ad9680 convert B

When I use ad9680, the configuration is M = 1, F = 2, L = 2. At this time, ad9680 selects converter A as the sampling channel by default. Since the impedance of converter A does not meet my requirements, I want to use converter B, but I don't know how to set it up. I tried to turn off converter A to power it down, but it had no effect, so I would like to ask how to use converter B to sample the data when a converter is selected.

Please reply me as soon as possible。

Thanks!