I am interested in use AD604 as the front end to an instrument, this instrument will work up to 1Mhz, I need lowest noise possible and Input Impedance of 20K
It depends on the application. If you have a 50ohm source it usually means the source needs to be terminated, and this is shown in fig 52, for example. In this instance the source isn't shown, but a great many US transducers were once 50ohms.
If your application isn't 50 ohms, or if it is terminated you may not need 50 ohms but you may need to terminate fo preserve waveform integrity. depending on the distance to the signal source.
thanks for your answer, let me explain what I am trying to do:
It is an instrument with a 2 o 3 feet coaxial cable probe, therefore length is not the problem, but I can not load the circuit I am measuring, therefore I am just guessing 20K will do, the measurements are from mV to 40 V rms, this are my questions: