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AD7678 ADC is not giving any value below 200mV

I am using 3 AD7678s in a daisy chain for a precise test system. I have some problems with the outputs of AD7678. First problem is; I cannot read any values below nearly 200mV. I get hex'00000' output or hex'20847' output below nearly 200mV, no other values. And I cannot read correct values values below nearly 500-550mV. But after nearly 600mV, I read correct values, and above 600mV to 3.3V(ref voltage) there is no problem.

I am comparing the values with "Fluke 289 The True RMS Multimeter". I am making the measurements with the same places as ADC. The ADC outputs are not changing when I am making measurements with Fluke.

Is there anyone can say anything about this weird problem.