ADC with programmable quantization levels
The continuous signal is a radar signal which is to be digitized with
  an ADC with controllable qunatization levels.


A lot of ADCs have an externally controllable reference voltage input - this controls what the full scale range for the ADC is. For example, some ADCs might use a +2.5V reference with an ADC input range from 0V to +2.5V - if it is a 10 bit device, each quantization step is 2.5V/1024 = about 2.5mV.

If a different reference were used (say) 1V, the quantization level will be about 1mV.

So, choose an ADC that can work with a variation in reference voltage that suits your application OR use a fixed reference and process the signal to the ADC with a programmable gain amplifier - you could even use a multiplying DAC to give you very fine gain adjustments making it almost the perfect programmable gain amplifier.

Another alternative is to the use highest resolution ADC you can find and then not worry about quantization levels assuming it is far greater than what your application demands.