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How to deal with the unused input of AD7606-4?

I use one AD7606-4 on my board. All four channels are configured as below:

But for now, I only need use CH1 and CH2. So I tried several methods:

  1. Short the input of CH3 and CH4 as below: But after that, my chip don't work well. The data sampled from all 4 channels are messed (my input are 50Hz AC signal).
  2. Leave the CH3 and CH4's Vx floating, And give CH1 and HC2 same signal (about 1V). The data sampled from CH1 and CH2 is good.
  3. Leave CH3 and CH4 floating, give CH1 a 20mV AC signal, but CH2 a even smaller one, nearly 0, the data sampled from CH1 occasionally appear a small ‘plateau’ at 0 crossing (crossing-over distortion).

I'm a little confused now. What I should do with the unused inputs?