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.

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  • Again I say this AD7678 is giving exactly clear and good results over 600mV. Even in these cases my IN- is nearly 0mV and my IN+ is 600mV to 3.3V. And also before this project I used another 16bit true differential ADC of another manufacturer. In that project I've read even differential 6 mV to 4.096V clearly. In that project my ref voltage was 4.096V and REFGND was nearly 0V again.

    I couldn't understand why you are insistintly saying common mode voltage must be Vref/2. If so, why I need a ADC for DC voltages. It is nonsense. I will try to find the problem by myself and if I find a solution I would also tell you to how to solve.

    And the ADCs you suggested are 16 bit. I need 18 bit. If 16 bit was enough for my design, I wouldn't change the ADC.

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  • Again I say this AD7678 is giving exactly clear and good results over 600mV. Even in these cases my IN- is nearly 0mV and my IN+ is 600mV to 3.3V. And also before this project I used another 16bit true differential ADC of another manufacturer. In that project I've read even differential 6 mV to 4.096V clearly. In that project my ref voltage was 4.096V and REFGND was nearly 0V again.

    I couldn't understand why you are insistintly saying common mode voltage must be Vref/2. If so, why I need a ADC for DC voltages. It is nonsense. I will try to find the problem by myself and if I find a solution I would also tell you to how to solve.

    And the ADCs you suggested are 16 bit. I need 18 bit. If 16 bit was enough for my design, I wouldn't change the ADC.

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