Find that ad7606 input BW has big difference between range in 5V(15Khz) and 10V(23Khz), why input range affect the input BW? It will affect the phase for some fast change in slow signal.
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The AD7606 has two input ranges , +/-10V and +/-5V. Each voltage ranges use specific resistance or impedance to have a correct input voltage range. This change in the resistance and impedance for the specific range also affect the anti aliasing filter which it turn out affect the -3dB bandwidth. This why the two input voltage has different 3dB bandwidth response.