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Why do we need 20 ohm resistor at the input IN-? enter image description here datasheet: http://www.ti.com/lit/ds/symlink/ads8370.pdf

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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.

The THS4031 has almost no effective output impedance so the two resistors help matching. See this figure from the THS4031 datasheet: enter image description here

Also notice that this forms a RC low-pass with a cut-off at a few MHz.

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