Why do we need 20 ohm resistor at the input IN-? datasheet: http://www.ti.com/lit/ds/symlink/ads8370.pdf
From the datasheet:
Care should be taken to ensure that the output impedance of the sources driving +IN and –IN inputs are matched. If this is not observed, the two inputs can have different settling times. This can result in offset error, gain error, and linearity error which vary with temperature and input voltage.
Also notice that this forms a RC low-pass with a cut-off at a few MHz.