The voltage at an Ch X pin of multiplexer appears exactly as it is on COM pin until it is below about 1V.
However, as it increases above 1 V, the offset(voltage drop) starts to gradually increase with input voltage.
2V at Ch X pin leads to 1.98V at COM pin.
The supply voltage for both ADC and analog multiplexer is 3.3V
The Ron of analog multiplexer is 125 Ohms but there is virtually no current flowing through it, since the input impedance of the ADC is 6M Ohms.
I tried removing 1K resistor connecting COM pin and ADC input, but the drop voltage increased to 30mV from 20mV.
The result is that There is about 1 % nonlinearity. What is wrong and How should I improve it?