the ADC. These include noise from the sensor, from the signal conditioning circuitry, and from the surrounding digital circuitry